Back in May I wroteabout a study by La Jolla Institute for Immunology, which raised the possibility that exposure to coronaviruses which cause the common cold could offer some degree of immunity to Covid-19. Scientists involved in the research had discovered a reaction to Sars-CoV-2 – the virus which causes Covid-19 – in the T cells of people who had not been infected with the virus, but at the time they weren’t sure whether it was a strong enough reaction to offer any effective immunity.