The Indian Council for Medical Research has found that two out of three Indians (or 67.6 percent of India’s population aged above six years) have SARS-CoV-2 antibodies.

While this data presented a ‘ray of hope’ amid fears of a third wave, it also offered a sobering reality that at least 40 crore people are still totally unprotected against the infection. And because such a large pool of vulnerable population is still amongst us, one out of three,  the overall vulnerability for the nation still remains very high.

It showed that as of mid-July, just over 13 percent of eligible volunteers were fully vaccinated while only 24 percent had taken their first shot. An overwhelming 62.2 percent of volunteers eligible for the vaccine had not taken a single shot.

The result of this study is an important tool to map the trajectory of the pandemic. But it is also extremely relevant because it comes in wake of the deadly second wave, and is the first study conducted after vaccination roll-out.

Here is all you need to know about the study.

What is a serosurvey?

A serosurvey is a population-wide sampling test done over a fixed period of time. It is traditionally done by taking blood samples from a random selection of people across ages and regions, to look for antibodies against SARS-CoV-2 – the virus that causes COVID-19 . The results, which indicate how much of the population is/was likely infected and how many have recovered, can help shape strategic decisions around the pandemic.

Since many patients don’t appear to exhibit symptoms and testing is not widespread enough to trace and test the contacts of every single known case, the serosurvey data holds special importance.

In general, serosurveys are expected to answer the following questions

  • What percentage of the general population has been exposed to and infected by the virus?
  • Who (age group/gender/ region) are at higher risk of getting an infection?
  • Areas (urban/rural) where containment efforts need to be strengthened?

What the serosurvey does not tell us

Serosurveys do not tell us if the population tested is protected from any further infections. It detects all antibodies present in the body and not all antibodies are protective in nature.

What are the main findings of the fourth serosurvey?

  • Two-thirds or 67.6 percent of India’s population had antibodies either via vaccine or by natural immunity developed after an infection 
  • 62.3 percent unvaccinated population above age of six have SARS-CoV-2 antibodies
  • 81 percent of those who received the first dose had antibodies.
  • Antibody prevalence was 89.8 percent for people who had received both shots.
  • But around 40 crore people are still vulnerable to COVID-19 infection. This means that they are neither vaccinated nor have they developed natural immunity against the virus.
  • At least 85 percent of the surveyed healthcare workers had antibodies against SARS-CoV-2 and one-tenth of HCWs were still unvaccinated
  • The survey covered 28,975 general population and 7,252 healthcare workers.
  • The fourth round of the survey was conducted in the 70 districts across 21 states where the previous three rounds had been carried out.

What did India’s previous sero-survey reveal?

In the first nationwide serosurvey carried out by ICMR, they reported that 0.73 percent of adults in India were exposed to SARS-CoV-2, adding up to some 6.5 million cases by May. The survey concluded that there were 80 to 130  infections per reported case by May 2020. The first survey was conducted between 11 May to 4 June across 400 districts and about 28,000 were sampled — most of them in rural India — to gauge the spread of the infection. The results were published in the Indian Journal of Medical Research on 11 September.

The second serosurvey, which was conducted between 17 August 2020 to 22 September 2020, found that the seroprevalence had risen to 7.1 percent. The survey found 26–32 infections per reported case by August 2020. The survey included random samples from volunteers in the same 700 villages/wards (in urban) from 70 districts from 21 states as the first survey.

The third serosurvey was done between 17 December and 8 January, two weeks before India kicked off its vaccination drive. It revealed that 24.1 percent of 28,589 people surveyed above 18 years of age showed exposure to the virus. Nearly one in five individuals aged ≥18 years from the general population as well as HCWs in India had been exposed to SARS-CoV-2 by December 2020. Twenty-seven infections per known case went undetected in December 2020.

How does the data from the fourth survey stand in comparison?

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As compared to the first serosurvey, when only 0.7 percent seropositivity was found in May at the start of the pandemic, the fourth survey reported the presence of antibodies in 67.6 percent population. The government has attributed the rise in seroprevalence to the role of delta variant in spreading the second wave and to the vaccination cover.

Another important factor is that while in May 2020, 80 to 130 cases per recorded case went undetected, this number had dropped to 27 by December.

The third survey had included children above 10 years while the latest edition also covered those aged six years and above, half of whom have shown past exposure to coronavirus.

According to the government data, the key takeaway from these surveys are:

  • Covid-appropriate behaviour and early vaccination is still the key defence against the pandemic.
  • Non-essential travel should be avoided by the population at large as there is a disparity in seroprevalence in states and the infection can still spread rapidly in previously unaffected parts.
  • The third wave cannot be ruled out especially in the face of a large population still unprotected.
  • States which in the serosurvey reported lower immunity are naturally at a higher risk of future waves.
  • Although the current survey sheds no light on this, there is evidence that reinfections and breakthrough infections are rare but possible.
  • There was no difference between seroprevelance in urban and rural areas, which means that villages and cities are equally at risk of further spread of the virus.
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