Outbreak of poliomyelitis due to type 3 poliovirus, northern India, 1999–2000: injections a major contributing factor

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Abstract
Background A large outbreak of poliomyelitis due to poliovirus type 3 (P3) occurred in India in 1999. This raised concerns about oral poliovirus vaccine (OPV) effectiveness, particularly the type 3 component, in preventing clinical disease and offered an opportunity to describe the epidemiology of a P3 outbreak. Methods We reviewed data collected by the National Polio Surveillance Project to describe the outbreak and conducted a case-control study to determine risk factors for the development of paralytic poliomyelitis. The P3 cases with paralysis onset in 2000 were enrolled with four controls per case, matched for age and neighbourhood. Results Of 1126 virologically confirmed poliomyelitis cases reported in 1999, 719 (64%) were due to P3. We enrolled 48 (80%) of 60 cases and 175 matched controls. Age (30.6 months, cases versus 30.4 months, controls) and vaccination status (median 5.8 OPV doses, cases versus 6.1 OPV doses, controls) were similar among cases and controls. The only significant difference between the groups was the proportion that received any injection in the last 30 days prior to paralysis onset or the corresponding reference date for controls (35.4% versus 12.3%, adjusted odds ratio [OR] = 3.9, 95% CI: 1.8–12.5). Conclusions Cases and controls had similar vaccination histories. The only significant risk factor for paralytic illness was having received any injection in the 30 days before onset. Our study confirms that injections administered during the poliovirus incubation period can provoke paralytic poliomyelitis. Injections in polio-endemic countries should only be indicated when other therapeutic options have failed or are not available.