A surfeit of sero surveys

While they are useful in detecting the prevalence of antibodies, frequent sero surveys are expensive and futile

Updated - December 04, 2021 10:32 pm IST

Published - August 25, 2021 12:15 am IST

A health worker takes blood sample for a serological survey. File

A health worker takes blood sample for a serological survey. File

A sero surveillance survey ascertains the prevalence of a past infectious disease in a population. In the case of COVID-19 , it helps to detect whether antibodies to SARS-CoV-2 are present in a population. The antibodies are like evidence in a crime scene and the virus is like a criminal. But to prevent the criminal from repeating the crime, we need to conduct RT-PCR and Rapid Antigen Tests (RAT). With this information, we can isolate the individual and prevent further infection and also manage complications of the infection if they arise.

Benefits of sero surveys

If anyone has had a severe case of COVID-19, can they walk around freely believing that they won’t get the infection again as they have developed adequate natural antibodies? No. They can still be asymptomatic carriers of the virus. They cannot be exempted from COVID-19-appropriate behaviour. And they must get vaccinated too. In other words, all sero positive individuals have to observe COVID-19-appropriate behaviour and get fully vaccinated. This means that sero surveys don’t personally benefit individuals.


For public health authorities, sero surveys are of varying use at different phases of the pandemic. Such surveys are widely used by the media and by epidemiologists to show under-reporting of cases and deaths due to COVID-19. Independent sero surveillance data can expose the level of data suppression.

After releasing the fourth sero survey results, the ICMR recommended acceleration of vaccination of the vulnerable population, especially yet-to-be-vaccinated health staff; tracking COVID-19 infection in SARI cases in hospitals; and identifying clusters of current cases and cases of clinical severity for genome sequencing which would help track mutations of the virus. But these are valid irrespective of the sero conversion levels.

The second use of sero surveys is to find out whether community transmission has taken place or not. The ICMR was right in refuting some accounts by the media in mid-2020 that community transmission had taken place without the system knowing about it. Many demanded RAT or RT-PCR tests on a large scale. That would have been a wrong public health action. The first sero prevalence survey in May-June 2020 showed that overall infection was 0.73%. The relevant public health actions to be taken then were rapid case detection, isolation and containment measures as vaccines were not ready then. In August 2020, the second survey showed that sero prevalence had increased to 7.1%. Even after the first wave, when evidence of rapid transmission was emerging, we needed an additional weapon to fight the virus apart from observing COVID-19-appropriate behaviour and that was vaccination.


The third use of sero surveys is to assess how far or close we are to herd immunity. Many virologists said that 60% of the population should be immune to COVID-19 for us to reach herd immunity. We were nowhere near a herd immunity level at that time. Though there were claims that parts of the urban poor population in Mumbai and Pune had 70% developing antibodies, the third survey found that after the first wave, only 21.4% Indians had SARS-CoV-2 antibodies. The degree of change was highest in rural areas at 19.1%. In non-slum urban areas, it was 26.2%, while in urban slums the prevalence had increased to 31.7%. The fourth survey showed that 67.6% of the population had developed antibodies against SARS-CoV-2 meeting the earlier prophesied threshold for herd immunity. Now, we have to change the goalpost to 80-90%. There are breakthrough infections everywhere. The Ahmedabad Municipal Corporation has reported that over 81% have developed antibodies against SARS-CoV-2 in Ahmedabad, but there continue to be cases.

Cost-benefit analysis

Under these circumstances, it is not rational for public health experts to advise governments to embark upon city-wise seroprevalence surveys to detect the presence of COVID-19 antibodies. Those surveys are no more useful than our COVID-19-appropriate behaviour and vaccinations to control the pandemic. For an academic documentation of the trend of the pandemic spread and penetration among communities, one national-level ICMR survey is good enough.


A survey of 5,000 people per city costs around ₹25 lakh. This is not a good investment for a country still grappling with the pandemic. If we do a cost-benefit analysis, frequent sero surveys are a poor use of staff time, technology and funds and divert attention from the core activities of screening, testing and other containment measures. The genome analysis of breakthrough cases in spite of vaccination is more beneficial for evidence-based policy changes.

K.R. Antony is a pediatrician and public health consultant based in Kochi. Views are personal

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