My understanding is that a policy of vaccination with killed
vaccine first followed by attentuated live virus vaccine has been adopted
as recommended policy. The basis is indeed the trend, at least in the
US, that most new poliomyelitis is the result of vaccination or virus
from vaccinees being transmitted to other susceptibles. It is important
to keep in mind that the attentuated vaccine is extraordinarily safe-
risk in healthy young children being less than 1:1,000,000 of disease.