explained | Why are serological surveys important?


Does knowing the extent of COVID-19 exposure help plan for the next wave? Why is ICMR unlikely to launch a fourth study anytime soon?

The story so far: Since the start of the pandemic in 2020, the Indian Council of Medical Research (ICMR) has conducted three serological surveys nationwide to measure the spread of infection from the SARS-CoV-2 virus. The third (from December 17 to January 8) showed that 21.5% of the population had been exposed to the virus. Until May 23, India had only been able to fully immunize 3% of the population and only 10.9% had received a dose. There are indications that the ICMR is unlikely to immediately undertake a fourth national serological survey to estimate the extent of exposure to the coronavirus since January of this year.

Why are serological surveys important?

Serological surveys take blood samples from participants and measure antibodies to check past exposure to the virus. From about 10 days after infection, antibodies reach significant numbers in most patients. As some studies show, they start to decrease after five to six months. Researchers are more interested in a class of antibodies called immunoglobulin G (IgG), which persist the longest and attach to the coronavirus to prevent them from overgrowing. Their longevity and number are therefore indicators of immunity against future infection. Since the SARS-CoV-2 virus is new, there is uncertainty about the actual duration of these antibodies. Serological surveys are therefore a crude measure of the proportion of a population likely to be protected against a second infection; this can be used by planners to decide the future capacity of health care and the opening of the economy.

How many serological surveys have been carried out so far?

The ICMR has conducted three national serological surveys since May 2020 and found that exposure to the virus was several times higher than that reported by confirmed cases. The third serological survey which measured the spread of the infection from December to early January found that 21.5% of India’s adult population and 25% of people aged 10 to 17 may have been infected. Prior to that in August-September, it was 7% and the first survey, May-June 2020, estimated the infection rate at 0.73%. The ICMR survey covered 70 districts, spread across 21 states, which were to include high and low prevalence of infection as well as the spread of infection in rural and urban areas of India and recruited about 28,000 people. Subsequent surveys were carried out in the same districts but included people aged 10 to 17 as well as health workers. There have also been several city-specific surveys in Pune, Delhi, Mumbai, Chennai – sometimes multiple rounds – to assess the distribution among different districts. Many of those surveys, at least before the second wave hit, seemed to suggest that more than 50% of the population in urban clusters may have been exposed to the virus.

Why is ICMR not likely to perform a fourth serological survey?

India’s devastating second wave began in mid-March amid a vaccination program that prioritized healthcare and frontline workers, as well as the elderly. The antibodies that are raised after infection with the SARS-CoV-2 virus are structurally indistinguishable from those after vaccination. However, studies show that people vaccinated after being exposed to the virus have much higher levels of antibodies than those vaccinated. Add to that the complication of increasing and decreasing antibody numbers. Dr Balram Bhargava, Director General of ICMR, said The Hindu, “Serological surveys lose their scientific relevance once vaccination begins in the population. ”

In theory, this could be corrected by eliminating the vaccinated people from the survey. However, health officials claimed that due to the vicious second wave, hospitals and health care infrastructure in several districts were overwhelmed by the increase in the number of cases. A serological survey is a demanding logistical exercise as detailed interviews must be conducted with the participants, qualified phlebotomists must be available and dedicated laboratories must be assigned to analyze blood tests. This would require significant contributions from the districts. In addition to infections, the regional focus should be on immunization, and India is clearly struggling to meet demand. Only 10.9% of the population received a single dose despite almost 20.4 crore of vaccines administered. It is quite possible that the ICMR will consider a future investigation when the situation is more favorable.

Can serological surveys reveal more than just exposure?

Surveys provide information about the questions they are designed to ask. Tellingly, the second serological survey indicated an increase in infections in rural India. These surveys showed that large parts of the country were not exposed to the virus and therefore at risk of infection – as the second wave highlighted.

Editorial | Focus on serology

Specific blood analysis tests, called assays, can be used to check whether the antibodies the body makes are targeting the peak region of the coronavirus or an inner envelope containing its genetic material called a nucleocapsid. It is believed that antibodies against the peak are more relevant in preventing future infections and therefore a better indicator of immunity. They can also thus provide inputs for vaccine improvement.


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