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COVID-19 is a respiratory disease with a clinical spectrum that ranges from no or few symptoms to acute respiratory failure, sepsis, and multiple organ dysfunction syndrome. More than 82 million cases of the illness have been reported worldwide. Most healthy people who contract COVID-19 are not hospitalized, however, making it difficult to determine the extent to which infection confers future immunity. A recent report indicates that even mild or asymptomatic cases of COVID-19 provide immunity against future reinfection.

The authors of the report conducted a cross-sectional case-control study to assess T cell and neutralizing antibody immunity approximately four months after the United Kingdom lockdown commenced in March 2019. They drew on a subset of data from the UK COVIDsortium, a longitudinal study of a London-based cohort of hospital healthcare workers. The study involved 76 healthcare workers who tested positive for the presence of SARS-CoV-2 (the virus that causes COVID-19) and 60 healthcare workers (of similar age, gender, and ethnicity) who tested negative. Those who tested positive completed self-reported health questionnaires to identify mild or asymptomatic infections around the time of onset. The investigators measured immune response via serial blood sampling.

They found that 89 percent of the healthcare workers who had tested positive had neutralizing antibodies against SARS-CoV-2, regardless of disease severity. The antibody responses did not always match T cell responses, which tended to be lower after asymptomatic infection than in symptomatic infection. The healthcare workers that lacked neutralizing antibodies and had undetectable T cell responses to the SARS-CoV-2 spike protein (the primary viral antigen) did have T cells that reacted with other SARS-CoV-2 antigens.

These findings suggest that most people who have mild or asymptomatic SARS-CoV-2 infection carry immunity against future reinfection at four months post-infection. Other research indicates that immunity may last as long as six months.

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