In mid-May, when India’s centrally imposed lockdown restrictions began to ease up, the Indian Council of Medical Research (ICMR) launched a nationwide cross-sectional serosurvey in over 26,000 individuals in 70 districts. The survey included only the containment zones of the 10 cities reporting the highest number of cases as of April 25.
The long-awaited results of the initial study — published in the Indian Journal of Medical Research on September 10, 2020 — arrived at the conclusion that the national seroprevalence rate was only 0.73% among the general population, excluding the hotspots of 10 cities.
The second national serosurvey, whose findings were published at the end of September, revealed that seropositivity levels had reached 7%. Despite the 10-fold jump in the rates, it highlighted the continued vulnerability of a massive seronegative population base.
The objective of the second study, which was carried out among the 29,082 people across 70 districts in 21 states between August 17 and September 22, was to estimate the prevalence of SARS-CoV-2 infection among individuals aged 10 years and above, ICMR said.
Around 6.6% of individuals aged 10 and above and 7.1 percent of the adult population aged 18 and above showed evidence of past exposure to the virus.
At 8.2%, the infection prevalence was found nearly double in urban non-slums compared to rural areas (4.4 percent). The infection rate was the highest at 15.6 percent in urban slums.
The country-wide prevalence of the virus was similar to that seen in the United States at the time, which was around 9.3%. Brazil and Spain had a prevalence of 2.8% and 4.6%, the ICMR said.
In May, the country’s apex body governing biomedical research advised states to conduct periodic serosurveys to measure coronavirus exposure in the population using the IgG ELISA test.
Municipal corporations of major cities in India, including Delhi, Mumbai and Ahmedabad, released the results of the COVID-19 seroprevalence surveys that they had conducted in association with research institutes.
The data from these surveys provide us some insight into the infection prevalence in urban Indian populations.
National Centre for Disease Control (NCDC), under the health ministry, in collaboration with the Delhi government carried out a community-based cross-sectional serosurvey in all the 11 districts of Delhi between June 27 and July 10.
After analysing 21,387 sera samples, the study found that average infection prevalence across Delhi from mid-June to the third week of the month was 22.86 percent. This indicated that about 4.6 million individuals out of over 20 million population in Delhi were infected, even though the official tally ranged from 43,000 to 82,000 during that period.
Though smaller in size, the Mumbai serosurvey was undertaken by a joint venture between Tata Institute of Fundamental Research (TIFR), Mumbai; Translational Health Science and Technology Institute (THSTI), Faridabad, Haryana; University of Chicago, United States; Duke University, North Carolina, US; ATE Chandra Foundation, a Mumbai-based philanthropic organisation; Kasturba Hospital, Mumbai; and IDFC Institute, a Mumbai-based public policy think tank.
The study has estimated an average prevalence of 56.5 percent in slums and 15.5 percent in non-slums in the three wards selected. Assuming that 42 percent of Mumbai’s population lives in slums, this works out to an overall seroprevalence rate of about 33 percent for the entire city, which is significantly higher than the value of about 23 percent found for Delhi.
On the basis of these results, the study inferred that asymptomatic infections are likely to constitute a high fraction of the total infections, like in the case of Delhi.
The Ahmedabad Municipal Corporation (AMC) claimed that the serosurvey conducted in the city was the world’s largest, but it released only as much information about the results as the Delhi survey. According to the AMC press release, the survey was conducted from June 16 to July 11, with 30,054 samples collected among the seven zones of the city. It found that about 16.7% of the samples returned positive for antibodies. The city has a population of around 6.3 million.
In these surveys, average prevalence numbers ranged from 50% in Mumbai to 29% in New Delhi, 32% in Chennai and 7.8% in Indore. Surveys in other cities are underway.
These numbers roughly suggest that about one out of every four or five individuals in the major cities have been infected. Yet, this is still far off from what is required for herd immunity to set in.