Neutralizing antibodies could protect people against reinfection from the same strain of coronavirus even six months after the first infection, a first-of-its-kind longitudinal cohort study on sero-positivity by Council of Scientific and Industrial Research (CSIR) has revealed. The study also found out that by September 2020, an estimated 10 crore Indians were likely to have been infected and in the study cohort; smokers, vegetarians and those with O positive blood group were observed to be associated with lower levels of infection.
A pre-print or non-peer-reviewed version of the study was published online on medRxiv.
Beginning in June last year, CSIR scientists collected blood samples of 10,427 adults working in 40 CSIR laboratories across 17 states and 24 cities to test for antibodies against coronavirus infection. As is done in a longitudinal study, the participants were tested twice at an interval of three months and six months since the first sampling to test for the presence of antibodies as well as neutralizing antibodies.
An aggregate sero-positivity arrived at was 10.14%. This data together with the geographical location and density of the cities indicated that by September 2020, there was wide but uneven spread of the pandemic and subsequently, it grew more rapidly in regions with low sero-positivity in September.
Government data on detected infections showed that coronavirus infections peaked in the middle of September and then began to plateau gradually. On September 16, the country had reported 97,894 cases. This was the peak of new confirmed cases recorded on a single day.
The Indian Council of Medical Research (ICMR) had done a sero-surveillance study around the same time as the CSIR study and its results largely indicated same levels of sero-positivity in the country’s population.
Presence of Neutralizing Antibodies
Dr Shantanu Sengupta, senior principal scientist, CSIR-Institute of Genomics and Integrative Biology (IGIB), said that these results were part of the first round of assessment. “We have begun the second round now. The main difference between our study and the ICMR sero-survey is the follow up with the same participants. This has helped us assess the effect of neutralising antibodies,” Dr Sengupta said.
Of the 10,427 (over 70% men), 10.14% or 1,058 individuals had antibodies against SARS-CoV-2. A follow-up on 346 of these individuals after three months showed stable to higher antibody levels against the virus and a declining plasma activity for neutralizing SARS-CoV2 receptor binding domain and ACE2 interaction.
Another follow-up among 35 individuals, six months after the original sample collection, revealed that antibodies were declining whereas neutralizing activity remained stable compared to the first follow-up three months before. About 75% of those who tested positive for antibodies did not recall even one of nine symptoms since March 2020. Fever was the most common symptom with one-fourth reporting loss of taste or smell.