Poliovirus vaccines - Progress toward global poliomyelitis eradication andchanging routine immunization recommendations in the United States

In the prevaccine era, poliovirus was endemic, each year infecting new coho rts of susceptible infants born in the community and exposing virtually all infants early in life. In this pattern, in which women of childbearing age almost universally possessed antibodies to all three poliovirus types, pas sive immunity was transferred from mothers to newborns, and many infants ex perienced their first infections in the first few months of life, while mat ernal antibodies still provided some protection. In the instances in which these infections took the paralytic form, many of these cases went unrecord ed in populations already faced with high infant and child mortality rates from many other causes. A change from endemic to epidemic transmission was observed in industrializ ed areas of the world late in the nineteenth century and early in the twent ieth century, probably because of improving hygiene, which delayed exposure to the virus until later in childhood. In the United States, the median ag e of poliomyelitis cases increased from less than 5 years at the turn of th e century to more than 10 years in the 1950s, immediately before poliovirus vaccine licensure.(37, 83) The vaccine era in the United States began in 1955 with licensure of inacti vated poliovirus vaccine (IPV). Widespread use of first IPV, and later oral poliovirus vaccine (OPV), rapidly controlled poliomyelitis in the United S tates. OPV and IPV do not contain thimerosal, an ethyl mercury preservative commonly added to inactivated vaccines. OPV has been the recommended vacci ne in routine use to prevent poliomyelitis since 1965. The total number of poliomyelitis cases decreased from a peak of more than 57,000 in 1952 to fe wer than 35 cases per year in 1969 to 1972 in the United States.(29) Labora tory surveillance for enteroviruses and poliomyelitis case surveillance sug gest that endemic circulation of indigenous wild-type polioviruses probably ceased in the United States in the 1960s.(57, 82) The last cases of indige nously acquired endemic and epidemic paralytic poliomyelitis in the United States were reported in 1979.(14, 87)