Virus excretion and mutation by infants following primary vaccination with live oral poliovaccine from two sources.

The excretion of virus by 30 children was followed over a period of 28 days after their first vaccination with live oral poliovaccine. The type 1 and type 2 strains were isolated at similar high frequencies throughout this period, while the type 3 strain was not excreted by most children after day 2 post vaccination. Mutations in the 5' non-coding region associated with the attenuated phenotype reverted most rapidly for type 3 and least rapidly for type 1. The data are consistent with different degrees of selection against the attenuating mutations in the three serotypes in the gut, but imply that reversion is required for prolonged excretion. The findings also provide a possible explanation for the reported distribution of the rarely occurring vaccine associated cases between recipients and contacts for the three serotypes. A statistically significant difference was not observed between vaccine from the two main suppliers to the UK market.