April 15 (UPI) -- COVID-19 infection does not completely protect young adults from getting the virus again, a study published Thursday by The Lancet Respiratory Medicine found.
About 10% of 18- to 20-year-olds included in the analysis developed a case of the virus despite being infected earlier in the pandemic, the data showed.
The findings suggest that, despite previous infection and the presence of antibodies against the virus in the blood, vaccination still is necessary to boost immune response, prevent new infection and reduce virus transmission, the researchers said.
"As vaccine rollouts continue to gain momentum, it is important to remember that ... young people can catch the virus again and may still transmit it to others," study co-author Stuart Sealfon said in a statement.
"Immunity is not guaranteed by past infection, and vaccinations that provide additional protection are still needed for those who have had COVID-19." said Sealfon, a professor and chair of the department of neurology at Icahn School of Medicine at Mount Sinai in New York City.
Just under 1% of people with an earlier history of COVID-19 will test positive again with a different strain of the virus, according to a study published by researchers in Denmark in March.
Cases of COVID-19 reinfection, or those among people who have recovered from the virus. but are infected again with a new strain, have been rare. But they have been cause for concern as the world seeks to contain the pandemic.
If people infected with the virus develop antibodies, or immune cells in the blood that help fight off pathogens, against it, they combined with those who get immunity through vaccination could help the world reach "herd immunity," experts have said.
Herd immunity, or widespread protection against the virus, could prevent new strains from emerging and new cases of infection, effectively bringing an end to the pandemic.
For this study, Sealfon and his colleagues analyzed data on nearly 2,500 U.S. Marine Corps recruits who completed an unsupervised quarantine at home for two weeks before entering a Marine-supervised quarantine facility for another two weeks between May and November last year.
All of the study participants underwent antibody testing to establish whether any had been infected previously with the coronavirus and had antibodies against it, the researchers said.
The participants also were tested for new infection at the beginning of their second quarantine, and then again one week and two weeks later, according to the researchers.
Following quarantine, recruits who did not have COVID-19 entered basic training and were tested for the virus every two weeks for six weeks, the researchers said.
Of the Marine recruits included in the analysis, 189 tested positive for antibodies and 2,247 tested negative at the start of the study, the data showed.
There were 1,098 new infections during the study, including 19 among participants who previously tested positive for antibodies, for a reinfection rate of 10%, the researchers said.
Reinfected participants were found to have lower antibody levels against the virus than those who did not become reinfected, according to the researchers.
Most of the new COVID-19 cases -- 16 out of 19 reinfected cases and 732 out of 1,079, or 68%, of new cases -- had no or mild symptoms, and none were hospitalized, the data showed.
"The takeaway message for all young people, including our military service members, is clear -- immunity resulting from natural infection is not guaranteed," study co-author Lt. Dawn Weir said in a statement.
"You still need to be vaccinated even if you have had COVID-19 and recovered." said Weir, a microbiologist at the Navy Medical Research Center in Bethesda, Md.