Updated: January 14, 2022 9:31:35 am
Natural infection and not vaccination in Delhi residents led to high prevalence of antibodies against Sars-CoV-2 virus that causes Covid, shows the complete report of the sixth round of sero-surveillance recently published in pre-print server medRxiv.
The sero-survey conducted in September-October after the second wave showed that 9 out of 10 Delhi (or 88.1%) residents were sero-positive or had Covid antibodies. The sero-prevalence stands at 97% when adjusted for the accuracy of the kits. The study, that is yet-to-be peer-reviewed, states that sero-prevalence among unvaccinated adults increased from 50.3% to 82% between the fifth and sixth rounds of survey, and in children from 52.4% to 81.7%, “which is indicative of natural infection being the primary driver of immune response in a large proportion of the population”.
75% of Delhi’s population had received at least one shot of their Covid vaccine by September, and 30% had received both. Vaccination coverage has since increased with everyone having received at least one shot.
This hybrid immunity — from natural infection and immunisation — led to not only high sero-positivity but also high titres of antibodies, conferring enhanced protection in residents, as per researchers. Antibody levels ranged from 1 to 22.8 among study participants, with anything over 4 being considered to be good enough protection against disease.
Among those positive for antibodies, high levels were found in 90.8% in those over the age of 50 years; 88.9% in those between the ages of 18 and 49 years; and 70.9% in those below the age of 18 years, the study shows.
“The high sero-prevalence along with high levels of antibodies provided very good protection to Delhi residents; there were very few cases being reported after the second wave. Now even with omicron, which is known to evade immunity from vaccination and infection, the disease is not very severe,” said Dr Ekta Gupta, one of the authors of the study and a professor of virology at the Institute of Liver and Biliary Sciences.
Another interesting finding was that sero-positivity was similar among those who had received either Covaxin or Covishield — between 93% to 95%. This is much better than reported in the nationwide ICMR study where 10-14% of vaccinated individuals remained sero-negative even after two vaccine doses.
Also, seropositivity was similar even among those who had received a single dose of either vaccine — with a difference of nearly 10 percentage points among the vaccinated and unvaccinated.
“The infection had already spread through the population even as immunisation drive was being carried out. The similar seropositivity among those who received one and those who received two doses means the first dose acted as a booster in many who had been unknowingly exposed to the infection before,” said Dr Pragya Sharma, first author of the paper and professor in the department of community medicine at Maulana Azad Medical College.
“Before the delta wave, a huge proportion of the population was still susceptible to the infection. Now, almost all have been either infected or vaccinated. This means that infection level in the community should remain low unless some other variant emerges that is very capable of evading immunity,” she said.
She said there is no reason to conduct another sero-survey as 100% of the population has been either vaccinated or exposed to the infection. It is to be conducted six-eight months after omicron wave, if there are no surges due to other variants in the interim, to ascertain what proportion of population retains immunity.
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