Explained: Seroprevalence survey and its impact

In May, the Indian Council of Medical Research (ICMR) had announced that in collaboration with other key stakeholders, it will be conducting a population-based serosurvey to estimate the prevalence of coronavirus infection among the Indian population.

According to experts, serosurveys can help ascertain if there has been a community transmission of the respiratory disease.

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Instead of waiting for the end of the epidemic, repeated serosurveys carried out at regular intervals can be a useful tool to monitor the epidemic precisely, ICMR said.

In Delhi — which has several pockets of the dense population — the study was conducted from June 27 to July 10 by the National Centre for Disease Control in collaboration with the Delhi government and involved testing 21,387 samples. It was found that around 23 percent of the people tested were exposed to the novel coronavirus in the city.

How was the seroprevalence survey done in Delhi?

A serosurvey involves testing of blood serum of individuals as well as monitoring of the presence or absence of specific substances in the blood serum of a sample population.

For the survey, blood samples were collected from selected individuals after taking written informed consent. They were then tested for IgG antibodies and infection using Covid Kavach ELISA approved by the ICMR. It was one of the largest seroprevalence studies conducted in the country using the ELISA testing.

How reliable is the seroprevalence survey?

News reports suggest that due to the uncontrolled spread of the Coronavirus, an ample amount of time was not spent on the development of the tests which are used to conduct the serosurvey. Therefore, epidemiologists might not have been able to rule out errors.

Once someone recovers, antibodies generally start appearing after two weeks of the onset of infection and last for several months. ICMR has earlier noted that the IgG test is not useful for detecting acute infection but indicates an episode of SARS-CoV-2 infection in the past.

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"The RT-PCR test is considered the gold standard frontline test for clinical diagnosis of Covid-19. The test is useful only when performed in the acute stage of infection (less than seven days). For several viral infections, antibody tests are useful for disease detection after 5–7 days of illness," ICMR said in a statement.

What were the key findings of the serosurvey?

While the serosurvey conducted in Delhi, showed that 23 percent of the people had exposure to the novel coronavirus, it also revealed the remaining 77 percent are still vulnerable. Therefore, ICMR stressed on rigorous disease and containment measures.

The health research body has suggested that depending upon the level of seroprevalence of infection, appropriate public health interventions can be planned and implemented for prevention and control of the disease.

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Adding that it will also help estimate the prevalence of community transmission of the infection, the ICMR said, "The objectives of this sero-surveillance are to estimate and monitor the trend of seroprevalence for SARS-CoV-2 infection in the general population and high-burden cities, determine the socio-demographic risk factors for the coronavirus infection and delineate the geographical spread of the infection in the general population and hotspot cities."

The Delhi government has planned to conduct more monthly surveys for sero-prevalence in the national capital. The next such exercise will be conducted from August 1-5.

The final results of the sero-survey conducted by the ICMR in May, which focused on infections of mid-April is still under process, Rajesh Bhushan, Officer on Special Duty in the Union Health Ministry said.

The ICMR had earlier said on the basis of the data compiled from 65 districts out of 83 districts across the country in the first part of the sero-surveillance study, 0.73 percent of the population surveyed showed evidence of past exposure to SARS-CoV-2.

(With agency inputs)