- The hypothesis that live oral poliovirus vaccine (OPV) confers immunity to persons in close contact with the vaccine recipient was tested by comparing cord blood antibody titers in 31 first-born neonates to those of 48 neonates with one or more siblings adequately immunized with OPV. A borderline or negative result (defined as a reciprocal titer of < or = 8) for at least one type was significantly more prevalent among first-born neonates than among neonates with one or more siblings [17/31 (55%) versus 17/48 (27%) respectively; p < 0.03]. This difference was consistent for all three poliovirus types. The geometric mean titer (GMT) was consistently higher for each serotype in infants with one or more siblings compared with first-born neonates: 134.9 versus 64.5 for poliovirus 1; 262.1 versus 95.6 for poliovirus 2; and 48.6 versus 19.4 for poliovirus 3, respectively. When cord blood of neonates with two or more siblings was compared to that of neonates with only one sibling, no difference in titers was observed. Since mothers of one or more infants were on average older and less educated, the results were adjusted accordingly, but the same trend was observed again. These findings support the notion that OPV is important, not only as a vaccine for the individual infants, but also as a means of conferring immunity to persons in close contact.