Herd Immunity Against Foot-and-Mouth Disease Under Different Vaccination Practices in India.

A systematic vaccination programme is ongoing in India to control the three prevailing serotypes (A, O, Asia1) of foot-and-mouth disease (FMD) virus. Under the programme, more than 120 million bovine (term bovine applicable to both cattle and buffalo in this study) population of 221 of the 666 districts in the country are being bi-annually vaccinated with trivalent vaccine since 2010. Although clinical disease has reduced in these districts because of the systematic vaccinations, an abrupt increase in the number of FMD cases was recorded in 2013. Hence, a longitudinal field study was conducted in the year 2014 to estimate the serological herd immunity level in bovines, the impact of systematic vaccinations and field efficacy of the vaccines used. Serum samples (n = 115 963) collected from 295 districts of the 18 states of the country were analysed to estimate antibody titres against structural proteins of the three serotypes. The efficacy of the vaccine was demonstrated in the control group (group-D) where animals of the group were identified by ear tags for the purpose of repeated sampling after vaccination. Progressive building of the herd immunity in the field after systematic vaccination was demonstrated. The mean antibody titre against the serotypes O, A and Asia1 was estimated as log 1.93 (95% CI 1.92-1.93), 2.02 (2.02-2.02) and 2.02 (2.02-2.02), respectively, in the states covered under the control programme. However, in other states herd immunity was significantly low [mean titre log 1.68 (95% CI 1.67-1.69), 1.77 (1.76-1.78) and 1.85 (1.84-1.86) against the three serotypes]. Inverse relationship between the herd immunity and FMD incidences was observed the states following different vaccination practices. The study helped in demarcation of FMD risk zones in the country with low herd immunity. Estimation of herd immunity kinetics in the field helped in refining the vaccination schedule under the control programme.