In 14 months, the rampaging SARS-CoV-2 has found its way to nearly two-thirds of Indians. But most of the states still have little clue on the volume of vulnerable population in the absence of granular data, since they didn’t care to conduct serosurveys which generate such information. And a few that had undertaken the exercise failed to continue it with regular periodicity.
With the threat of a third wave looming large, the states now have little time to complete their surveys that can help identify the areas where the vaccination drive needs to be intensified.
Earlier this week, Union Health Secretary Rajesh Bhushan wrote to states asking them to launch their own seropositive surveys, which were essential in formulating localised public health response measures. The Centre also released the Indian Council of Medical Research's (ICMR) fourth national serosurvey’s state-wise data that shows widespread interstate variations — from 79% in Madhya Pradesh to 44% in Kerala.
The national average was 67.6%, signalling the exposure to two-thirds of Indians to the virus. This, however, doesn’t mean all of them had the disease. It may have occurred to a small percentage of the population (around 3.16 crore).
The serosurveys are inexpensive and follow a simple method. The ICMR, for instance, took samples from 70 districts in 21 states for each of the four rounds of the survey it conducted since last year. Blood samples were analysed to check the presence of SARS-CoV-2-specific antibodies.
The results demonstrate a silent spread of the virus — from 0.7% in May-June 2020 to a whopping 67.6% in June-July 2021 in the wake of the ferocious second wave.
The ICMR survey also reveals the presence of 40 crore vulnerable people who need to be identified quickly for vaccination.
But such an identification needs granular data within the state because the national survey masks district-wise variations. Experts said states’ inability to realise the importance of such surveys, coupled with limited manpower, has led to a situation where most of the states are unable to properly map their vulnerable residents.
"Till supply constraints are completely resolved, the results from the serosurveys will help deploy the scarce vaccine to places which deserve it the most so that mortality can be reduced," said Giridhara Babu, an epidemiologist at the Public Health Foundation of India (PHFI).
Some states with stronger health systems carried out state-level surveys, both because they understood their value and had the public health workforce to conduct them. Others with weaker health systems couldn’t carry out such an exercise due to the limited health workforce which was already stretched by tasks of Covid-19 prevention and management.
"The erroneous perception that there would be no second wave resulted in the lack of interest in surveys initially. Then the second wave struck with a ferocity that kept all states too busy with emergency response to think of conducting such surveys. The attitude may change now after the second wave," PHFI president K Srinath Reddy told DH.
Virus spread
The few state-level surveys threw up interesting results on how the virus spread into the population. Delhi, for instance, conducted as many as six surveys since June 2020 and discovered that nearly 56% of its population had been exposed to the virus following the second surge, in contrast to less than 30% exposure in the first wave (till October).
On the other hand, the Ahmedabad Municipal Corporation, which conducted five serosurveys so far, found antibodies in 81.63% of people (including the vaccinated lot) in its latest report in May-June this year. This is a more than 3.5 times jump from the third survey, which found seropositivity in only 24.20% people in the Gujarat capital a few months ago. The result of the fourth survey is yet to be released officially.
Down south, Karnataka conducted two surveys in the last one year with both showing that less than 30% people in the state were exposed to the virus. But results of the second one, carried out on 35,000 people in January 2021, are yet to be officially disclosed.
Tamil Nadu conducted three surveys in October-November 2020, March-April 2021 and June 2021. While the first survey included all 37 districts of the state, the second exercise was conducted in 36 districts excluding Chennai. Separately, the Greater Chennai Corporation conducted two serosurveys to check the prevalence of Covid-19 in the city, with one in coordination with the ICMR.
Also read: Here’s why CDC recommends indoor masks even if you’ve been fully vaccinated against Covid-19
The first survey found that nearly one-third of the state’s population (31.6%) had been exposed to the virus.
The second survey showed 23% of the population in Tamil Nadu, excluding Chennai, was exposed to the virus. Poonamallee, a suburb of the capital, recorded the highest seroprevalence of 50.6%, followed by Tiruvallur (48%) and Chengalpattu (42.5%).
The third survey conducted between June and July showed the seroprevalence rate is 66.2% in the state.
Critical tool
"Serosurveys are very important in any pandemic. It is a very critical tool to help the authorities plan their next course of action. If a serosurvey says 80% of the total population have antibodies, then the impact in the case of a third wave will be very less,” a public health expert who advises the Tamil Nadu government told DH.
Unfortunately, most of the state governments didn't get the message. West Bengal, for instance, hasn’t made any attempt to undertake such a survey, whereas Maharashtra — the epicentre of the pandemic in India — didn’t conduct a survey covering the entire state.
The surveys were carried out in and around two of its biggest cities, Mumbai and Pune, but they didn’t give any clue to the extent of the vulnerable population in coastal and western Maharashtra, which were severely hit by the second wave.
"There can be marked differences between highly rural or hilly districts and highly urban and industrial districts. The level of restrictions we place on people’s mobility as well as prioritisation of districts for immunisation coverage can be guided by understanding the differences in transmission dynamics and the percentage of susceptible population still existing in each district," said Reddy.
Since serosurvey studies done over a period of time provide the threshold of population immunity, they offer an evidence-based approach for decisions regarding several public health measures through the course of the pandemic.
"For example, this can be useful in deciding where and how unlocking can be done after the surge and thereby help to restart economic activities," noted Babu. "Since there are likely to be considerable variations between states and among districts within each state, context-relevant decision-making requires granular data," added Reddy.
On the ground, the realities are, however, starkly different.
(With inputs from Mrityunjay Bose, Satish Jha, E T B Sivapriyan, Akhil Kadidal and Soumya Das)