One of the most concerning issues since the emergence of the Covid-19 virus has been whether those who have had it can get it a second time – and what that means for immunity. On Monday, both Sir Patrick Vallance, the government’s chief scientific adviser, and Prof Chris Whitty, Boris Johnson’s chief medical adviser, sought to reassure the public. Those who have had the virus once will develop some immunity, they said – and it is rare to get an infectious disease again. The questions first arose last month, after Japanese authorities said a woman who had had the virus, and been declared virus-free, had tested positive again. Scientists were left confused by the news and and also uneasy. Prof Mark Harris, an expert in virology at Leeds University, said reinfection in that case was “unlikely”, but added that “there is some evidence in the scientific literature for persistent infections of animal coronaviruses (mainly in bats)”. When Vallance was asked on Monday if the Japanese case meant herd immunity was no longer achievable, he replied that some people do catch infectious diseases a second time, but that it is a rare occurrence. There was no evidence to suggest that it would occur with the coronavirus, he added. Prof Whitty explained that with diseases, even if there is no long-term immunity, there is normally some short-term immunity. Prof Jon Cohen, emeritus professor of infectious diseases at Brighton and Sussex Medical School, said: “The answer is that we simply don’t know [about reinfection] yet because we don’t have an antibody test for the infection, although we will have soon. “However, it is very likely, based on other viral infections, that yes, once a person has had the infection they will generally be immune and won’t get it again. There will always be the odd exception, but that is certainly a reasonable expectation.”