A past COVID-19 infection does not completely protect against reinfection in young people, according to an observational study of more than 3,000 healthy members of the US Marines Corps most of whom were aged 18-20 years, published in The Lancet Respiratory Medicine journal.

The authors say that despite previous infection and the presence of antibodies, vaccination is still necessary to boost immune responses, prevent reinfection, reduce transmission, and that young people should take up the vaccine wherever possible.

In the study, between May and November 2020, around 10% (19 out of 189) of participants who were previously infected with SARS-CoV-2 (seropositive) became reinfected, compared with new infections in 50% (1,079 out of 2,247) of participants who had not previously been infected (seronegative).

Although the study was in young, fit, mostly male Marine recruits, the authors believe that the risk of reinfection found in their study will apply to many young people, but that the exact rates of reinfections will not be applicable elsewhere (owing to the crowded living conditions on a military base and close personal contact required for basic training). For example, a study of 4 million people in Denmark also found that the risk of infection was five times higher in people who had not before had COVID-19, but they found that only 0.65% of people who had COVID-19 during Denmark's first wave tested positive again during the second wave, compared with 3.3% of people who tested positive after initially being negative.

In addition, a preprint study including British healthcare workers found that those who had been not previously infected had a five times higher risk of being reinfected than people who had a past infection.