Re: “Postinfectious Immunity After COVID-19 and Vaccination Against SARS-CoV-2” by Krsak et al .

To the Editor:

We read with interest the communication by Krsak et al., (3) where the authors highlighted the benefits of naturally acquired immunity. Conflicting reports on the persistence of antibody levels in individuals recovered from coronavirus disease 2019 (COVID-19) infection suggest that the immunity against COVID-19 may not be lasting for long (1,6). In India, by June 30, 2021, not less than 30 million people were infected with COVID-19 and 0.39 million people were reported to have lost their life to the disease in India (5). Three rounds of serosurvey in the state of Odisha have demonstrated an increasing trend of population level exposure to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (4). We followed up with a subsample of our previous serosurvey participants to assess whether natural immunity against SARS-CoV-2 was associated with a reduced risk of reinfection.

We conducted telephonic interview of a total of 3,038 participants, out of which 2,238 participants responded and five participants were found to be not alive, as conveyed by their close relatives. There was a nonresponse rate of 26.1%. Out of the 2,238 participants, 1,170 were seropositive and 1,068 were seronegative for antibody against COVID-19. Our survey found that only three individuals in the seropositive group got infected with COVID-19, whereas 127 individuals reported contracting the infection the seronegative group. Taking into account a median follow-up duration of 258 days (66–319 days) and considering a confidence interval (CI) of 95%, the incidence of COVID-19 among the seropositive was 4.09/1,000 person-years (95% CI: 0.84 to 11.94), whereas that among the seronegative was 173.68/1,000 person-years (95% CI: 144.78 to 206.64), thus making a rate difference of −169.59/1,000 person-years (95% CI: −200.15 to −139.03) and a rate ratio of 0.023 (95% CI: 0.007 to 0.073). This suggests that antibody produced in response to natural infection with COVID-19 is likely to reduce the risk of COVID reinfection by 97.7%. This is congruent with another similar study reporting that previous infection with COVID-19 decreases the risk of reinfection by 84%, with the median duration of protection being 7 months (4). Interestingly, from the 127 seronegative individuals who later contracted COVID-19 infection, 30 needed hospitalization, out of which 12 were on oxygen therapy, 4 in intensive care unit (ICU), and 1 was on ventilator. In contrast, from the three seropositives reinfected with COVID-19, one had hospitalization but did not require oxygen support or critical care.

These findings reinforce the strong plausibility that development of antibody after natural infection not only protects against reinfection by the virus to a great extent (2), but also safeguards against progression to severe COVID-19 disease. However, further follow-up must continue to draw concrete inferences on the protection conferred by naturally developing immunity after COVID-19 infection and its ability to protect against other emerging variants of the virus. Adherence to COVID-19 preventive protocols and the maintenance of universal masking remain necessary ingredients in a multipronged strategy for infection reduction in a large dense demography such as India.