Updated: April 20, 2021 8:23:52 am
A past Covid-19 infection does not completely protect against reinfection in young people, according to a new observational study published in The Lancet Respiratory Medicine.
The study was among more than 3,000 healthy members of the US Marines Corps, most of whom were in the age group 18-20. Of 2,346 Marines followed long enough for this analysis of reinfection rate, 189 were seropositive and 2,247 were seronegative at the start of the study, between May and November 2020. Around 10% of participants who were previously infected with SARS-CoV-2 (19 out of 189) became reinfected, compared with new infections in 50% (1,079 out of 2,247) of participants who had not previously been infected.
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Implications beyond cohort
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.
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. This is owing to the crowded living conditions on a military base and close personal contact required for basic training likely contributing to a higher overall infection rate than seen elsewhere.
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. 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 infected than people who had a past infection.
Antibodies & viral load
To understand why reinfections occurred in the study of Marine recruits, the authors studied the reinfected and not infected participants’ antibody responses. They found that, among the seropositive group, participants who became reinfected had lower antibody levels against the SARS-CoV-2 virus than those who did not become reinfected. In addition, in the seropositive group, neutralising antibodies were less common.
Comparing new infections between seropositive and seronegative participants, the authors found that viral load in reinfected seropositive recruits was on average only 10 times lower than in infected seronegative participants, which could mean that some reinfected individuals could still have a capacity to transmit infection, but the authors note that this will need further investigation.
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