A region can achieve herd immunity against Covid-19 with fewer people than previously estimated, a new study said on Wednesday. The study, published in the journal Science and conducted by mathematicians from the University of Nottingham and University of Stockholm, categorized people into separate groups reflecting their age and social activity levels. Their model found that the threshold percentage of population who need to be infected for the chain of transmission to be broken – known as herd immunity – was 43% and not 60%, as thought earlier. The study says that herd immunity can be achieved in two ways: by vaccinating enough number of people or if enough people are infected and develop immunity against the disease. However, experts have warned that the humanitarian and medical costs of taking the route continue to remain high, especially in India.