New Delhi: The Indian Council of Medical Research (ICMR) on Monday warned that introduction of a community transmission of Covid-19 cases may take anywhere from a minimum of 20 days to a few months to be visible. In a mathematical modelling study published on Monday in the Indian Journal of Medical Research, the ICMR has highlighted that the control of epidemic would be difficult in “pessimist" scenarios in metropolitan cities such as Delhi, Mumbai, Kolkata, Bengaluru, Chennai, Hyderabad and Kochi, if people who have a travel history to infected countries or contact with the infect don’t isolate themselves.