Covid19Phase 2NCT04668339

A Trial Evaluating the Safety and Effects of an RNA Vaccine ARCT-021 in Healthy Adults

A Phase 2 study testing ARCT-021 single dose priming, ARCT-021 two lower dose priming and ARCT-021 two higher dose priming in people with Covid19 and SARS-CoV Infection, run by Arcturus Therapeutics, Inc. at 15 sites in 2 countries.

Terminated

Stopped early. The study ended before it was planned to and will not resume.

Official record

Search all trials

Trial summary

This is a Phase 2, randomized, placebo-controlled, and observer-blind study in healthy adults. The study will evaluate the safety, tolerability, and immunogenicity of the SARS-CoV-2 RNA vaccine candidate against COVID-19: As 2 doses (at two different dose levels), separated by 28 days or as 1 dose In adults 18 years… Read more about this study

Who is it for?: Ages 18 years and olderHealthy volunteers can take part
Conditions the study is for: Covid19, SARS-CoV Infection, Corona Virus Infection
What is being tested?: ARCT-021 single dose priming, ARCT-021 two lower dose priming and ARCT-021 two higher dose priming
Could I receive a placebo?: Yes, some participants may receive a placebo
How many people: 581 participants
Where: 15 sitesin 2 countries
Study dates: 7 Jan 2021to 1 Mar 2022
Phase: Phase 2
Are there results?: Yes, posted in the registrySummary in the doctors' view

Key requirements

Who can take part, as listed by the sponsor. Only the study team can confirm whether you are eligible, so take this list to your doctor.

Ages 18 years and older
All sexes
Healthy volunteers accepted
Adult, Older adult

Who may be able to join

Individuals who:
are able to provide consent
agree to comply with all study visits and procedures
are willing and able to adhere to study restrictions
are sexually active and willing to adhere to contraceptive requirements
are male or female ≥18 or (in Singapore) ≥21 years of age
are medically stable

Who cannot join

Individuals who:
have had SARS-CoV-2 infection or COVID-19 disease.
have had cancer except for cancers that were treated and that have low risk of returning
have chronic kidney disease
have some chronic lung diseases
have some heart conditions

Show all 26Show fewer

have compromised immune systems
are obese
have sickle cell disease or some other blood disorders
are current smokers and/or use illegal drugs
have Type 2 diabetics
are immunocompromised, immunodeficient or have had a transplant
have autoimmune disease
have other severe or uncontrolled diseases or disease that may interfere with the interpretation of the study
have a positive test for hepatitis B or C or human immunodeficiency virus
have had a severe reaction to previous investigational vaccines
have a fever or are feeling sick close to the time of the first vaccination of the study
have positive drug test at screening
are pregnant
are breastfeeding
have a bleeding disorder
have previously received an investigational coronavirus vaccine (SARS-CoV(1) or MERS) or who plan to be in other COVID-19 studies
have recently been vaccinated with other vaccines
have recently received blood products
who work at one of the clinic sites participating in this study, work at Arcturus, who work at other companies that monitor the study or close family members to the sites, Arcturus, or partners involved in study monitoring
other restrictions may apply

Trial locations and contacts

15 sites in 2 countries.

Country

City, postcode or hospital

15 sites in 2 countries

United States12 sites

Arcturus Investigational Site 102
Anderson, South Carolina, 29621
Arcturus Investigational Site 111
Austin, Texas, 78705
Arcturus Investigational Site 103
Chandler, Arizona, 85224
Arcturus Investigational Site 108
Dallas, Texas, 75234
Arcturus Investigational Site 104
Melbourne, Florida, 32934
Arcturus Investigational Site 105
Orlando, Florida, 32806
Arcturus Investigational Site 101
Peoria, Illinois, 61614
Arcturus Investigational Site 106
Pinellas Park, Florida, 33781
Arcturus Investigational Site 110
Rockville, Maryland, 20850
Arcturus Investigational Site 112
San Diego, California, 92108
Arcturus Investigational Site 109
The Villages, Florida, 32162
Arcturus Investigational Site 107
Tucson, Arizona, 85741

Other trials in United States

Singapore3 sites

Arcturus Investigational Site 204
Singapore, 117599
Arcturus Investigational Site 201
Singapore, 169608
Arcturus Investigational Site 203
Singapore, 308433

Other trials in Singapore

About this study

Official title: A Phase 2 Randomized, Observer-Blind, Placebo-Controlled Study to Assess the Safety, Reactogenicity, and Immunogenicity of the SARS CoV-2 Vaccine ARCT-021 in Healthy Adult Participants

This is a Phase 2, randomized, placebo-controlled, and observer-blind study in healthy adults.

The study will evaluate the safety, tolerability, and immunogenicity of the SARS-CoV-2 RNA vaccine candidate against COVID-19:

As 2 doses (at two different dose levels), separated by 28 days or as 1 dose

In adults 18 years of age and older

Read the detailed description

This is a multiregional, multicenter, Phase 2, randomized, observer-blind study designed to evaluate the safety, reactogenicity, and immunogenicity of the study vaccine in younger and older adult participants. Enrolled participants will be randomly assigned to receive either study vaccine ARCT-021 or placebo (sterile saline).

Approximately 600 participants (300 each in younger [18 to <56 years of age in United States or 21 to <56 years of age in Singapore] and older [≥56 years of age] participants) will be enrolled (including at least 50% of participants in the older cohort ≥65 years of age). Participants will be stratified by age and then randomly assigned (3 ARCT-021:1 placebo) to receive 2 doses of study vaccine separated by 28 days. At 180 days after second study vaccination (Day 208), participants in Study Groups 1, 2, and 3 will be randomly assigned again to receive a single booster dose of study vaccine (randomly assigned as 1 ARCT-021:1 placebo). Study Group 4 will not be randomized but will receive 1 dose of placebo at Day 208. Study Groups are summarized in Table 1. Study vaccine will be administered in an observer-blind fashion. Participants will be followed for safety and immunogenicity through 180 days after booster vaccination (Day 388). At a subset of clinical sites, all enrolled participants will also undergo blood sampling for evaluation of CMI responses.

Vaccine doses will be assigned as follows:

Younger Age Cohort:

Study Group 1: n= 75 participants, ARCT-021 7.5 µg (first dose), Placebo (second dose Study Group 2: n= 75 participants, ARCT-021 5.0 µg (first dose), 5.0 µg (second dose) Study Group 3: n= 75 participants, ARCT-021 7.5 µg (first dose), 7.5 µg (second dose) Study Group 4: n= 75 participants, Placebo (first dose), Placebo (second dose)

Booster Vaccine:

Study Groups 1, 2, 3: 113 participants, ARCT-021 5.0 µg or 7.5 µg, 112 participants, Placebo Study Group 4: n= 75 participants, Placebo

Older Age Cohort:

Booster Vaccine:

Study Groups 1, 2, 3: 113 participants, ARCT-021 5.0 µg or 7.5 µg, 112 participants, Placebo Study Group 4: n= 75 participants, Placebo

A DSMB will be in place to independently review the safety data of participants. Pausing Rules are also utilized in this study to reduce risk to study participants.

The expected duration of participation for an individual participant is approximately 14 months, inclusive of the Screening period.

Conditions: Covid19, SARS-CoV Infection, Corona Virus Infection
Being tested: ARCT-021 single dose priming, ARCT-021 two lower dose priming, ARCT-021 two higher dose priming, Placebo (two doses), priming, Randomized booster
Keywords: SARS-CoV-2 Vaccine, Coronavirus Virus Diseases, RNA COVID 19, COVID 19 Vaccine Arcturus, self amplifying RNA vaccine
Medical subject headings: COVID-19, Severe Acute Respiratory Syndrome, Coronavirus Infections

What participants receive

What participants receive. In a randomised study each person is assigned to one group by chance.

ARCT-021 single dose primingBiological
ARCT-021 higher dose (one dose) + placebo (one dose)
ARCT-021 two lower dose primingBiological
ARCT-021 lower dose (two doses, Day 0 and Day 28)
ARCT-021 two higher dose primingBiological
ARCT-021 higher dose (two doses, Day 0 and Day 28)
Placebo (two doses), primingBiological
Placebo (two doses, Day 0 and Day 28)
Randomized boosterBiological
ARCT-021 (single dose) OR placebo, booster
Placebo boosterBiological
Placebo (single dose)

Study groups in NCT04668339
Study group	Role	What they receive
Study Group 1, Younger Adult Participants	Experimental	Participants will receive one dose of ARCT-021 on Day 0, one dose of Placebo (saline) on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo Receives: Biological: ARCT-021 single dose priming; Biological: Randomized booster
Study Group 2, Younger Adult Participants	Experimental	Participants will receive one dose of ARCT-021 on Day 0, a second dose of ARCT-021 on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo Receives: Biological: ARCT-021 two lower dose priming; Biological: Randomized booster
Study Group 3, Younger Adult Participants	Experimental	Participants will receive one dose of ARCT-021 on Day 0, a second dose of ARCT-021 on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo Receives: Biological: ARCT-021 two higher dose priming; Biological: Randomized booster
Study Group 4, Younger Adult Participants	Placebo comparator	Participants will receive one of Placebo (Saline) on Day 0, one dose of Placebo on Day 28, and one dose of Placebo on Day 208 Receives: Biological: Placebo (two doses), priming; Biological: Placebo booster
Study Group 1, Older Adult Participants	Experimental	Participants will receive one dose of ARCT-021 on Day 0, one dose of Placebo (saline) on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo Receives: Biological: ARCT-021 single dose priming; Biological: Randomized booster
Study Group 2, Older Adult Participants	Experimental	Participants will receive one dose of ARCT-021 on Day 0, a second dose of ARCT-021 on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo Receives: Biological: ARCT-021 two lower dose priming; Biological: Randomized booster
Study Group 3, Older Adult Participants	Experimental	Participants will receive one dose of ARCT-021 on Day 0, a second dose of ARCT-021 on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo Receives: Biological: ARCT-021 two higher dose priming; Biological: Randomized booster
Study Group 4, Older Adult Participants	Placebo comparator	Participants will receive one dose of Placebo (saline) on Day 0, a second dose of Placebo on Day 28 and a third dose of Placebo on Day 208 Receives: Biological: Placebo (two doses), priming; Biological: Placebo booster

What is being measured

The primary outcome is the main question the study is designed to answer.

Percentage of Participants Reporting Solicited Local Adverse Events Within 7 Days Post Each Priming Vaccination
Per prespecified analysis, solicited local adverse events were defined as pain, erythema, swelling, or injection site tenderness. Solicited adverse reactions were recorded by the participant in an eDiary. Per prespecified analysis, solicited adverse events were not evaluated for severity (such as, serious or non-serious). A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Measured: For 7 days following each dose administration (Day 0 up to Day 7 for Vaccination 1 and Day 29 up to Day 35 for Vaccination 2)
Percentage of Participants Reporting Solicited Local Adverse Events Within 7 Days Post Booster Vaccination
Per prespecified analysis, solicited local adverse events were defined as pain, erythema, swelling, or injection site tenderness. Solicited adverse reactions were recorded by the participant in an eDiary. Per prespecified analysis, solicited adverse events were not evaluated for severity (such as, serious or non-serious). A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Measured: For 7 days following each dose administration (Day 208 up to Day 215)
Percentage of Participants Reporting Solicited Systemic Adverse Events Within 7 Days Post Each Priming Vaccination
Per prespecified analysis, solicited systemic adverse events were defined as fever, headache, fatigue, myalgia, arthralgia, nausea, chills, diarrhea, dizziness, and vomiting. Per prespecified analysis, solicited adverse events were not evaluated for severity (such as, serious or non-serious). A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Measured: For 7 days following each dose administration (Day 0 up to Day 7 for Vaccination 1 and Day 29 up to Day 35 for Vaccination 2)
Percentage of Participants Reporting Solicited Systemic Adverse Events Within 7 Days Post Booster Vaccination
Per prespecified analysis, solicited systemic adverse events were defined as fever, headache, fatigue, myalgia, arthralgia, nausea, chills, diarrhea, dizziness, and vomiting. Per prespecified analysis, solicited adverse events were not evaluated for severity (such as, serious or non-serious). A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Measured: For 7 days post booster dose administration (Day 208 up to Day 215)
Percentage of Participants Reporting Unsolicited Adverse Events Up to 28 Days Post Each Priming Vaccination
An unsolicited AE was defined as any AE (serious and nonserious) occurring after administration of first dose of study vaccine and before the end of study. A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Measured: 28 days following each dose administration (Day 0 up to Day 28 for Vaccination 1 and Day 29 up to Day 56 for Vaccination 2)
Percentage of Participants Reporting Unsolicited Adverse Events Up to 28 Days Post Booster Vaccination
An unsolicited AE was defined as any AE (serious and nonserious) occurring after administration of first dose of study vaccine and before the end of study. A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Measured: 28 days following each dose administration (Day 208 up to 236 days)
Percentage of Participants Reporting Treatment-Emergent Serious Adverse Events (SAE), Medically Attended Adverse Events (MAAE) and New Onset of Chronic Disease (NOCD) Post Each Priming Vaccination
SAEs were defined as any event that resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in a persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions, resulted in a congenital anomaly or birth defect, or was an important medical event. An NOCD was defined as any AE that led to the new diagnosis of a chronic medical condition that was not present or suspected prior to enrollment. An MAAE was an AE that led to an unscheduled visit (including a telemedicine visit) to a healthcare practitioner. A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Measured: Up to Day 207
Percentage of Participants Reporting Treatment-emergent Serious Adverse Events, Medically Attended Adverse Events and New Onset of Chronic Disease Post Booster Vaccination
SAEs were defined as any event that resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in a persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions, resulted in a congenital anomaly or birth defect, or was an important medical event. An NOCD was defined as an AE that led to the new diagnosis of a chronic medical condition that was not present or suspected prior to enrollment. An MAAE was an AE that led to an unscheduled visit (including a telemedicine visit) to a healthcare practitioner. A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Measured: Day 208 to early termination (up to 396 days)
Geometric Mean Titer (GMT) of Serum Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Neutralizing Antibodies Post Priming Vaccination
Measured: Day 1
GMT of SARS-CoV-2 Neutralizing Antibodies Post Priming Vaccination
Measured: Day 56
GMT of SARS-CoV-2 Neutralizing Antibodies Post Booster Vaccination
Measured: Day 208
GMT of SARS-CoV-2 Neutralizing Antibodies Post Booster Vaccination
Measured: Day 236
Geometric Mean Fold Rise (GMFR) in SARS-CoV-2 Neutralizing Antibody Titers Post Priming Vaccination
GMFR is reported as a ratio to baseline (Day 0).
Measured: Day 56
Geometric Mean Fold Rise (GMFR) in SARS-CoV-2 Neutralizing Antibody Titers Post Booster Vaccination
GMFR is reported as a ratio to baseline (Day 0).
Measured: Day 208
GMFR in SARS-CoV-2 Neutralizing Antibody Titers Post Booster Vaccination
GMFR is reported as a ratio to baseline (Day 0).
Measured: Day 236
Percentages of Participants Achieving Greater Than or Equal to 2-fold and 4-fold Increase From Before Vaccination (Baseline) in SARS-CoV-2 Serum Neutralizing Antibody Levels at Day 56
Measured: Baseline up to Day 56
Percentages of Participants Achieving Greater Than or Equal to 2-fold and 4-fold Increase From Before Vaccination (Baseline) in SARS-CoV-2 Serum Neutralizing Antibody Levels at Day 208
Measured: Baseline up to Day 208
Percentages of Participants Achieving Greater Than or Equal to 2-fold and 4-fold Increase From Before Vaccination (Baseline) in SARS-CoV-2 Serum Neutralizing Antibody Levels at Day 236
Measured: Baseline up to Day 236

Secondary outcomes (4)

Geometric Mean Concentration of SARS-CoV-2 Anti-S1, Anti-RBD, and Anti-N Binding Antibody Levels Post Priming Vaccination
Measured: Days 0 and 56
Geometric Mean Concentration of SARS-CoV-2 Anti-S1, Anti-RBD, and Anti-N Binding Antibody Levels Post Booster Vaccination
Measured: Day 208 and 236
Geometric Mean Fold Rise (GMFR) in SARS-CoV-2 Anti-S1, Anti-RBD, and Anti-N Binding Antibody Levels Before Vaccination to Day 56
Measured: Day 56
Percentage of Participants Achieving Greater Than or Equal to 2-fold and 4-fold Increase From Before Vaccination (Baseline) in SARS-CoV-2 Anti-S1, Anti-RBD, and Anti-N Binding Antibody Levels
Measured: Day 56

Timeline

From the start of the study to its completion; the mark is today. Estimated dates are the sponsor’s plan.

7 Jan 20211 Mar 2022

First posted16 Dec 2020
Study start7 Jan 2021
Primary completion1 Mar 2022
Study completion1 Mar 2022

Why it stopped: A decision was made to terminate the study for operational/business reasons. The study was not terminated for reasons of safety or immunogenicity.

Resources

Official record on ClinicalTrials.govThe registry entry this page is built from, with its full change history.
Posted resultsParticipant flow, outcome measures and adverse events reported by the sponsor.
Other trials for Covid19Studies of the same condition, open and under way.
What the trial phases meanHow a treatment moves from first tests in people to approval.

More details

For doctors and researchers: how the study is designed, who runs it, and its registry IDs.

Study design

Study type: Interventional
Phase: Phase 2
Primary purpose: Prevention
Allocation: Randomised
Intervention model: Parallel groups
Model description: sequential assignment
Masking: Quadruple blind
Who is masked: Participant, Care provider, Investigator, Outcomes assessor
Enrolment: 581 participants (actual)

IDs, dates and publications

ClinicalTrials.gov ID: NCT04668339
Sponsor's protocol ID: ARCT-021-04
Status last verified: May 2025
First submitted: 25 Nov 2020
Last updated: 15 May 2025
Results posted: Yes, on ClinicalTrials.gov
Data monitoring committee: Yes
FDA-regulated drug: Yes
FDA-regulated device: No
Individual data sharing: No: Individual participant data will only be made available to study investigators at this time.

Want to know more?

The sponsor has not listed a contact. The official record may have more details.

Talk to your doctor first and take this page with you. Only the study team can confirm whether you can join.

Questions to ask the study team

What is the study trying to find out?
What would I need to do, and how often would I visit?
Could I receive a placebo instead of the treatment?
What are the possible risks and benefits for me?
Will it cost me anything? Are travel or time paid for?
Can I keep seeing my own doctor during the study?
What happens when the study ends?

See locations

Not yet recruitingPhase 1
Phase I Open-Label Safety Trial of Pembrolizumab for Neurological Post- Acute Sequelae of SARS-CoV-2 (PD1-PASC I)
Post-Acute COVID-19 Syndrome
Active, not recruitingPhase 1
A Safety and Immunogenicity Trial of OCU500, ChAd36 Vector Encoding SARS-CoV-2 Spike Vaccine Via Intranasal and Inhalational Routes in Previously Vaccinated Adults
COVID-19
Active, not recruiting
Intervention to Address Disparate Mental Health Consequences of COVID-19 Pandemic on Latinx and African Newcomers
Mental Health Issue, Mental Health Disorder
Recruiting
Home-Based Exercise Tele-Rehabilitation After COVID-19
Post SARS-CoV2 (COVID-19)
Not yet recruitingPhase 1
A Safety, Reactogenicity and Immunogenicity Trial of RVX-sCPD9 Booster Intranasal COVID-19 Vaccine
COVID -19

All Covid19 trials

Study at a glance

From the registry record, last updated by the sponsor on 15 May 2025. Recruitment status is as the sponsor reported it: confirm with the study team.

Official title: A Phase 2 Randomized, Observer-Blind, Placebo-Controlled Study to Assess the Safety, Reactogenicity, and Immunogenicity of the SARS CoV-2 Vaccine ARCT-021 in Healthy Adult Participants
ClinicalTrials.gov ID: NCT04668339
Acronym / sponsor ID: ARCT-021-04
Conditions: Covid19, SARS-CoV Infection, Corona Virus Infection
Keywords: SARS-CoV-2 Vaccine, Coronavirus Virus Diseases, RNA COVID 19, COVID 19 Vaccine Arcturus, self amplifying RNA vaccine
Phase: Phase 2
Study type: Interventional
Recruitment status: TerminatedReason stopped: A decision was made to terminate the study for operational/business reasons. The study was not terminated for reasons of safety or immunogenicity.
Enrolment: 581 participants (actual)
Lead sponsor: Arcturus Therapeutics, Inc.Industry (pharma, biotech, devices)
Study start: 7 Jan 2021 (actual)
Primary completion: 1 Mar 2022 (actual)
Study completion: 1 Mar 2022 (actual)
First posted: 16 Dec 2020 (actual)
Results: Posted 15 May 2025

Clinical summary

The research question as the sponsor registered it. This describes what the study plans to test, not what it found.

This is a Phase 2, randomized, placebo-controlled, and observer-blind study in healthy adults.

The study will evaluate the safety, tolerability, and immunogenicity of the SARS-CoV-2 RNA vaccine candidate against COVID-19:

As 2 doses (at two different dose levels), separated by 28 days or as 1 dose

In adults 18 years of age and older

Detailed description

Vaccine doses will be assigned as follows:

Younger Age Cohort:

Booster Vaccine:

Study Groups 1, 2, 3: 113 participants, ARCT-021 5.0 µg or 7.5 µg, 112 participants, Placebo Study Group 4: n= 75 participants, Placebo

Older Age Cohort:

Booster Vaccine:

Study Groups 1, 2, 3: 113 participants, ARCT-021 5.0 µg or 7.5 µg, 112 participants, Placebo Study Group 4: n= 75 participants, Placebo

A DSMB will be in place to independently review the safety data of participants. Pausing Rules are also utilized in this study to reduce risk to study participants.

The expected duration of participation for an individual participant is approximately 14 months, inclusive of the Screening period.

Study design

Primary purpose: Prevention
Allocation: Randomised
Intervention model: Parallel groups
Masking: Quadruple blind (participant, care provider, investigator, outcomes assessor)
Number of arms: 8
Model description: sequential assignment

Arm	Type	Receives
Study Group 1, Younger Adult Participants	Experimental	Biological: ARCT-021 single dose priming, Biological: Randomized boosterParticipants will receive one dose of ARCT-021 on Day 0, one dose of Placebo (saline) on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo
Study Group 2, Younger Adult Participants	Experimental	Biological: ARCT-021 two lower dose priming, Biological: Randomized boosterParticipants will receive one dose of ARCT-021 on Day 0, a second dose of ARCT-021 on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo
Study Group 3, Younger Adult Participants	Experimental	Biological: ARCT-021 two higher dose priming, Biological: Randomized boosterParticipants will receive one dose of ARCT-021 on Day 0, a second dose of ARCT-021 on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo
Study Group 4, Younger Adult Participants	Placebo comparator	Biological: Placebo (two doses), priming, Biological: Placebo boosterParticipants will receive one of Placebo (Saline) on Day 0, one dose of Placebo on Day 28, and one dose of Placebo on Day 208
Study Group 1, Older Adult Participants	Experimental	Biological: ARCT-021 single dose priming, Biological: Randomized boosterParticipants will receive one dose of ARCT-021 on Day 0, one dose of Placebo (saline) on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo
Study Group 2, Older Adult Participants	Experimental	Biological: ARCT-021 two lower dose priming, Biological: Randomized boosterParticipants will receive one dose of ARCT-021 on Day 0, a second dose of ARCT-021 on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo
Study Group 3, Older Adult Participants	Experimental	Biological: ARCT-021 two higher dose priming, Biological: Randomized boosterParticipants will receive one dose of ARCT-021 on Day 0, a second dose of ARCT-021 on Day 28 and on Day 208 either one dose of ARCT-021 or one dose of placebo
Study Group 4, Older Adult Participants	Placebo comparator	Biological: Placebo (two doses), priming, Biological: Placebo boosterParticipants will receive one dose of Placebo (saline) on Day 0, a second dose of Placebo on Day 28 and a third dose of Placebo on Day 208

Who may qualify

Ages: 18 years and older
Sex: All
Healthy volunteers: Accepted
Age groups: Adult, Older adult

Key inclusion criteria (7 listed)

are able to provide consent
agree to comply with all study visits and procedures
are willing and able to adhere to study restrictions
are sexually active and willing to adhere to contraceptive requirements
are male or female ≥18 or (in Singapore) ≥21 years of age

Key exclusion criteria (26 listed)

have had SARS-CoV-2 infection or COVID-19 disease.
have had cancer except for cancers that were treated and that have low risk of returning
have chronic kidney disease
have some chronic lung diseases
have some heart conditions

Eligibility is decided by the study team after screening; this overview does not establish that a patient qualifies. Full inclusion and exclusion criteria.

Interventions

Intervention	Type	Arms
ARCT-021 single dose primingARCT-021 higher dose (one dose) + placebo (one dose)	Biological	Study Group 1, Older Adult Participants, Study Group 1, Younger Adult Participants
ARCT-021 two lower dose primingARCT-021 lower dose (two doses, Day 0 and Day 28)	Biological	Study Group 2, Older Adult Participants, Study Group 2, Younger Adult Participants
ARCT-021 two higher dose primingARCT-021 higher dose (two doses, Day 0 and Day 28)	Biological	Study Group 3, Older Adult Participants, Study Group 3, Younger Adult Participants
Placebo (two doses), primingPlacebo (two doses, Day 0 and Day 28)	Biological	Study Group 4, Older Adult Participants, Study Group 4, Younger Adult Participants
Randomized boosterARCT-021 (single dose) OR placebo, booster	Biological	Study Group 1, Older Adult Participants, Study Group 1, Younger Adult Participants, Study Group 2, Older Adult Participants, Study Group 2, Younger Adult Participants, Study Group 3, Older Adult Participants, Study Group 3, Younger Adult Participants
Placebo boosterPlacebo (single dose)	Biological	Study Group 4, Older Adult Participants, Study Group 4, Younger Adult Participants

Primary outcome

Percentage of Participants Reporting Solicited Local Adverse Events Within 7 Days Post Each Priming Vaccination
Time frame: For 7 days following each dose administration (Day 0 up to Day 7 for Vaccination 1 and Day 29 up to Day 35 for Vaccination 2)
Per prespecified analysis, solicited local adverse events were defined as pain, erythema, swelling, or injection site tenderness. Solicited adverse reactions were recorded by the participant in an eDiary. Per prespecified analysis, solicited adverse events were not evaluated for severity (such as, serious or non-serious). A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Percentage of Participants Reporting Solicited Local Adverse Events Within 7 Days Post Booster Vaccination
Time frame: For 7 days following each dose administration (Day 208 up to Day 215)
Per prespecified analysis, solicited local adverse events were defined as pain, erythema, swelling, or injection site tenderness. Solicited adverse reactions were recorded by the participant in an eDiary. Per prespecified analysis, solicited adverse events were not evaluated for severity (such as, serious or non-serious). A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Percentage of Participants Reporting Solicited Systemic Adverse Events Within 7 Days Post Each Priming Vaccination
Time frame: For 7 days following each dose administration (Day 0 up to Day 7 for Vaccination 1 and Day 29 up to Day 35 for Vaccination 2)
Per prespecified analysis, solicited systemic adverse events were defined as fever, headache, fatigue, myalgia, arthralgia, nausea, chills, diarrhea, dizziness, and vomiting. Per prespecified analysis, solicited adverse events were not evaluated for severity (such as, serious or non-serious). A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Percentage of Participants Reporting Solicited Systemic Adverse Events Within 7 Days Post Booster Vaccination
Time frame: For 7 days post booster dose administration (Day 208 up to Day 215)
Per prespecified analysis, solicited systemic adverse events were defined as fever, headache, fatigue, myalgia, arthralgia, nausea, chills, diarrhea, dizziness, and vomiting. Per prespecified analysis, solicited adverse events were not evaluated for severity (such as, serious or non-serious). A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Percentage of Participants Reporting Unsolicited Adverse Events Up to 28 Days Post Each Priming Vaccination
Time frame: 28 days following each dose administration (Day 0 up to Day 28 for Vaccination 1 and Day 29 up to Day 56 for Vaccination 2)
An unsolicited AE was defined as any AE (serious and nonserious) occurring after administration of first dose of study vaccine and before the end of study. A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Percentage of Participants Reporting Unsolicited Adverse Events Up to 28 Days Post Booster Vaccination
Time frame: 28 days following each dose administration (Day 208 up to 236 days)
An unsolicited AE was defined as any AE (serious and nonserious) occurring after administration of first dose of study vaccine and before the end of study. A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Percentage of Participants Reporting Treatment-Emergent Serious Adverse Events (SAE), Medically Attended Adverse Events (MAAE) and New Onset of Chronic Disease (NOCD) Post Each Priming Vaccination
Time frame: Up to Day 207
SAEs were defined as any event that resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in a persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions, resulted in a congenital anomaly or birth defect, or was an important medical event. An NOCD was defined as any AE that led to the new diagnosis of a chronic medical condition that was not present or suspected prior to enrollment. An MAAE was an AE that led to an unscheduled visit (including a telemedicine visit) to a healthcare practitioner. A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Percentage of Participants Reporting Treatment-emergent Serious Adverse Events, Medically Attended Adverse Events and New Onset of Chronic Disease Post Booster Vaccination
Time frame: Day 208 to early termination (up to 396 days)
SAEs were defined as any event that resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in a persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions, resulted in a congenital anomaly or birth defect, or was an important medical event. An NOCD was defined as an AE that led to the new diagnosis of a chronic medical condition that was not present or suspected prior to enrollment. An MAAE was an AE that led to an unscheduled visit (including a telemedicine visit) to a healthcare practitioner. A summary of all unsolicited adverse events and solicited adverse events regardless of causality is located in the Reported Adverse Events module.
Geometric Mean Titer (GMT) of Serum Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Neutralizing Antibodies Post Priming Vaccination
Time frame: Day 1
GMT of SARS-CoV-2 Neutralizing Antibodies Post Priming Vaccination
Time frame: Day 56
GMT of SARS-CoV-2 Neutralizing Antibodies Post Booster Vaccination
Time frame: Day 208
GMT of SARS-CoV-2 Neutralizing Antibodies Post Booster Vaccination
Time frame: Day 236
Geometric Mean Fold Rise (GMFR) in SARS-CoV-2 Neutralizing Antibody Titers Post Priming Vaccination
Time frame: Day 56
GMFR is reported as a ratio to baseline (Day 0).
Geometric Mean Fold Rise (GMFR) in SARS-CoV-2 Neutralizing Antibody Titers Post Booster Vaccination
Time frame: Day 208
GMFR is reported as a ratio to baseline (Day 0).
GMFR in SARS-CoV-2 Neutralizing Antibody Titers Post Booster Vaccination
Time frame: Day 236
GMFR is reported as a ratio to baseline (Day 0).
Percentages of Participants Achieving Greater Than or Equal to 2-fold and 4-fold Increase From Before Vaccination (Baseline) in SARS-CoV-2 Serum Neutralizing Antibody Levels at Day 56
Time frame: Baseline up to Day 56
Percentages of Participants Achieving Greater Than or Equal to 2-fold and 4-fold Increase From Before Vaccination (Baseline) in SARS-CoV-2 Serum Neutralizing Antibody Levels at Day 208
Time frame: Baseline up to Day 208
Percentages of Participants Achieving Greater Than or Equal to 2-fold and 4-fold Increase From Before Vaccination (Baseline) in SARS-CoV-2 Serum Neutralizing Antibody Levels at Day 236
Time frame: Baseline up to Day 236

4 secondary outcomes

Geometric Mean Concentration of SARS-CoV-2 Anti-S1, Anti-RBD, and Anti-N Binding Antibody Levels Post Priming Vaccination
Time frame: Days 0 and 56
Geometric Mean Concentration of SARS-CoV-2 Anti-S1, Anti-RBD, and Anti-N Binding Antibody Levels Post Booster Vaccination
Time frame: Day 208 and 236
Geometric Mean Fold Rise (GMFR) in SARS-CoV-2 Anti-S1, Anti-RBD, and Anti-N Binding Antibody Levels Before Vaccination to Day 56
Time frame: Day 56
Percentage of Participants Achieving Greater Than or Equal to 2-fold and 4-fold Increase From Before Vaccination (Baseline) in SARS-CoV-2 Anti-S1, Anti-RBD, and Anti-N Binding Antibody Levels
Time frame: Day 56

Results and safety

Results posted to the registry on 15 May 2025, as reported by the sponsor. Figures are copied from the record, not re-analysed. Full results on ClinicalTrials.gov.

Participant flow

Group	Started	Completed
Treatment A	146	0
Treatment B	145	0
Treatment C	145	0
Treatment D	145	0

Primary outcome: Percentage of Participants Reporting Solicited Local Adverse Events Within 7 Days Post Each Priming Vaccination

Time frame: For 7 days following each dose administration (Day 0 up to Day 7 for Vaccination 1 and Day 29 up to Day 35 for Vaccination 2). Number. Unit: percentage of participants

Measure	Treatment A n = 142	Treatment B n = 142	Treatment C n = 144	Treatment D n = 146
Vaccination 1: Pain	52.8	48.6	57.6	8.2
Vaccination 1: Erythema (Redness)	14.8	19.7	11.1	7.5
Vaccination 1: Swelling (Hardness)	11.3	15.5	9.0	5.5
Vaccination 1: Injection Site Tenderness	79.6	75.4	83.3	19.9
Vaccination 2: Pain	7.9	33.1	44.5	5.9
Vaccination 2: Erythema (Redness)	7.1	14.6	8.0	8.9
Vaccination 2: Swelling (Hardness)	4.7	11.5	4.4	3.0
Vaccination 2: Injection Site Tenderness	15.0	67.7	75.2	15.6

Population analysed: The Reactogenicity analysis set included all participants who received any dose of priming study vaccine (ARCT-021 or placebo) and provided at least 1 reactogenicity diary report for the time period evaluated. Here, number analyzed signifies those participants who were evaluable in specified categories only.

Primary outcome: Percentage of Participants Reporting Solicited Local Adverse Events Within 7 Days Post Booster Vaccination

Time frame: For 7 days following each dose administration (Day 208 up to Day 215). Number. Unit: percentage of participants

Measure	Treatment E1 n = 41	Treatment F1 n = 36	Treatment G1 n = 38	Treatment H1 n = 39
Pain	51.2	36.8	39.5	5.1
Erythema (Redness)	9.3	7.9	7.9	2.6
Swelling (Hardness)	11.6	5.3	7.9	0
Injection Site Tenderness	81.4	65.8	73.7	12.8

Population analysed: The Booster Reactogenicity analysis set included all participants who received any dose of booster vaccination (ARCT-021, ARCT-145, ARCT-156 or placebo) and provided at least 1 diary entry for the time period evaluated. Participants were analyzed according to the vaccine actually received. Here, overall number of participants analyzed signifies those who were evaluable for this outcome measure and number analyzed signifies those participants who were evaluable in specified categories only.

Primary outcome: Percentage of Participants Reporting Solicited Systemic Adverse Events Within 7 Days Post Each Priming Vaccination

Time frame: For 7 days following each dose administration (Day 0 up to Day 7 for Vaccination 1 and Day 29 up to Day 35 for Vaccination 2). Number. Unit: percentage of participants

Measure	Treatment A n = 142	Treatment B n = 142	Treatment C n = 144	Treatment D n = 146
Vaccination 1: Fever	4.2	2.1	2.8	0
Vaccination 1: Headache	43.0	34.5	39.6	19.2
Vaccination 1: Fatigue	47.2	36.6	40.3	20.5
Vaccination 1: Myalgia	44.4	28.9	39.6	8.2
Vaccination 1: Arthralgia	21.1	15.5	17.4	8.2
Vaccination 1: Nausea	10.6	8.5	6.3	7.5
Vaccination 1: Chills	28.9	16.9	22.9	2.1
Vaccination 1: Diarrhea	12.0	12.7	16.0	9.6

Primary outcome: Percentage of Participants Reporting Solicited Systemic Adverse Events Within 7 Days Post Booster Vaccination

Time frame: For 7 days post booster dose administration (Day 208 up to Day 215). Number. Unit: percentage of participants

Measure	Treatment E1 n = 41	Treatment F1 n = 35	Treatment G1 n = 38	Treatment H1 n = 38
Fever	11.6	0	5.3	2.6
Headache	60.5	34.2	31.6	25.6
Fatigue	69.8	34.2	50.0	25.6
Myalgia	62.8	31.6	42.1	12.8
Arthralgia	48.8	21.1	31.6	12.8
Nausea	27.9	7.9	10.5	2.6
Chills	44.2	18.4	21.1	7.7
Diarrhea	25.6	18.4	10.5	10.3

4 secondary outcomes are reported in the full results.

Adverse events

Time frame: Vaccination 1 and 2: Day 1 up to Day 207; Booster Vaccination: Day 208 to early termination (up to 396 days). Groups E-H1 also include participants who did not receive booster dose. As pre-specified, if a participant didn't receive booster dose but had safety data collected for the booster vaccination period, the participant was included in the placebo booster group for summarization (Group H1).. Other adverse events reported at 2.5% frequency or more

Group	Serious adverse events	Other adverse events	Deaths
Treatment A	0 of 145 (0%)	49 of 145 (33.8%)	0 of 145
Treatment B	3 of 144 (2.1%)	15 of 144 (10.4%)	0 of 144
Treatment C	1 of 145 (0.7%)	27 of 145 (18.6%)	0 of 145
Treatment D	3 of 146 (2.1%)	11 of 146 (7.5%)	0 of 146
Treatment E1	0 of 44 (0%)	10 of 44 (22.7%)	0 of 44
Treatment F1	0 of 41 (0%)	9 of 41 (22%)	0 of 41
Treatment G1	1 of 42 (2.4%)	11 of 42 (26.2%)	0 of 42
Treatment H1	0 of 99 (0%)	19 of 99 (19.2%)	0 of 99
Treatment A - Reactogenicity Analysis	0 of 142 (0%)	113 of 142 (79.6%)	0 of 142
Treatment B - Reactogenicity Analysis	0 of 142 (0%)	107 of 142 (75.4%)	0 of 142
Treatment C - Reactogenicity Analysis	0 of 144 (0%)	120 of 144 (83.3%)	0 of 144
Treatment D - Reactogenicity Analysis	0 of 146 (0%)	30 of 146 (20.5%)	0 of 146
Treatment E1 - Reactogenicity Analysis	0 of 43 (0%)	35 of 43 (81.4%)	0 of 43
Treatment F1- Reactogenicity Analysis	0 of 38 (0%)	25 of 38 (65.8%)	0 of 38
Treatment G1 - Reactogenicity Analysis	0 of 38 (0%)	28 of 38 (73.7%)	0 of 38
Treatment H1 - Reactogenicity Analysis	0 of 39 (0%)	10 of 39 (25.6%)	0 of 39

Loading the posted results from ClinicalTrials.gov…

Research links

Official record on ClinicalTrials.govLast updated by the sponsor 15 May 2025.
Posted resultsParticipant flow, baseline, all outcome measures and adverse events.
Individual participant dataSharing planned: No. Individual participant data will only be made available to study investigators at this time.

Related trials for Covid19

Not yet recruitingPhase 1
Phase I Open-Label Safety Trial of Pembrolizumab for Neurological Post- Acute Sequelae of SARS-CoV-2 (PD1-PASC I)
Post-Acute COVID-19 Syndrome
Active, not recruitingPhase 1
A Safety and Immunogenicity Trial of OCU500, ChAd36 Vector Encoding SARS-CoV-2 Spike Vaccine Via Intranasal and Inhalational Routes in Previously Vaccinated Adults
COVID-19
Active, not recruiting
Intervention to Address Disparate Mental Health Consequences of COVID-19 Pandemic on Latinx and African Newcomers
Mental Health Issue, Mental Health Disorder
Recruiting
Home-Based Exercise Tele-Rehabilitation After COVID-19
Post SARS-CoV2 (COVID-19)
Not yet recruitingPhase 1
A Safety, Reactogenicity and Immunogenicity Trial of RVX-sCPD9 Booster Intranasal COVID-19 Vaccine
COVID -19

Referral and contact

AzTrials does not enrol participants or take referrals. Contact the study team or a site listed below; they decide who can take part. This study is not recruiting at the moment.

The record lists no contact. Use the official record to reach the sponsor.

15 sites in 2 countries: see every site and its status.

Questions about this trial

What is this clinical trial studying?

The full registered description is in the About this study section and the official record.

Who may be eligible to take part?

The sponsor lists 7 inclusion and 26 exclusion criteria. Eligibility is decided by the study team after screening, so read the full criteria and contact a listed site.

Is this clinical trial currently recruiting?

The record says: stopped early, as of the sponsor's last update on 15 May 2025. Status can change between updates, so confirm with the study contact before making plans.

Where is the study taking place?

The record lists 15 sites in United States, Singapore. Whether a site is taking part right now should be confirmed with that site. See every site.

What are the possible benefits and risks?

They depend on the study and the treatment, and no study can promise a benefit. Some participants in this study may receive a placebo. The study team explains the known risks during informed consent, before anyone joins; the record itself does not list them.

What does this phase mean?

Phase 2 studies give the treatment to a larger group to see whether it has the intended effect and to keep checking safety.

Not every study follows the same path, and some studies (observational studies, device studies) have no phase.

Are results available for this study?

Yes. Results are posted in the registry (first posted 15 May 2025). A summary is in the For doctors view, and the full results are on ClinicalTrials.gov.

How can a doctor refer a patient or contact the research team?

The record lists no contact. The official record names the sponsor, who can be asked directly. AzTrials does not enrol participants or take referrals.

What does the primary outcome measure?

Percentage of Participants Reporting Solicited Local Adverse Events Within 7 Days Post Each Priming Vaccination (time frame: For 7 days following each dose administration (Day 0 up to Day 7 for Vaccination 1 and Day 29 up to Day 35 for Vaccination 2)). The study lists 18 primary outcomes in all. It is what the study was designed to measure, not a finding.

Where can I read the original study information?

On the ClinicalTrials.gov record for NCT04668339.

Does a listing mean the treatment is approved or proven to work?

No. A listing means the study is registered. It does not show that the treatment is safe, works, or is approved by a regulator.

How often is this information updated?

AzTrials copies changed records from ClinicalTrials.gov every night and reloads the whole registry every week. The sponsor last updated this record on 15 May 2025.