Previous COVID-19 Infection in Young Adults Does Not Fully Protect Against Reinfection


Study finds a 10% reinfection rate amongst a mostly 18-20 year old United States Marine population in a closed setting.


Young people are vulnerable to COVID-19 reinfection and should consider vaccination when eligible says authors from a new study. In the study looking at US Marine recruits, 10% (19 out of 189) of participants were reinfected with the virus.

“Our study shows that some individuals with lower levels of neutralizing antibodies were reinfected, indicating that it is possible that previously infected and recovered people are susceptible to new SARS-CoV-2 infection at a later time,” coauthor Lieutenant Dawn Weir, PhD, of the Navy Medical Research Center, said.

The participants included 2346 Marines, with 189 being seropositive and 2247 being seronegative at the start of the study. Across both cohorts, there were 1098 (45%) new infections during the study.

These findings were reported in The Lancet Respiratory Medicine.

In order to learn why reinfections were occurring, the investigators examined the antibodies of both the infected and the non-infected participants. 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, neutralizing antibodies were less common—neutralizing antibodies were detected in 45 (83%) of 54 uninfected, and in six (32%) of 19 reinfected participants during the six weeks of observation.

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 noted that it would need further investigation to confirm.

And new infections occurred in approximately 48% (1079 out of 2247) of participants who had not previously been infected.

The study found most new COVID-19 cases were asymptomatic: 84% (16 out of 19 participants) in the seropositive group vs 68% (732 out of 1079 participants) in the seronegative group. Some experienced mild symptoms but none were hospitalized.

In addition, the study’s authors point out the reinfection rate might not be replicated because of the nature of the environment amongst marines living in tight living quarters and close contact for training. They point to another study in Denmark that included 4 million people which found the risk of reinfection was only 0.65%. However, in the same study, the risk of infection was also five times higher in people who had not previously had COVID-19. 

This marine study was funded by the Defense Health Agency and Defense Advanced Research Projects Agency, and It was conducted by researchers from the Naval Medical Research Center, Icahn School of Medicine, Mount Sinai, New York, Naval Medical Research Unit SIX, Peru, University of Texas Medical Branch Galveston, University of Vermont and the Naval Medical Readiness and Training Command Beaufort.

“The takeaway message for all young people, including our military service members, is clear. Immunity resulting from natural infection is not guaranteed; you still need to be vaccinated even if you have had COVID-19 and recovered,” Weir concluded.

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