Influenza, HumanHIIVUNCT03748160
Herd Immunity and Influenza Vaccine Uptake
A study testing Letter in people with Influenza, Human, run by Finnish Institute for Health and Welfare at 1 site in Finland.
Trial summary
The aim of this study is to test the effectiveness of communicating the concept of herd immunity on actual influenza vaccine uptake using a randomized controlled trial. Read more about this study
- Who is it for?
- Ages 65 years and olderHealthy volunteers can take part
- Conditions the study is for
- Influenza, Human
- What is being tested?
- Letter
- Type of treatment
- Vaccines
- Could I receive a placebo?
- No, everyone receives an active treatment
- How many people
- 48,125 participants
- Where
- 1 siteFinland
- Study dates
- 1 Oct 2018to 31 Dec 2019
- Phase
- Interventional
- Are there results?
- None posted in the registry
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 65 years and older
- All sexes
- Healthy volunteers accepted
- Older adult
Who may be able to join
- At least 65 year old citizens in the municipalities of Espoo, Maalahti, Korsnäs, Kristiinankaupunki, Kaskinen and Närpiö
Who cannot join
- -
Trial locations and contacts
1 site in 1 country.
About this study
Official title: Herd Immunity Communication and Influenza Vaccine Uptake
The aim of this study is to test the effectiveness of communicating the concept of herd immunity on actual influenza vaccine uptake using a randomized controlled trial.
Read the detailed description
The investigators run a large-scale randomized controlled trial (RCT) to understand behavioural processes that guide decisions to vaccinate and test alternative strategies to communicate the concept of herd immunity. The experiment varies the text of postal letters send to individuals eligible for free influenza vaccines. The experiment is run in collaboration with the municipal health authorities.
The investigators identify from the Population Register all individuals who are born in year 1953 or before and resided in the participating municipality on October 01, 2018. The investigators will obtain the postal addresses of these individuals using the Population Register and randomly assign individuals to treatment conditions that differ in the wording of the information material and reception of a text message. Importantly, the extract from the Population Register will also include an individual identifier (social security number) that can be used to match the received type of letter (treatment) with the actual vaccination decision recorded in the Care Register for Health Care to objectively evaluate the impact of received information on vaccine uptake. The matching of postal addresses and vaccination data will be conducted using pseudonymized identifiers. The pseudonymization of individual identifiers will be performed separately for both datasets by professional data management and data security staff of the National Institute for Health and Welfare. The investigators conducting the data analysis will be able to access only pseudonymous data without a possibility to directly identify individuals from the data set. The data will be stored according to the data protection legislation and regulations of the National Institute for Health and Welfare during the research project.
The investigators randomly assign individuals to treatment conditions that differ in the wording of the letters. The authority's standard letter serves as the baseline condition (T1). This letter is extended by communicating the concept of herd immunity and appealing to pro-social preferences by highlighting the fact that vaccinations may effectively prevent infections among people who are not able to vaccinate themselves (especially babies) (T2). To summarize, this study includes a control treatment (T0, no letter) and two active treatment arms with the following contents:
T1 (Mailing) - Standard letter T2 (Herd) - Herd immunity letter
- Conditions
- Influenza, Human
- Being tested
- Letter
- Medical subject headings
- Influenza, Human
What participants receive
What participants receive. In a randomised study each person is assigned to one group by chance.
LetterOther
Intervention consists of two different types of postal letters reminding elderly citizens (65 years and above) about influenza vaccines that are available free of charge from the local health centers
| Study group | Role | What they receive |
|---|---|---|
| Control | No intervention | There is no intervention (letter) in the control arm. There is no control arm in the city of Espoo. 1/3 of subjects in all other municipalities are assigned to the control arm. |
| Mailing | Active comparator | This treatment arm consists of a standard letter reminding elderly citizens (65 years and above) about influenza vaccines that are available free of charge from the local health centers. Receives: Other: Letter |
| Herd | Active comparator | This treatment arm consists of a letter that highlights the herd immunity effects of vaccination and reminds elderly citizens (65 years and above) about influenza vaccines that are available free of charge from the local health centers. Receives: Other: Letter |
What is being measured
The primary outcome is the main question the study is designed to answer.
Vaccine uptake
Number of influenza vaccinations in study population
Measured: up to six months (beginning from November 1 / until April 30)
Timeline
From the start of the study to its completion; the mark is today. Estimated dates are the sponsor’s plan.
- First posted20 Nov 2018
- Study start1 Oct 2018
- Primary completion1 Jun 2019
- Study completion31 Dec 2019
Resources
- Official record on ClinicalTrials.govThe registry entry this page is built from, with its full change history.
- Publications (1)Papers the sponsor has linked to this study.
- Other trials for Influenza, HumanStudies 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 N/A
- Primary purpose
- Prevention
- Allocation
- Randomised
- Intervention model
- Parallel groups
- Model description
- Cluster (household) RCT with three treatment arms
- Masking
- Quadruple blind
- Who is masked
- Participant, Care provider, Investigator, Outcomes assessor
- Enrolment
- 48,125 participants (actual)
Sponsor and investigators
- Lead sponsor
- Finnish Institute for Health and Welfare
- Sponsor type
- Other government
- Collaborators
- RWTH Aachen University, University of Erfurt
- Responsible party
- Sponsor
- Investigators
- Cornelia Betsch, PhD, Principal investigator, University of ErfurtRobert Böhm, PhD, Principal investigator, RTWH Aachen UniversityHanna Nohynek, PhD, Principal investigator, Finnish Institute for Health and WelfareSaila Pitkänen, MSc, Principal investigator, Finnish Institute for Health and WelfareHeini Salo, PhD, Principal investigator, Finnish Institute for Health and WelfareLauri Sääksvuori, PhD, Study director, Finnish Institute for Health and WelfareJonas Sivelä, PhD, Principal investigator, Finnish Institute for Health and Welfare
IDs, dates and publications
- ClinicalTrials.gov ID
- NCT03748160
- Sponsor's protocol ID
- 240317
- Acronym
- HIIVU
- Status last verified
- Aug 2025
- First submitted
- 1 Nov 2018
- Last updated
- 20 Aug 2025
- Results posted
- No
- Data monitoring committee
- No
- FDA-regulated drug
- No
- FDA-regulated device
- No
- Individual data sharing
- Undecided: This study is based in administrative register follow-up. We will share pseudonymized data according to current data availability regulations.
Publications
Saaksvuori L, Betsch C, Nohynek H, Salo H, Sivela J, Bohm R. Information nudges for influenza vaccination: Evidence from a large-scale cluster-randomized controlled trial in Finland. PLoS Med. 2022 Feb 9;19(2):e1003919. doi: 10.1371/journal.pmed.1003919. eCollection 2022 Feb.
PubMed 35139082
Study at a glance
From the registry record, last updated by the sponsor on 20 Aug 2025. Recruitment status is as the sponsor reported it: confirm with the study team.
- Official title
- Herd Immunity Communication and Influenza Vaccine Uptake
- ClinicalTrials.gov ID
- NCT03748160
- Acronym / sponsor ID
- HIIVU / 240317
- Conditions
- Influenza, Human
- Study type
- Interventional
- Recruitment status
- Completed
- Enrolment
- 48,125 participants (actual)
- Lead sponsor
- Finnish Institute for Health and WelfareOther government
- Collaborators
- RWTH Aachen University, University of Erfurt
- Study start
- 1 Oct 2018 (actual)
- Primary completion
- 1 Jun 2019 (actual)
- Study completion
- 31 Dec 2019 (actual)
- First posted
- 20 Nov 2018 (actual)
- Results
- Not posted in the source record
Clinical summary
The research question as the sponsor registered it. This describes what the study plans to test, not what it found.
The aim of this study is to test the effectiveness of communicating the concept of herd immunity on actual influenza vaccine uptake using a randomized controlled trial.
Detailed description
The investigators run a large-scale randomized controlled trial (RCT) to understand behavioural processes that guide decisions to vaccinate and test alternative strategies to communicate the concept of herd immunity. The experiment varies the text of postal letters send to individuals eligible for free influenza vaccines. The experiment is run in collaboration with the municipal health authorities.
The investigators identify from the Population Register all individuals who are born in year 1953 or before and resided in the participating municipality on October 01, 2018. The investigators will obtain the postal addresses of these individuals using the Population Register and randomly assign individuals to treatment conditions that differ in the wording of the information material and reception of a text message. Importantly, the extract from the Population Register will also include an individual identifier (social security number) that can be used to match the received type of letter (treatment) with the actual vaccination decision recorded in the Care Register for Health Care to objectively evaluate the impact of received information on vaccine uptake. The matching of postal addresses and vaccination data will be conducted using pseudonymized identifiers. The pseudonymization of individual identifiers will be performed separately for both datasets by professional data management and data security staff of the National Institute for Health and Welfare. The investigators conducting the data analysis will be able to access only pseudonymous data without a possibility to directly identify individuals from the data set. The data will be stored according to the data protection legislation and regulations of the National Institute for Health and Welfare during the research project.
The investigators randomly assign individuals to treatment conditions that differ in the wording of the letters. The authority's standard letter serves as the baseline condition (T1). This letter is extended by communicating the concept of herd immunity and appealing to pro-social preferences by highlighting the fact that vaccinations may effectively prevent infections among people who are not able to vaccinate themselves (especially babies) (T2). To summarize, this study includes a control treatment (T0, no letter) and two active treatment arms with the following contents:
T1 (Mailing) - Standard letter T2 (Herd) - Herd immunity letter
Study design
- Primary purpose
- Prevention
- Allocation
- Randomised
- Intervention model
- Parallel groups
- Masking
- Quadruple blind (participant, care provider, investigator, outcomes assessor)
- Number of arms
- 3
- Model description
- Cluster (household) RCT with three treatment arms
| Arm | Type | Receives |
|---|---|---|
| Control | No intervention | –There is no intervention (letter) in the control arm. There is no control arm in the city of Espoo. 1/3 of subjects in all other municipalities are assigned to the control arm. |
| Mailing | Active comparator | Other: LetterThis treatment arm consists of a standard letter reminding elderly citizens (65 years and above) about influenza vaccines that are available free of charge from the local health centers. |
| Herd | Active comparator | Other: LetterThis treatment arm consists of a letter that highlights the herd immunity effects of vaccination and reminds elderly citizens (65 years and above) about influenza vaccines that are available free of charge from the local health centers. |
Who may qualify
- Ages
- 65 years and older
- Sex
- All
- Healthy volunteers
- Accepted
- Age groups
- Older adult
Key inclusion criteria (1 listed)
- At least 65 year old citizens in the municipalities of Espoo, Maalahti, Korsnäs, Kristiinankaupunki, Kaskinen and Närpiö
Key exclusion criteria (1 listed)
- -
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 |
|---|---|---|
| LetterIntervention consists of two different types of postal letters reminding elderly citizens (65 years and above) about influenza vaccines that are available free of charge from the local health centers | Other | Herd, Mailing |
Primary outcome
Vaccine uptake
Time frame: up to six months (beginning from November 1 / until April 30)
Number of influenza vaccinations in study population
Results and safety
The study is completed, but results are not available in the source record. That does not mean the treatment did or did not work.
Research links
- Official record on ClinicalTrials.govLast updated by the sponsor 20 Aug 2025.
- Derived from this study: PubMed 35139082Saaksvuori L, Betsch C, Nohynek H, Salo H, Sivela J, Bohm R. Information nudges for influenza vaccination: Evidence from a large-scale cluster-randomized controlled trial in Finland. PLoS Med. 2022 Feb 9;19(2):e1003919. doi: 10.1371/journal.pmed.1003919. eCollection 2022 Feb.
- Individual participant dataSharing planned: Undecided. This study is based in administrative register follow-up. We will share pseudonymized data according to current data availability regulations.
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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.
Cornelia Betsch, PhD principal investigator
University of Erfurt
Robert Böhm, PhD principal investigator
RTWH Aachen University
Hanna Nohynek, PhD principal investigator
Finnish Institute for Health and Welfare
1 site in 1 country: see every site and its status.
Questions about this trial
What is this clinical trial studying?
A study testing Letter in people with Influenza, Human, run by Finnish Institute for Health and Welfare at 1 site in Finland.
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 1 inclusion and 1 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: finished, as of the sponsor's last update on 20 Aug 2025. Status can change between updates, so confirm with the study contact before making plans.
Where is the study taking place?
The record lists 1 site in Finland. 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. The study team explains the known risks during informed consent, before anyone joins; the record itself does not list them.
Are results available for this study?
The study is completed, but results are not available in the source record. That does not mean the treatment did or did not work.
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?
Vaccine uptake (time frame: up to six months (beginning from November 1 / until April 30)). 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 NCT03748160, which also lists 1 linked publication.
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 20 Aug 2025.