# Sustained Immune Persistence Five Years Post-Completion of Four-Dose sIPV Vaccination

## TL;DR

A study finds that the sIPV vaccine provides strong and lasting immunity against polio for five years after four doses, with antibody levels remaining above protective thresholds.

## Contribution

This study provides long-term evidence of immune persistence for sIPV compared to wIPV five years post-vaccination in children.

## Key findings

- Seropositivity rates for nAbs against all three polio serotypes exceeded 98% in both sIPV and wIPV groups.
- GMTs for nAbs were significantly higher in the sIPV group compared to the wIPV group for all serotypes.
- Non-inferiority of sIPV compared to wIPV in immune persistence was confirmed for all serotypes.

## Abstract

Background: The previous study assessed the immune durability of the Sabin strain inactivated poliovirus vaccine (sIPV) at four years of age; an update on its long-term persistence is warranted. Methods: This Phase IV, open-label, parallel-controlled observational study, required by China’s National Medical Products Administration (NMPA), involves 6.5-year-old children who received four doses of sIPV or Salk IPV (wIPV) at 2, 3, 4, and 18 months during the Phase III trial. Participants are recruited in a 2:1 ratio and contribute blood samples for polio-neutralizing antibody (nAb) assays to determine non-inferiority of immune persistence. Results: The study enrolled 483 participants aged 6.5 years in the 5-year Immune Persistence Set (IPS2), with 318 in the sIPV group and 165 in the wIPV group. Additionally, 387 participants (255 sIPV, 132 wIPV) with samples at six-time points were included in the Full Sequence Immune Persistence Set (IPS3). In IPS2, seropositivity rates (SPRs) for nAbs against serotypes 1–3 were over 99% in sIPV and 98% in wIPV. At 6.5 years, geometric mean titers (GMTs) were significantly higher in the sIPV group 543.96, 179.59 and 362.72 compared to the wIPV group 190.75, 81.05 and 203.95 for serotypes 1, 2 and 3, respectively. Participants in IPS3 demonstrated comparable SPRs and GMTs to IPS2, with values of 566.01 vs. 187.41 for serotype I, 177.55 vs. 78.01 for serotype II, and 365.47 vs. 190.31 for serotype III in the sIPV and wIPV groups, respectively. From one-month post-booster to 6.5 years, nAb GMTs showed declines: 19.35-fold for serotype I, 28.12-fold for serotype II, and 32.45-fold for serotype III in the sIPV group, and 23.42-fold, 23.83-fold, and 34.54-fold in the wIPV group, respectively. Non-inferiority of nAb SPRs and GMTs for sIPV compared to wIPV among participants aged 6.5 years was confirmed for all serotypes in IPS2 and IPS3. Conclusions: The sIPV maintains good immunological persistence five years after four doses of vaccination, with nAb GMT exceeding the seroprotecting threshold, suggesting that booster doses might be currently unwarranted.