Young people who have already had coronavirus are not necessarily protected from contracting it again, researchers have warned amid new data.
An observational study of more than 3,000 recruits of the US Marine Corps – aged between 18 and 20, between May and November 2020 – found that around 10 per cent of participants who had previously been infected with the Sars-CoV-2 virus became reinfected at a later date.
This was compared with new infections in 50 per cent of participants who had not previously been infected.
While the study – published this week in The Lancet Respiratory Medicine journal – was carried out among young, fit, mostly male recruits, researchers believe the risk of reinfection applies to many young people.
They did, however, acknowledge that the study’s rate of reinfection might not apply in other surroundings due to the crowded living conditions on a military base – and the close personal contact required for basic training likely to contribute to a higher overall infection rate than elsewhere.
For example, a study of four million people in Denmark found that the risk of infection was five times higher in people who had not had the disease before.
Researchers there found that just 0.65 per cent of Danes who had Covid during the country’s first wave tested positive again during the second wave – compared with 3.3 per cent of people who tested positive after initially being negative.
In addition, a preprint study including British healthcare workers found that those who had not been previously infected were five times more likely to be infected than people who had a past infection.
Professor Stuart Sealfon, of Icahn School of Medicine at Mount Sinai, in New York, and senior author of the US Marine Corps study, said despite vaccine rollouts around the world it is important to remember that, even if they have been infected before, “young people can catch the virus again and may still transmit it to others”.
“Immunity is not guaranteed by past infection, and vaccinations that provide additional protection are still needed for those who have had Covid-19,” Prof Sealfon told the PA news agency.
Recruits who tested positive for a second infection during the study were isolated and researchers followed up with additional testing.
Levels of neutralising antibodies were also taken from subsequently infected seropositive (those who had previously been infected with Covid) and selected seropositive participants who were not reinfected during the study period.
Of the 2,346 participants followed long enough for this analysis, 189 were seropositive and 2,247 were seronegative (participants who had not previously been infected) at the start of the study.
Across both groups, there were 1,098 new infections during the study – 45 per cent.
Among the seropositive participants, 19 tested positive for a second infection, and of the recruits who were seronegative 1,079 became infected.
The authors studied reinfected and non-infected participants’ antibody responses to understand why these reinfections occurred. They found that, among the seropositive group, participants who became reinfected had lower antibody levels against the virus than those who did not.
Comparing new infections between seropositive and seronegative participants, the authors found that viral load – the amount of measurable virus – in reinfected seropositive recruits was on average only 10 times lower than in infected seronegative participants.
This suggests some reinfected individuals could still be able to transmit the virus, but researchers noted this will need further analysis.
Most new cases in the study were asymptomatic – 84 per cent in the seropositive group, and 68 per cent in the seronegative group – or had mild symptoms. None were admitted to hospital.
Researchers said they also could have missed detectable infections that occurred between the PCR testing every two weeks during the study, and urged young people to take up the vaccine whenever possible.
Additional reporting by PA