Karachi January 15, 2021 Covid-19 cases have started surging again in Karachi as its current cold weather with moderate humidity is most suitable for the spread of the virus, while its dense population with millions living in slums and regularly-held “super spreading events” are other contributing factors behind the surge, experts said on Thursday. “Cold weather with moderate humidity in the range of 45-60 per cent is the ideal weather for the spread of the coronavirus and that is why its cases are surging in Karachi where its positivity was 15 per cent today as compared to national positivity rate of 7 per cent,” Dr Shahzad Ali Khan, a professor of public health at the Health Service Academy, Islamabad, told The News on Thursday.