Low Level of Cross-Reactive Antibodies to Pandemic Influenza (H1N1) 2009 Virus in Humans in Pre-Pandemic Period in Maharashtra, India

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Abstract
In India, the first outbreak of pandemic influenza (H1N1) 2009 (H1N1pdm) was reported from Panchgani, Maharashtra, in June 2009. Studies from several countries have revealed different levels of pre-existing immunity to H1N1pdm 2009 in various age groups. This study was undertaken using age-stratified pre-pandemic human sera to understand baseline cross-reactivity of antibodies against H1N1pdm. Using cut off antibody titers 20 and 40, overall cross-reactivity was 2.1 and 0.9% respectively by microneutralization assay; 1.2% and 0.7% by haemagglutination inhibition assay, respectively. Results showed higher baseline antibodies and cross-reactive antibodies in the 0–19 age group whereas the elderly age group (≥60) showed no cross-reactivity to H1N1pdm. The higher baseline and cross-reactive antibodies in 0–19 years age group could be because of higher positivity to seasonal H1N1 in that age group. Overall, low level of cross-reactive antibodies to H1N1pdm virus were found in humans in pre-pandemic period in Maharashtra, India.