Even though the number of individuals diagnosed with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is high, serological studies have revealed that the actual number of infected persons is even higher. With the ongoing worldwide mass vaccination program and the increased number of SARS-CoV-2 recovered individuals, scientists believe that it is important to understand the risk of reinfection. This would be immensely helpful for modeling the coronavirus disease 2019 (COVID-19) pandemic, which could aid in formulating vaccination strategies and predicting the onset of herd immunity.
In most cases, the SARS-CoV-2 recovered individuals sustain a serological response. Scientists have reported that in response to the infection, IgG antibodies are developed, which peak several weeks after infection. Subsequently, the IgG undergoes a decline phase and ultimately stabilizes. However, the humoral immune response is variable, and it depends on the individual’s immune system.
Previous studies have revealed that SARS-CoV-2 IgG and neutralizing antibodies protect individuals against subsequent infections. IgG antibodies against SARS-CoV-2 can be detected in the serum of most individuals several months post-infection. However, it is not so in all cases. For 2.5% to 28% of infected individuals, circulating antibodies could not be detected beyond a certain period. Further, around 10% of individuals failed to develop measurable neutralizing activity even after the primary development of protein-specific IgG antibodies. In this case, scientists have estimated full-length spike protein-specific IgG antibody titers lower than 1:320. In certain instances, SARS-CoV-2 reinfection was reported for individuals who had already recovered from the previous infection and also in seropositive individuals. In another similar study that included healthcare workers, scientists found two infected individuals among 1,265 seropositive participants and 223 infected individuals among 11,364 seronegative participants.
Researchers believe that the young population, who are predominantly asymptomatic, can transmit the infection to vulnerable populations. Therefore, evaluating the protection against subsequent SARS-CoV-2 infection for seropositivity in the young population is extremely important. This would also highlight the need for vaccinating this group.
New research has been published in The Lancet Respiratory Medicine, which focuses on evaluating the effect of SARS-CoV-2 seropositivity on the possibility of developing SARS-CoV-2 infection in a young and healthy population. This study is based on the longitudinal prospective cohort study that involves the COVID-19 Health Action Response for Marines (CHARM).
The current study included male recruits, from the US Marine Corps, aged between 18 and 20 years. They first underwent a two-week unsupervised home quarantine, following which, on return to the marine facility, they were again subjected to a two-week supervised quarantine. The participants were assessed for a baseline SARS-CoV-2 IgG seropositivity. Other details such as demographic, risk factors, medical history, COVID-19 symptoms, etc., were obtained through a questionnaire filled in by the participants.
Researchers evaluated the SARS-CoV-2 infection in the participants periodically (sampling at 0, 1st, and 2nd weeks of quarantine, using PCR). The participants were also asked to fill in a similar questionnaire during each evaluation. Participants who contracted COVID-19 were excluded from the study. The recruits who reported three negative swab PCR tests during quarantine and were identified as seronegative or seropositive for SARS-CoV-2, via serum serology test before the onset of supervised quarantine, were allowed to participate in the basic training program at Marine Corps Recruit Depot, Parris Island. Following this, three PCR tests were conducted on the 2nd, 4th, and 6th week for both seropositive and seronegative groups. They were also asked to fill in a detailed questionnaire, similar to the previous section of this study. The serum serology test was also performed on all participants.
In this study, the researchers have reported that around 10% of seropositive participants have shown at least one positive PCR test for SARS-CoV-2, and 48% of seronegative participants tested positive. The incidence rate ratio was found to be 0.18. While studying the viral load, researchers found that the infected seropositive participants contained much lesser viral load, i.e., approximately 10 times lower, than infected seronegative participants. During the six-week quarantine study, researchers found that the neutralizing titer, among the seropositive participants, was 83% (uninfected) and 32% (infected).
The current study reflects upon the fact that although antibodies offer protection against infections, there is no guarantee of effective immunity against subsequent infections. Scientists revealed that for marine recruits, extensive training might also suppress immunity. Thereby, this is a limitation as the findings are based on the data, where the frequency and intensity of exposure to the virus and the susceptibility of the host might differ from the general conditions. Another limitation of the study is that the role of cell-mediated immunity or host, environment, and viral factors leading to reinfection were not considered.
The researchers believe that understanding the rates of infection and the influence of seropositivity in reducing the risk of infection are essential factors for developing an accurate epidemiological model. It could also help predict herd immunity levels. The analysis of the rate at which reinfection occurs after vaccination or acquisition of natural immunity is important for gauging the number of individuals that need to be vaccinated to contain the pandemic and other risk factors. The study concludes that vaccinating previously infected young adults is necessary to contain the pandemic.