COVID-19 Reinfections Rare, Natural Immunity Lasts For Months: Pune Study

3D print of a SARS-CoV-2 virus particle. Photo: NIAID/Flickr CC BY 2.0

New Delhi: Reinfection among people who have recovered from COVID-19 is rare and natural immunity after illness lasts around nine months, a study involving long term follows up of more than 1,000 people in Pune has found.

According to the Times of India, a study conducted by epidemiologists and community medicine experts from the DY Patil Medical College and Hospital in the city tracked 1,081 people who showed SARS-CoV-2 antibodies during a serosurvey in September last year until this past June. During the nine months of observation, just 13 participants – just 1.2% – were reinfected. The study, which is still in pre-print, also found that all those who were reinfected only had mild cases of COVID-19 and made a full recovery.

The study is perhaps the first of its kind in India and its findings are in line with those conducted in other parts of the world. A similar study by Australian scientists, published in the journal PLOS Medicine on July 7, found that natural immunity against the coronavirus may last up to seven months long.

However, the scientists who conducted that study warned that the level of immunity over time is dependent on disease severity and the viral variant. Antibodies developed during the first wave of COVID-19 had reduced effectiveness against the six variants that have been identified since then, the study said.

From the Times of India report, it is not evident if researchers at the DY Patil Hospital tracked the efficacy of antibodies against variants.

The study’s lead investigator Amitav Banerjee told the newspaper that since reinfection is rare, India’s vaccination policy could be modified to prioritise those who have never contracted COVID-19. “The bottom line is, given the low rate of reinfection, people who have recovered from … infection should be at the back of the queue,” he said.

Infectious diseases expert Sanjay Pujari, member of the National Task Force on Covid-19, told the Times of India, “The results are in line with emerging data on reinfections from around the world. Reinfections are rare among those who have recovered from Covid-19.”

However, he said more research is needed to accurately determine the risk of reinfections, especially in the context of new variants. “Until more data is available, individuals who have recovered need to adhere to Covid-appropriate behaviour and get vaccinated in due time,” he told the newspaper.

In a brief issued in May, the World Health Organisation said that the strength and duration of the body’s natural immune responses to SARS-CoV-2 are not completely understood yet. Available data suggests that it varies by age and the severity of symptoms, the UN body said, adding that most people’s immune responses remain protective against reinfection for at least 6-8 months after infection.

“Four large studies from the United Kingdom, the United States of America and Denmark estimated that infection with SARS-CoV-2 provided 80-90% protection from reinfection up to 7 months, and up to 94% protection against symptomatic disease,” the WHO said.

The brief also added that variants of the virus with key changes in the spike protein have reduced susceptibility to neutralisation by antibodies in the blood.

Even though natural immunity is protective against COVID-19 for months, studies have shown that vaccination is still necessary not just to boost immune response but also to reduce disease transmission.

A small study by researchers at the Rockefeller University in the US made the case for people who have recovered from COVID-19 to be vaccinated, saying even a single dose boosts natural immunity “so much that they are likely protected even from the emerging variants”.

The pre-print study observed 63 people who were infected last year, finding that over time, antibodies produced by the immune system’s memory B cells got better at neutralising SARS-CoV-2. Antibodies were further enhanced among 26 people in the group who had received at least one dose of the Moderna or Pfizer vaccine.

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