NEW YORK — As the number of global cases of COVID-19 continues to climb, a crucial question for guiding public health efforts during the pandemic is whether natural infection with SARS-CoV-2, the viral cause of disease, results in immunity to reinfection. New research, unfortunately, suggests this may not be the case for everyone.
The research conducted by scientists at the Icahn School of Medicine at Mount Sinai and the Naval Medical Research Center evaluates, for the first time, the risk of reinfection in young adults. Study authors conclude that antibodies generated by natural infection may not be universally protective against reinfection.
“Our findings indicate that reinfection by SARS-CoV-2 in healthy young adults is common,” says senior author, Dr. Stuart Sealfon, in a statement. “Young people should get the vaccine whenever possible, since vaccination is necessary to boost immune responses, prevent reinfection, and reduce transmission.”
The study, which is part of the COVID-19 Health Action Response for Marines (CHARM), measured reinfection in more than 3,000 young, healthy U.S. Marines recruits over the course of six weeks. Study participants were predominantly male (92%) between the ages 18 and 20.
Following an unsupervised two-week home quarantine, the recruits traveled, while adhering to masking and social distancing, to a supervised quarantine facility, where they were offered the opportunity to enroll in the study. Upon enrollment, nasal swabs were obtained from each of the 3,249 study participants and tested by PCR for active SARS-CoV-2 infection. Blood samples were also assessed for baseline antibodies to SARS-CoV-2.
Of these participants, 3,168 who were negative for SARS-CoV-2 infection by PCR and had a conclusive baseline antibody test continued into the two-week supervised quarantine period. During this time, recruits were swabbed and tested by PCR two more times for infection. In all, 3,076 participants who had three negative tests at the end of the second quarantine period were followed for an additional six weeks after their transfer to basic training.
Over the six-week study period, participants were again tested for new SARS-CoV-2 infection on weeks two, four, and six. Of the initial 3,076 participants, 640 were either lost to follow-up or excluded due to inconclusive test results. That left a total of 2,436 recruits who were followed to the end of the study.
Seropositive vs. seronegative
Study participants were classified as either seropositive or seronegative, based on their baseline antibody tests. Seropositive individuals had a positive antibody test, indicating a prior infection. Seronegative individuals had a negative baseline antibody test, suggesting they had likely not been previously infected.
The researchers report that seropositive young adults have about one-fifth the risk of getting reinfected compared to seronegative individuals. Of the 189 participants who were seropositive at the start of the study period, 19 (10%) tested positive by PCR at least once. By contrast, 1,079 (48%) of the 2,247 seronegative participants tested positive.
Within the seropositive group, the researchers look for an association between baseline levels of antibodies and the risk of infection. They find that participants who are reinfected with SARS-CoV-2 have lower antibody levels than those who are not reinfected.
Looking specifically at levels of neutralizing antibodies, which are antibodies capable of blocking infection, results show lower levels in those who are reinfected compared to those who are not. Only 32% of reinfected seropositive individuals had detectable levels of neutralizing antibodies compared to 83% of uninfected seropositive individuals.
The researchers also evaluated differences in measurable SARS-CoV-2, or viral load, between seropositive and seronegative participants. They report that reinfected seropositive individuals have a viral load only 10 times lower on average than infected seronegative individuals. The authors note that this finding suggests that some reinfected individuals may transmit the infection to others at a similar rate as those who are infected for the first time.
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“Despite a prior COVID-19 [virus] infection, young people can catch the virus again and may still transmit it to others,” Dr. Sealfon explains. “This is an important point to know and remember as vaccine rollouts continue.”
The authors report that clinical outcomes across the two groups are similar, with a majority (84% in the seropositive group and 68% in the seronegative group) being asymptomatic. None of the study participants were hospitalized, and none died.
As a study limitation, the authors caution that these results may underestimate the risk of SARS-CoV-2 infection in previously infected individuals. This is because it is unknown whether any of the seronegative participants may have been previously infected, but did not have high enough levels of antibody to be detected.
The study is published in The Lancet Respiratory Medicine.