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Abstract
India has a population of over 1.3 billion, with several densely populated megacities such as Mumbai and Delhi. A biological attack caused by a respiratory-transmissible agent such as smallpox could have serious consequences in a highly populated city like Mumbai, with a population over 20 million. I estimated the impact of a smallpox attack in Mumbai, India, using a modified SIER (susceptible, exposed, infected, recovered) model. The effect of past vaccination, age-specific immunosuppression and contacts rates, and Indian population age structure were estimated and incorporated in the model. I examined the impact of case isolation and ring vaccination on the epidemic and tested the health system capacity. The estimated duration of an outbreak was ranged from 127 days to 8 years under different scenarios, and the number of doses needed for ring vaccination was ranged from 16,813 to 8,722,400 in the best and worst case scenarios. In the worst case scenario, the available hospital beds in Mumbai would not be enough, even if all were used only for smallpox cases. Compared to high income settings, the impact of a smallpox epidemic may be severe in Mumbai, but can be reduced with rapid response, high rates of case finding, case isolation and ring vaccination. Plans for surge capacity to meet the need for hospital beds to isolate cases, and additional staffing will be essential.