Experience with poliovaccines in the control of poliomyelitis in India.

OPV was introduced in the National Immunisation Programme (NIP) in 1979. The reported number of cases of poliomyelitis was about 20,000 in 1979, 38,000 in 1981; it gradually declined to the pre-immunisation level of 20,000 in 1986, but increased to 28,000 in 1987. In 1989, 10 years after NIP, the reported number of cases fell below the pre-immunisation level, and has further declined to 5669 in 1991. The 3-dose OPV coverage during infancy was estimated to be about 80% in 1991. If reporting efficiency is 10%, then about 60,000 cases would have occurred in 1991, for an incidence of disease of about 7/100,000 in the total population of 840 million. Three questions arise: (1) Why did it take over 10 years of NIP to reduce the disease burden? (2) Why is the disease occurring at a high incidence level in spite of 80% immunisation coverage? (3) Will we be able to eliminate poliomyelitis and eradicate polioviruses by the year 2000 by sustaining the high immunisation coverage using 3 doses of OPV? In most countries, the incidence of disease has declined immediately after instituting NIP including OPV. The vaccine efficacy (VE) of 3 doses of OPV in India is 70-93%. Hence the lack of decline of incidence during 10 years of NIP was most probably because sufficient proportions of infants were not being immunised. The incidence remains high because the VE and herd effect of 3 doses of OPV are insufficient. Vaccine failure cases will account for 2-9 cases per 100,000 per year since pre-immunisation incidence was 30 and VE is 70-93%.(ABSTRACT TRUNCATED AT 250 WORDS)