Delhi: Herd immunity only through vaccination

The third sero survey in Delhi, which has revealed that seroprevalence has declined to 25.1%, instead of going up, has made one thing clear: we cannot depend on Covid-19 to run its course in order to develop herd immunity.

Delhi: Herd immunity only through vaccination
NEW DELHI: The third sero survey in Delhi, which has revealed that seroprevalence has declined to 25.1%, instead of going up, has made one thing clear: we cannot depend on Covid-19 to run its course in order to develop herd immunity.

Epidemiologists say it may take months to achieve herd immunity through the natural process and this may entail loss of many more lives. Therefore, experts say, it is important to wear masks in crowded places and follow social distancing.

Herd immunity refers to a point in an epidemic situation when a virus can no longer spread easily because enough people are immune to it.


According to Dr N K Mehra, ICMR emeritus scientist (honorary) and former dean of AIIMS, at least 60% to 70% of the population needs to get infected to achieve herd immunity against Covid-19. “If you consider the mortality rate of the disease to be 1% or less as suggested in multiple studies, it would mean loss of thousands of more lives in the country,” he said.

Already, data shows, more than 98,000 people have died due to Covid-19 across India. In Delhi, 5,401 people have died after contracting the infection. Delhi reported its first case of Covid-19 on March 2 when a 45-year-old man with a history of travel to Italy tested positive for the disease.

“Preventive vaccination is the only way to develop herd immunity against Covid-19. People should wear masks and follow physical distancing norms to protect themselves from contracting the disease until a vaccine is available,” said Dr S K Sarin, director of Institute of Liver and Biliary Sciences (ILBS).
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One of the lead researchers of the sero survey told TOI that antibodies against Covid-19 weren’t detectable in many people. “We tested around 300 people who had tested positive for Covid-19 through RT-PCR. Nearly 34% of them did not have detectable Covid antibodies,” the researcher said. This may explain why the percentage of people having Covid antibodies has reduced in the latest survey.

In the sero surveys carried out by the Delhi government in July and August, the seroprevalence stood at 22.8% and 28.7%, respectively. However, the latest survey conducted in the first week of September shows that only about 25.1% had detectable Covid antibodies. “It is possible that the antibodies waned in many people. Also, the survey carried out this time involved more people from urban areas which could explain the difference,” the researcher said.

Dr Mehra, however, said the waning of antibodies in persons who have tested positive for Covid-19 shouldn’t cause concern. “Antibody responses always decline after the infection. But there is an effective pool of other cells (B cells) that maintain immunological memory against the pathogen. These are enough to protect against re-infection,” he added.
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