Persistence of IPV Immunity

Background and Rationale

Importance of Poliomyelitis and Polioviruses and vaccine

There are three polioviruses types, 1, 2 and 3, with minimal cross-immunity. About 1/200 infections (depending on the type of poliovirus) produce paralytic poliomyelitis. An estimated 5-10% of individuals with paralytic poliomyelitis die and the remaining suffer from lifelong paralysis of one or more limbs without a cure. The presence of detectable antibodies in blood against each type protects against paralytic poliomyelitis, but intestinal immunity develops only after exposure to live poliovirus (vaccine or wild). Oral poliovirus vaccines that contain attenuated poliovirus strains and inactivated poliovirus vaccine both induce humoral immunity and protect against paralysis. OPV can also immunize or boost immunity of close contacts through secondary spread and trivalent OPV (tOPV) with poliovirus types 1, 2 and 3, was the vaccine of choice for polio eradication.

The immunological response to poliovirus vaccines is evaluated by measuring type-specific poliovirus antibodies using neutralization assays and can be detected as early as 1 to 3 days after infection with WPV or receipt of OPV or IPV. Antibody titers usually decline in the first two years (10- to 100-fold reduction) and then plateau, persisting for many years. However, administration of additional doses of vaccine or new exposures to wild poliovirus, induces a quick rise in antibody titers, with a peak reached within one week after the dose due to induction of "priming" or immunological memory.

Changes in polio vaccine use with progress in global polio eradication

Global Polio Eradication Initiative (GPEI) reduced polio cases enormously with the use of the tOPV in routine immunization and campaigns and WPV circulation is now limited to a few areas of the world. However, problems related to vaccines emerged. OPV strains may occasionally cause paralysis in vaccinated children and their close contacts (vaccine-associated paralytic poliomyelitis or VAPP). OPV strains can circulate among susceptible individuals for long time in areas with absence of wild poliovirus transmission and suboptimal coverage with routine immunization resulting in low population immunity. Prolonged person-to-person transmission can result in genetic changes and the emergence of circulating vaccine-derived polioviruses (cVDPV) with neurovirulence and transmissibility characteristics of WPV. Type 2 poliovirus was responsible for about 40% of annual VAPP cases reported, and 85% paralytic cases caused by cVDPVs during 2000-2015. Based on SAGE recommendation this led to removal of type 2 by switching from tOPV to bOPV in primary immunization and introduction of "at least one dose of IPV in routine immunization" in most of the countries of the world.

Rationale for the study and expected outcomes and benefits for the GPEI

Several recent studies have demonstrated that, although a single dose would result in seroconversion to type 2 for a limited number of infants (32% to 80% depending on the age of administration and study), a high proportion (>90%) of those infants who are seronegative, may actually "primed" by that single dose. Although it is not clear whether priming may protect directly against paralysis if a type 2 cVDPV 2 outbreak emerges, primed children should develop protective antibody levels quickly following a new OPV or IPV dose provided as a response to the outbreak.

Studies assessing long-term immunity to IPV or OPV were conducted in individuals receiving three or more doses of poliovirus vaccines who could have been exposed to circulating wild or vaccine poliovirus. It is unknown whether the proportion of children who were seropositive or primed following a single dose of IPV will stay positive or primed, and for how long this immune response will persist. Additionally IPV delivered in clinics and outreach sites in low-resource countries may not produce the same response to those observed in clinical trials due to programmatic issues.

Furthermore, global shortage of IPV resulting in IPV stock out in many countries including Nepal led many children to miss out their IPV dose in routine immunization. Reliable estimates of population immunity in a country or region based upon coverage and estimated immunogenicity of the type of vaccine and vaccination schedules received, are crucial to guide programmatic decisions and manage vaccine supply for outbreak responses to type 2 poliovirus.

Therefore, a cross-sectional study is being conducted to determine whether the immune response provided by a single dose of IPV persists for more than a year, by assessing the proportion of children born after the tOPV-bOPV switch and vaccinated with a single dose of IPV at about 14 weeks in routine immunization who are still seropositive or primed at around two years of age. These new estimates will inform the GPEI on vaccine choices for responding to type 2 VDPV outbreaks and guide decisions on polio immunization schedules after cessation of all OPV types.

Objectives

The study primarily will compare the proportion of infants vaccinated with one dose of IPV after 14 weeks of age who are seropositive or primed against type 2 poliovirus, either > 21 months after vaccination (study group), or one month after vaccination (control group). Also we would determine the proportion of children seropositive to types 1 and 3 following a sequential bOPV-IPV or bOPV alone schedule, delivered through routine immunization services in a low resource country.

Study Design/ Procedures

This is an open-label phase IV clinical trial assessing immunogenicity to IPV. Study participants will be identified through screening of children who attend outpatient clinics at the study sites for well-child visits, immunization or minor illness. After screening, confirmation of eligibility and obtaining consent, children will be allocated to one of the two study arms, and given a dose of IPV after collecting clinical information and obtaining a blood sample. Children will be followed up according to the schedule for study arms to assess study objectives.

Blood sample will be centrifuged within 24 hours of collection, serum aliquoted into two cryovials and stored at Institute of Medicine (IOM) laboratory at -20*C until final shipment to Centers for Disease Control and Prevention(CDC), Atlanta. Determination of poliovirus antibodies will be conducted using a microneutralization testing. Titers below 1:8 will be considered negative and the highest detectable titer will be 1:1448.

Sample size and analytic plan:

The sample size will be powered to address the primary objective based on previous trials that have shown 90 - 100% of children to develop detectable immunity to type 2 poliovirus after IPV dose. Using a one-sided test for differences between proportions with a continuity correction Z-test using pooled variance (PASS v14), 237 children will be required in each group to detect a one-sided difference of ≥10% with 90% power and 0.05 alpha. To account for potential 5% drop-outs, the sample will be rounded up to 250 per group or 500 children total.