When The Past COVID-19 infection can protect against reinfection, as good as 2 vaccine doses: The Lancet

HOW LONG-LASTING IS PROTECTION AFTER NATURAL COVID-19 INFECTION?

      Since January 2021, several studies and reviews have reported the effectiveness of past COVID-19 infection in reducing the risk of reinfection and the time taken for immunity to wane. This is the largest review of how long protection after natural COVID-19 infection can last and how durable that protection will be against different variants. Researchers from the Institute for Health Metrics and Evaluation (IH ME) at the University of Washington’s School of Medicine, USA conducted a review and meta-analysis of previous studies that compared the reduction in risk of COVID-19 among unvaccinated individuals against a SARS-CoV-2 reinfection to non-vaccinated individuals without a previous infection up to September 2022. They reviewed 65 studies from 19 countries (including India) and evaluated the effectiveness of past infection by outcome (infection, symptomatic disease, and severe disease), variant, and time since infection. Studies examining natural immunity in combination with vaccination (hybrid immunity) were excluded from the analyses. Their review showed that protection from past infection and any symptomatic disease was high for ancestral, alpha, beta, and delta variants, but was substantially lower for the omicron BA.1 variant

      The study also suggests that the level and duration of protection against reinfection, symptomatic disease and severe illness is at least on a par with that provided by two doses of the mRNA vaccines (Moderna, Pizer-Bio tech) for ancestral, Alpha,  Delta and Omicron BA.1 variants. It did not include data on infection from Omicron XBB and its sublineage. The study says past infection with pre-Omicron variants provided substantially reduced natural immunity protection against reinfection with Omicron BA.1 (36 per cent at 10 months after infection). Analysis of data from 21 studies estimated that protection against reinfection from a pre-Omicron variant was about 85 per cent at one month — and fell to about 79 per cent at 10 months. Protection from a pre-Omicron variant infection against reinfection from the Omicron BA.1 variant was lower (74 per cent at one month) and declined more rapidly to 36 per cent at around 10 months. “Nevertheless, the analysis of five studies reporting on severe disease (hospitalization and death) found that protection remained universally high for 10 months: 90 per cent for ancestral, Alpha, Beta and Delta, and 88 per cent for Omicron BA.1,” the study reported. Six studies evaluating protection against Omicron sub-lineages, specifically BA.2 and BA.4/BA.5, suggested significantly reduced protection when the prior infection was a pre-Omicron variant. But when the past infection was Omicron, protection was maintained at a higher level.

          VACCINATION STILL THE SAFEST WAY TO ACQUIRE IMMUNITY

    Researchers, however, caution that while recognizing the natural immunity in people, who have recently been infected with COVID-19, their findings should not discourage vaccination because it is the safest way to acquire immunity. “Vaccination is the safest way to acquire immunity, whereas acquiring natural immunity must be weighed against the risks of severe illness and death associated with the initial infection,” says lead author, Dr Stephen Lie, Institute for Health Metrics and Evaluation (IH ME) at the University of Washington’s School of Medicine, US. As IH ME co-author, Dr Caroline Stein, explains in the study: “Vaccines continue to be important for everyone in order to protect high-risk populations such as those who are over 60 years of age and those with comorbidities. This also includes populations that have not previously been infected and unvaccinated groups, as well as those who were infected or received their last vaccine dose more than six months ago. Decision makers should take both natural immunity and vaccination status into consideration to obtain a full picture of an individual’s immunity profile”

      WHY HYBRID IMMUNITY SHOULD REMAIN IN FOCUS

     Experts say the review is an important one as it summarizes the protection against severe disease by past infections. “While considering natural infection as almost equivalent to two doses of high quality vaccines, the translational value of this evidence is challenging,” says Dr Sanjay Punjabi, expert member of the Indian Council of Medical Research (ICMR) task force on COVID-19. Noted virologist, Dr Gagandeep Kong, also points out that criteria like “age structure of the population, the kind of vaccines used, history of infection and by which variant need to be considered to determine the duration of protection.” Both Dr Kong and Dr Punjabi say that most populations across the world may have hybrid immunity that has not been considered in the study.

     Protection afforded by hybrid immunity may be more robust and the dynamics of its waning may be different from natural or vaccine-induced immunity, says Dr Punjabi. Segregating people at population levels by determining past infection history and its timing complicates the vaccine program as timing of booster doses may need to be individualized, he adds. “Determining asymptomatic past infections is challenged by the need to perform antibody tests. While the behavior of new emerging variants for immune escape is difficult to predict, specially vulnerable populations, like immunocompromised hosts, may need different vaccine schedules than that for the general population. The need then is to focus on completing vaccine schedules, genomic surveillance and development of better vaccines like nasal and broad Coronavirus vaccines, which are important for future control of pandemics,” Dr Punjabi says.