NEW DELHI: As India launched the world's largest Covid-19 vaccine exercise, a pan-India sero surveillance said that about a quarter of Indian population is expected to be already infected by SARS-CoV2 and that could be behind declining number of positive cases in the country.
The Council of Scientific and Industrial Research (CSIR) piloted an assessment of seropositivity against SARS-CoV2 in over 40 labs and centres spread across 24 Indian cities. The aggregate seropositivity of 10.14 per cent suggested that more than a hundred million Indians were infected by September 2020, primarily in larger cities but rapidly spreading all over the nation.
"By September 2020, there were likely more than a hundred million infected and recovered Indians with effective neutralising antibodies with a life of 6 months or more. Probably 2-3 times of that by now," said Anurag Agarwal, Director, CSIR-Institue of Genomics and Integrative Biology.
It confirms that by September 2020, there was wide but uneven spread of the pandemic, with the epidemic, subsequently growing more rapidly in regions with low seropositivity in September.
"We find that large cities have high seropositivity rates, consistent with local surveys, but the majority of India continues to have low to moderate seropositivity. Most seropositive individuals had occupational exposure risk and significant symptoms were recalled by only 25 per cent, with 75 per cent recalling no symptoms at all," it added.
About 10,427 adult individuals working in CSIR laboratories and their family members based on voluntary participation were assessed for antibody presence and stability was analyzed over 6 months.
As many as 1058 individuals (10·14 per cent) had antibodies against SARS-CoV2. A follow-up on 346 seropositive individuals after three months revealed stable to higher antibody levels against SARS-CoV2 but declining plasma activity for neutralizing SARS-CoV2 receptor binding domain and ACE2 interaction.
A repeat sampling of 35 individuals, at six months, revealed declining antibody levels while the neutralizing activity remained stable compared to three months. The majority of seropositive individuals (75 per cent) did not recall even one of nine symptoms.
"Such information is invaluable towards a well-informed vaccination program, especially in a setting where shortages are expected. Although India has been among one of the most affected nations in this SARS-CoV2 pandemic, such data is not yet available. The present study, which recruited individuals from 24 cities, provides an important and timely snapshot across multiple geographies," said the study.