A study of more than 3,000 Marine recruits indicates that having a case of COVID-19 doesn't necessarily keep young people from getting the illness again.
The research, conducted during six months last year at the Marine Corps' recruit depots, found that 10% of 189 recruits known to have previously had COVID-19 became reinfected with the virus while in quarantine or during boot camp.
The risk of reinfection was five times lower than the infection risk among those who hadn't previously had the virus, but shows that people can get COVID-19 more than once and can pass it along to others, wrote researchers from the Naval Medical Research Center and the Icahn School of Medicine at Mount Sinai in New York.
Read Next: Two Guardsmen Return to Duty After Being Shot in Minneapolis; 3K Troops Brace for Unrest
"Our findings indicate that reinfection by SARS-CoV-2 in healthy young adults is common," said senior author Dr. Stuart Sealfon, a neurology professor at the medical school, using the technical name for the disease. "Despite a prior COVID-19 infection, young people can catch the virus again and may still transmit it to others."
The study, published April 15 in The Lancet Respiratory Medicine, assessed 3,249 18- to 20-year-old Marine recruits, 90% of whom were male, who were tested for COVID-19 antibodies after arriving for supervised restriction of movement at boot camp.
During the six-week study period, 2,346 were followed for the duration and were tested for COVID-19 every two weeks after quarantine.
Initial antibody testing found that 189 were "seropositive" on arrival at boot camp, indicating they previously had a COVID-19 infection. Of those, 19 contracted COVID-19 during the study period. Of the recruits who showed no evidence of a previous coronavirus infection, 1,079, or 48%, were diagnosed with COVID during the six-week study period.
The viral loads of the reinfected recruits were roughly 10 times lower than those in the newly infected, and most of the reinfected had asymptomatic or mild cases of COVID-19, according to the report.
The authors said the study is significant because it can help determine the rate at which reinfection occurs following natural immunity or vaccine-induced immunity. That calculation could help determine when the U.S. population may reach "herd immunity" -- the proportion of the population with immunity developed by either having the illness or receiving a vaccine -- that can deprive the coronavirus of hosts, leading to a decline in prevalence and cases.
"Despite a prior COVID-19 infection, young people can catch the virus again and may still transmit it to others," Sealfon said. "This is an important point to know and remember as vaccine rollouts continue. Young people should get the vaccine whenever possible, since vaccination is necessary to boost immune responses, prevent reinfection, and reduce transmission."
The authors also said the study had some limitations, including that it likely underestimated the risk of reinfection in previously infected individuals because it didn't include individuals with low levels of antibodies.
In commentary accompanying the research, two infectious disease experts also noted that without having sequenced the virus in both the first and second infections, it's difficult to determine whether the repeat cases were actually reinfections and not the result of the patient harboring a persistent COVID strain.
But the results do show that having had a COVID-19 infection does provide "an important, albeit limited protection, for new infections," even as it does not provide an "almost universal and long-lasting protective immunity unlike that seen in measles, for example," wrote Dr. Maria Velasco of Hospital Universitario Fundacion Alcorcon, and Dr. Carlos Guijarro of Rey Juan Carlos University, both in Madrid.
They added that the results show that even young people with a previous COVID-19 infection should be vaccinated to target transmission from asymptomatic individuals and improve immune response.
"Reports suggest that vaccine-induced immune response might be higher than that elicited by SARS-CoV-2 infection, suggesting that vaccination might be more effective in preventing new infections," they wrote.
As of Monday, there have been 183,875 cases of COVID-19 diagnosed in U.S. service members, including 66,598 in the Army, 37,379 in the Navy, 30,538 in the Air Force and 21,353 Marines. Nearly 27,000 cases have been tallied in the National Guard and 1,105 additional cases among personnel assigned to Defense Department agencies.
Twenty-four service members have died, including three active-duty Army soldiers, five active-duty Navy sailors, six Army Reserve members, two Navy Reserve members and eight Air Force or Army National Guard members.
-- Patricia Kime can be reached at Patricia.Kime@Monster.com. Follow her on Twitter @patriciakime.
Related: Navy Will Make COVID-19 Vaccination Mandatory 'As Soon as We Can:' 3-Star Admiral
