The story so far: The fourth national serology survey by the Indian Council of Medical Research (ICMR) to estimate the spread of the coronavirus infection in India reported this month that two-thirds of Indians had antibodies to SARS-CoV-2. As part of this, it also released figures that showed that antibody prevalence was lowest in Kerala and highest in Madhya Pradesh, along with data from 19 other States.
What do the findings imply?
Seroprevalence studies are meant to roughly estimate what percentage of the population may have been exposed to the virus. Kerala, which is now recording the sharpest rise in cases in the country fuelled by the Delta variant, had a seroprevalence of only 44.4%, implying that 56.6% of the population lacked antibodies and remained vulnerable to the infection. Madhya Pradesh has a seroprevalence of 79%, followed by Rajasthan (76.2%) and Bihar (75.9%).
After Kerala, the least antibody prevalence was in Assam (50.3%) and Maharashtra (58%). Assam and several other northeastern States are registering a sharp rise in COVID-19 infections. It is important to note that the antibodies may have been produced in response to a natural infection or from vaccination. The ICMR has not published a detailed research paper on the findings from the fourth survey.
How was the survey conducted?
Conceived in May 2020, the ICMR surveys are based on testing close to 400 people in 70 districts in 21 States. The districts were chosen based on their population. Hence, some States have more places represented and, all in all, nearly 28,500 people were part of the survey. Unlike the three previous editions of the survey, conducted from May 2020 to January 2021, the latest — covering June and July this year — included children and adolescents ranging from six to 17 years.
In the case of Kerala and Madhya Pradesh, three districts were selected to extrapolate the extent of the SARS-CoV-2 antibody prevalence, while nine districts were selected in Uttar Pradesh and six in Bihar.
Why is there a disparity among States?
The ICMR has stated that for a State to accurately capture the prevalence of antibodies, it must conduct its own regional survey and the results must only be taken as a guide to future policies. There are, however, different interpretations of the seroprevalence data. Kerala has said the lowest seroprevalence among States testifies to its public health measures through all of last year that prevented the infection from spreading to the majority of the population. Therefore, it points out that the Centre must increase allocation of vaccines to the State. Several experts agree, but say that the recent spike in cases is a consequence of Kerala loosening its grip.
The ICMR data also points to the extent of under-reporting by States. Public health expert and epidemiologist Chandrakant Lahariya cited the seroprevalence data and compared it to the officially reported cases from the 21 States. As of June 25, by his estimates, Bihar, with a seroprevalence of 75.9%, should have had an estimated 427.6 lakh cases, but it officially reported only 7.2 lakh cases — an under-reporting factor of 59. This means in Bihar, for every case that was reported, 59 cases were missed. Similarly, Uttar Pradesh and Madhya Pradesh had an under-reporting factor of 98 and 83 respectively. Kerala’s worked out to six and Maharashtra’s to 12.
Are the surveys useful?
The fourth serosurvey was unique because it was conducted after the vaccination programme began. The ICMR was initially reluctant to conduct such a survey because the antibody test they used to detect antibody prevalence cannot differentiate between the vaccinated and unvaccinated. The principle that inoculating a large fraction of the population could confer herd immunity does not entirely hold given detections of breakthrough infections and that available vaccines, while significantly able to reduce hospitalisation and mortality, were less effective in curbing transmission of the newer variants.
Several countries, including the U.S., the U.K. and Korea, are battling the Delta variant, which ravaged India during the second wave. However, seroprevalence surveys could help with planning targeted vaccination drives, especially when India is battling a shortage with fewer than expected doses available to fully inoculate the eligible adult population by the year-end.