20-30% people lose natural immunity against Covid in 6 months, new study finds

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How long does so-called natural immunity against the coronavirus disease last?

That’s a question everyone, especially those who contracted Covid-19 and recovered, ask.

Well it turns out it lasts for 6-7 months at least, but between 20% and 30% of those infected lose this immunity after 6 months, according to research by the Institute of Genomics and Integrative Biology (IGIB).

“The key finding of 20-30% of subjects losing virus neutralizing activity, despite staying seropositive, at 6 month follow-up helps understand why the large second wave has not spared cities like Mumbai with high seropositivity,” Dr Anurag Agarwal, the director of IGIB said in a tweet.

The research is important because it could explain the timing of the second wave of the disease — such as the one India is witnessing right now.

It is also important because it emphasizes the importance of vaccines. Research is ongoing, but most vaccines currently in use are believed to protect people from severe infection and death for at least a couple of years.

Also Read| As vaccination drive picks up pace, challenges remain

The researchers say that the finings can explain why cities such as Mumbai and Delhi are witnessing a sharp spike in cases of the viral infection despite having high seropositivity – or antibodies. Delhi was found to have an average seropositivity of just over 56% in January, which doctors in the city believe was the reason for the slowing of the pandemic after the November surge.

On Saturday, Delhi registered 7,897 new cases, and Mumbai, 9,327.

The IGIB study also established that seropositvity was inversely proportional to test positivity rate. This means, a higher prevalence of antibodies leads to a decline in transmission.

“In September, when we conducted a sero-survey across CSIR (Council for Scientific and Industrial Research) laboratories, just over 10% of the participants were found to have antibodies against the virus. Then, we followed up with a fraction of these participants for three and five to six months and conducted a quantitative test to check their antibody levels,” said Dr Shantanu Sengupta, senior scientist from IGIB and one of the authors of the study accepted for publication in eLife journal on Saturday.

“At five to six months, nearly 20% of the participants had lost the neutralisation activity despite having antibodies; the neutralisation activity for the rest was also on the decline.” Neutralisation is the ability of the antibody to either kill the virus or completely prevent it from entering a cell.

Of the 10,427 study participants, 1,058 or 10.14% tested positive for antibodies in September last year. The researchers tracked 175 of the 1058 for five to six months and found that 31 or 17.7% lost neutralising activity and another eight (4.6%) longer had antibodies.

Also Read| Delhi’s Covid-19 positivity rate jumps to 10.2%; 7,897 new cases recorded

A shorter three-month exercise tracking 607 of the 1058 found that only 5.6% lost their neutralising activity and a mere 2.8% did not have antibodies any longer.

The study was conducted on the Phenome India cohort that includes permanent staff, their family members, students, and temporary employees providing support services in CSIR labs across 17 states and union territories.

“Widespread asymptomatic SARS-CoV2 infections affected nearly 100 million Indians by September 2020 with a subsequent decline in new cases which may be attributable to increased population immunity albeit seeing reduced neutralization activity at six months this respite may be temporary,” read the impact statement of the study.

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