Serotype‐specific mucosal immune response and subsequent poliovirus replication in vaccinated children

A total of 32 unimmunized children (median age 5 months) living in an orphanage in Minsk, Belarus, were vaccinated with three doses of oral poliovirus vaccine (OPV) with a 60-day interval between the doses. Blood samples were drawn before the immunizations and 45-50 days after each vaccine dose. Excretion of the vaccine viruses was followed by examining fecal specimens collected weekly after each vaccine dose. All children seroconverted by the second dose of OPV at the latest to all three serotypes of poliovirus but differences were seen regarding the intestinal responses. The strongest responses in both neutralizing antibodies and virus-binding IgA in fecal suspensions were seen toward poliovirus Type 2. Consistent with this, relatively little poliovirus Type 2 excretion was seen after the second and third doses of OPV as compared to that of poliovirus Type 1 or Type 3. The delayed development of functional intestinal immunity against the latter serotypes was associated with relatively weaker intestinal antibody responses compared to poliovirus Type 2. In the case of poliovirus Type 3, about 10% of children were still excreting the vaccine virus 9 weeks after administering the third dose. These results are consistent with epidemiological observations of the serotype-specific efficacy of OPV immunizations. The proportion of specimens showing nonpolio enteric viruses gradually increased through the 6-month study period. This may reflect the possibility that after the first dose of OPV, intensive replication of the vaccine viruses may out compete the nonpolio viruses in the intestines of the vaccinees or, alternatively, mask nonpolio viruses during the cell culture isolation procedure.