Scientists have found that the baseline immune statuses in men previously infected with SARS-CoV-2 was altered in ways that changed the response to an exposure different from SARS-CoV-2. The team of researchers systematically analyzed immune responses of healthy people who had been given the flu vaccine. From that data, they then compared the responses between those who had never been infected by SARS-CoV-2, the virus that causes covid-19, and those who experienced mild cases but recovered. The team led by John Tsang, an immunobiologist from Yale University, US, found that immune systems of men who had recovered from mild cases of covid-19 responded more robustly to flu vaccines than women who had had mild cases or men and women who had never been infected. ”This was a total surprise,” said Tsang. ”Women usually mount a stronger overall immune response to pathogens and vaccines, but are also more likely to suffer from autoimmune diseases,” said Tsang. The long-term effects of infection on the immune system had long intrigued Tsang. After the body has faced down a pathogen, does the immune system return to the previous baseline? Or does a single infection change it in ways that alter how it will respond not only to a familiar virus but also to the next new viral or bacterial threat it faces? Tsang, a professor at Yale University, had long believed that the immune system reverts to the previous stable baseline after viral infection. The emergence of the covid-19 pandemic in 2020 allowed him and colleagues to test that theory. The answer, they found, depends on the individual’s sex. The findings have been published in the journal Nature.