Immuno-persistence after the fourth and fifth doses of inactivated polio vaccines in school-aged children.

Objectives: This study aimed to assess the long-term persistence of neutralizing antibodies (nAb) titres and seroprotection proportions after the fourth and fifth doses of inactivated polio vaccine (IPV).

Methods: Serum samples from 299 children in Hong Kong were collected and used to estimate the persistence of nAb titres and seroprotection proportions by neutralisation test.

Results: The mean nAb titres against poliovirus types 1, 2, and 3 (PV1, PV2, and PV3) 1 month after receiving the fourth dose of IPV at 19 months of age were 2068 (95% credible interval, 1517-2864); 4705 (3439-6436); and 2758 (1894-4086); respectively, but declined substantially in 4 years to 268 (222-325), 751 (630-900), and 411 (323-521), respectively. Administration of the fifth dose of IPV restored nAb titres among children aged 6 to 7 years, and the decline in nAb titres was slightly slower with the estimated mean titres of 355 (272-462), 538 (427-681), and 548 (378-786) against PV1, PV2, and PV3 at 4 years post the fifth dose. We estimated that the proportion of children who were seroprotected against PV1, PV2, and PV3 would drop below 90%: (i) 8.2, 10.8, and 8.7 years after the fourth dose; and (ii) 11.6, 11.2, and 11.0 years after the fifth dose.

Discussion: The results revealed the immuno-persistence after the fourth and fifth doses of IPV and highlighted the importance of completing immunization series to ensure high vaccination coverage, particularly among children in the developing countries affected by the COVID-19 pandemic.