Covid-19 immunity may last longer: Study

New Delhi: Immunity to the coronavirus disease, or Covid-19, might last for much longer than what was previously known, a new study from the United States has suggested, a critical finding at a time when vaccine candidates are racing against time to reach the public.

The study, conducted on blood samples of 185 patients between 19 to 81 largely from New York and California, was posted on the pre-print server biorxiv on Monday and first reported by the New York Times. The majority of subjects had a mild case of Covid-19 and 92% of subjects were never hospitalised.

The researchers looked for four things: antibodies of the immunoglobulin G (IgG) variety that bestow long-lasting immunity, two kinds of T-cells that kill infected cells and prevent the virus from spreading and B cell that releases antibodies into the blood.

The researchers found that IgG was relatively stable over 6+ months, the B cells were more abundant at six months than at one month and the two T-cells declined with a half-life of 3-5 months.

“Immune memory consisting of at least three immunological compartments was measurable in ~90% of subjects > 5 months PSO [post-symptom onset], indicating that durable immunity against Covid-19 disease is a possibility in most individuals,” the researchers wrote. The study has not been peer reviewed yet.

The finding confirms a clutch of recent studies that show that immunity to the infection may last for six months or longer, even years.

Earlier this month, a study by American scientists on 87 Covid-19 patients showed that even though antibodies waned to a fifth of their peak levels over six months after the infection, the ones that remained were far more potent, and helped maintain the immune system’s memory of the infection – which helped generate antibodies at a much faster rate than usual.

Another study, published in Cell’s immunity journal’s November issue, found that neutralising antibodies are produced for at least five to seven months after the 2 infection.

These recent findings are heartening, especially after initial studies in June and July found that antibodies decreased drastically a few months after the infection. In October, a study led by Imperial College London of 365,000 people across England, conducted in three rounds between June and September, found the number of people testing positive for antibodies dropped by 26.5% across the study period.

The recent results come at a time when two firms– Pfizer and Moderna – have announced impressive early results from their human trials of Covid-19 vaccine candidates, and announced their intention of applying for emergency regulatory approval for public distribution. While Pfizer’s vaccine candidate was 95% effective no significant safety problems so far, Moderna announced its vaccine candidate was 94.5% effective. Results from a third vaccine candidate, being developed by Oxford-AstraZeneca, is also due soon.

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