London, June 24 : Contrary to decades-old theory that herd immunity against a pandemic is achieved when at least 60 per cent population is infected, new research shows it can be done even with 43 per cent when differences in age and social activity are considered. Herd immunity happens when so many people in a community become immune to an infectious disease that stops the disease from spreading. This happens by people contracting the disease and building up natural immunity.When a large percentage of the population becomes immune to a disease, the spread of that disease slows down or stops and the chain of transmission is broken. According to mathematicians from the University of Nottingham and University of Stockholm, herd immunity to Covid-19 could be achieved with less people being infected than previously estimated. "Our findings have potential consequences for the current Covid-19 pandemic and the release of lockdown and suggests that individual variation (in activity level) is an important feature to include in models that guide policy," the authors wrote. However, the figure of 43 per cent should be interpreted as an illustration rather than an exact value or even a best estimate, said the study published in the journal Science. "By taking this new mathematical approach to estimating the level for herd immunity to be achieved, we found it could potentially be reduced to 43 per cent and that this reduction is mainly due to activity level rather than age structure," explained Professor Frank Ball from University of Nottingham. The more socially active the individuals are the more likely they are to get infected than less socially active ones, and they are also more likely to infect people if they become infected. "Consequently, the herd immunity level is lower when immunity is caused by disease spreading than when immunity comes from vaccination," Ball explained. To reach this conclusion, the teams devised a simple model categorising people into groups reflecting age and social activity level. When differences in age and social activity are incorporated in the model, the herd immunity level got reduced from 60 per cent to 43 per cent. This research takes a new mathematical approach to estimating the herd immunity figure for a population by an infectious disease, such as the current COVID-19 pandemic. For Covid-19, it is often stated that this is around 60 per cent, a figure derived from the fraction of the population that must be vaccinated (in advance of an epidemic) to prevent a large outbreak. /IANS