Being infected again with the new coronavirus may not be as rare as previously believed, suggests a new study.
The researchers found that, among young Marine Corps recruits who had tested positive, more than 10 percent had a second positive test during a six-week follow-up.
Furthermore, the group that was infected again had lower levels of antibodies than those that were infected for the first time. and most lacked neutralizing antibodies detectable against COVID-19.
The team – from the Naval Medical Research Center in Silver Springs, Maryland, and the Icahn School of Medicine on Mount Sinai in New York City – say the findings provide evidence that antibodies induced by an initial infection are largely protective, but do not guarantee immunity against a subsequent infection.
The researchers analyzed 3,249 Marine Corps recruits aged 18 to 20 years during a 14-day quarantine and six-week basic training, 189 of whom had already been infected with COVID-19 and 2,247 who had not. Pictured: US Navy recruits receive coronavirus health checkups at the Marine Corps Recruitment Depot in San Diego, California, April 2020
More than 10% of the previously infected group had a second positive test during basic training (yellow line) and 48% of the uninfected group tested positive (black line)
For the study, published on the pre-printed website medRxiv.org, the team used data from the COVID-19 Health Action Response for Marines (CHARM) study.
The study looked at newcomers from the Navy since May 4, testing them every week for the first two weeks and then every two weeks thereafter.
A total of 3,249 marine recruits, mostly male, aged between 18 and 20 years old, were tested for antibodies against coronavirus on arrival for a period of 14 days Quarantine.
They were tested for the virus before, in the middle and at the end of the quarantine and then followed up for six weeks, being tested every two weeks.
The researchers say the three negative tests during the quarantine helped to ensure that the infections diagnosed during basic training were not persistent infections, but new infections.
The neutralizing antibodies were undetectable in one third of the reinfected group (left) and more than 80% in those with a positive test (right) before quarantine, but not again
Among recruits, 189 had already been infected with COVID-19 before reaching the quarantine period and 2,247 did not.
The results showed that 19 recruits from the previously infected group, or 10.1 percent, had at least one positive test during the six-week follow-up.
This is much higher than the rate among the general public, where only three people known in the United States out of more than 32 million have tested positive twice.
In comparison, 1,079 recruits from the uninfected group, equivalent to 48 percent, tested positive during the follow-up period.
“Our results indicate that, although infection-induced antibodies are largely protective, they do not guarantee effective immunity against subsequent infections,” wrote the authors.
The team then tested viral loads and antibody levels between the two groups.
They found that recruits in the newly infected group with a positive test for the first time had viral loads 10 times higher than those with a positive test again.
In addition, 84.2 percent of the reinfected group was asymptomatic compared to 67.8 percent of the newly infected group.
However, perhaps surprisingly, antibody levels were higher among the group tested positive for the first time.
The results of the antibody test used distinct dilutions defined at 1: 150, 1: 450, 1: 1350, 1: 4050 or 1: 12150, with each score indicating the number of times a scientist can dilute a patient’s blood and still be able to detect the presence of antibodies.
Titles of 1: 150 were categorized as low; 1: 450 and 1: 350 as moderate; and 1: 4050 or 1: 12150 as high.
The reinfected group had 26% in the low range, 57.9% in the moderate range and 15.8% in the high range.
In comparison, in the newly infected group, 3.5 percent were in the low range, 76.4 percent were in the moderate range and 20 percent were in the high range.
The neutralizing antibodies were also undetectable in six of the 19 recruits – or 31.6 percent – who were infected again.
Even among 54 patients who tested positive before basic training and were not infected again, 83.3 percent had undetectable neutralizing antibodies.
The team says that this study shows the importance of vaccinating both those who have never been infected and those who have been.