Randomised, controlled trial with the trypsin-modified inactivated poliovirus vaccine: assessment of intestinal immunity with live challenge virus.

The enhanced potency inactivated poliovirus vaccine (E-IPV) was modified to contain trypsin-treated type 3 poliovirus (PV3), strain Saukett, as the type 3 component (TryIPV). This pilot vaccine was previously shown to redistribute the vaccine-induced antibody specificities in mice to mimic those seen in man after poliovirus infection. Groups of infants were then immunised with three doses of TryIPV or E-IPV in a randomised, double-blind trial. Six months after the third dose, at the age of 18 months, the children were challenged with one dose of oral monovalent type 3 poliovirus vaccine. Intestinal immunity was evaluated by assessing the length and extent of PV3 excretion through determination of PV3 titres in 9 successive faecal specimens (2-42 days after challenge). No significant difference in the length or extent of virus excretion was seen between the groups. The results indicate that TryIPV, under the conditions used, was no more potent than the regular E-IPV in inducing resistance to intestinal poliovirus infection.