The Indian Council of Medical Research (ICMR) is planning to conduct a fourth serological survey across the country to assess the prevalence of Sars-Cov-2, the virus that causes Covid-19, following the massive second wave in the country, said experts familiar with the plan.
“It is currently being discussed and will be announced formally by the director general of ICMR,” said Dr Samiran Panda, head, epidemiology and communicable diseases division, ICMR.
Experts say sero surveys are the key to knowing disease prevalence and taking countermeasures. In the current scenario, a fresh sero survey can help the government decide when to ease restrictions on movement. “Quick rounds of sero surveys can be conducted to know the antibody prevalence that will help in opening up accordingly. I suspect that the positivity rate, which has gone down, is largely due to natural infection,” said Dr Sanjay Rai, professor, community medicine department, All India Institute of Medical Sciences, Delhi.
Experts also say that the fourth survey will be different from the other three surveys as apart from a sudden, massive surge, a section of the population has also been vaccinated in the past four months. “What we look for in a sero survey is the presence of antibodies against the virus, and at this stage the antibodies seen could be because of not only infection but also vaccination,” Dr Panda said.
He also pointed out that among the infected are those who were asymptomatic or mildly symptomatic and some who got seriously sick. “It will be a mixed kind that we will get this time, while earlier there was infection/disease transmission dynamics,” Panda said.
Blood samples in a sero survey are tested for the presence of IgG (Immunoglobulin G) antibodies that determine a past infection due to the virus. Sero surveys are also important in determining whether the disease has entered the community transmission stage or not.
ICMR’s National Institute of Epidemiology in Chennai has been the nodal agency for all past surveys and will most likely supervise the fourth one as well.
The earlier sero surveys suggested that a significant size of the population was still susceptible to catching the viral infection, and India was far from achieving herd immunity. The first one was designed as a cross-sectional survey of adults aged 18 years or more from 21 states, where the districts were categorised into four strata according to the reported Covid-19 cases per million population.
the second and third rounds, all other parameters were the same except for the age bracket — samples were taken from people aged 10 and above.
The third sero survey, conducted between December 17, 2020, and January 8, 2021, found 21.4% of those aged 10 and above infected with the virus.
In the second round, nearly one in 15 (6.6%) people above the age of 10 was found to be exposed to the virus till August 2020.
The first survey conducted in May last year had found the infection rate to be 0.73% nationally.