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EXPLAINED: ICMR Says 2/3rds Of Indians May've Had Covid. Here's How Sero Surveys Work And What They Mean For Herd Immunity

The 4th ICMR sero survey suggests that more than 67 per cent of Indians have antibodies against the novel coronavirus

The 4th ICMR sero survey suggests that more than 67 per cent of Indians have antibodies against the novel coronavirus

While the majority of Indians may have antibodies against the novel coronavirus, a big chunk of the population remains vulnerable to the disease

Close to 68 per cent of Indians aged over 6 years could have picked up the Covid-19 disease and recovered from it. So says the latest serological survey conducted by the Indian Council of Medical Research (ICMR). These surveys, which track the presence of antibodies against the novel coronavirus among a target population, are seen as a helpful tool in gauging the spread of a disease. It is widely held that the actual number of Covid-19 infections is substantially higher than that captured in tests, so here’s what sero surveys say about the trajectory of the pandemic.

How Many National Surveys Have Been Held In India?

The first nationwide sero survey was conducted in May last year and found that less than 1 per cent of the people tested had antibodies against the novel coronavirus. ICMR’s fourth, and latest, nationwide antibody survey was held between June and July reportedly in the same 70 districts across 21 states in which the first three rounds were conducted. The total sample size for the fourth round was 28,975 people.

The second sero survey, which had commenced in August last year, showed a positivity rate of 6.6 per cent. The third survey, between December 2020 and January this year, found that more than a fifth of the people tested had contracted and recovered from Covid-19.

The third survey had included children above 10 years while the latest edition also covered those aged 6 years and above.

Why Are Sero Surveys Held?

The key implications of a sero survey for public health experts lie in the status of the spread of an infection among a particular sample group. According to ICMR, serosurveys can provide data on “the proportion of population exposed to the novel coronavirus, including asymptomatic individuals”.

But that is not the only takeaway. Surveys among specific groups, for instance, high-risk or vulnerable populations like health and frontline workers, immunocompromised individuals or those in containment zones can also be used to assess who all may be more vulnerable compared with the others. That can allow health authorities to plan pointed interventions that are tailored to a specific group, or area’s, health needs.

Soumya Swaminathan, the WHO Chief Scientist, adds that sero surveys also provide a chance to track how long immunity lasts after an individual has recovered from Covid-19. Sero surveys can be used to “test the same group of people over a period of time to understand more about the immunity against this virus”.

Importantly, sero surveys also tell us who has not been infected with the novel coronavirus, which is to say that health authorities can track what proportion of the population is still vulnerable to getting infected and how far a particular location is from achieving herd immunity.

What Is Herd Immunity? Has India Reached It?

When enough people in a given population have developed antibodies to an infection — either by contracting the disease and recovering from it or through vaccination — it can be assumed that the disease will be halted in its track as it will become difficult for it to find new people to infect. This is known as herd immunity, that is, when exposure in a sufficient section of the population ensures that the remaining people are protected.

This proportion varies for different diseases. For instance, for a highly contagious disease like measles, the threshold for herd immunity is a high 94 per cent. Which means that more than nine out of 10 people should have either recovered from the disease or been vaccinated against it to ensure there are no further cases. But since vaccination for measles and other diseases is common, we don’t hear of big outbreaks of the disease even though stray cases keep getting reported.

For the novel coronavirus, herd immunity is estimated to lie at around the 70-80 per cent level. That is, about four out of every five people should have got antibodies to the infection to arrest its spread. However, experts point out that it is not as simple as that for a new infection that humanity has never encountered before, which is what the novel coronavirus is.

The rise of new strains that can cause reinfections can make herd immunity a non-starter. Also, the ability of any strains to beat vaccine antibodies may pose a challenge to achieving herd immunity.

“Two-thirds of the general population that is above the age of six years had SARS-CoV-2 infection. More importantly, a third of the population did not have any antibodies… 40 crore population of this country is still vulnerable,” ICMR Director General Dr Balram Bhargava said while announcing the fourth sero survey results. While it means that an even larger proportion of Indians may now be said to have antibodies against the disease, the need to keep our guards up is still very much there.

“The implications of this large sero survey clearly show that there is a ray of hope. But there is no room for complacency. We must maintain Covid-appropriate behaviour,” Bhargava added.

So, What Happens In A Sero Survey?

According to ICMR, the antibodies tracked in a serosurvey, called IgG antibodies, typically start appearing “after two weeks of onset of infection, once the individual has recovered after infection and last for several months”. So, an antibody test is not useful for detecting infection, which is mainly done in India via the RT-PCR or rapid antigen test (RAT), for which an oral and nasal swab sample is collected by health workers.

The serological test, on the other hand, uses a blood sample to test for antibodies. Further, RT-PCR and RAT look for the presence of the actual virus whereas the antibody test checks for antibodies in the blood.

According to the US drugs regulator, “the performance of these tests is described by their ‘sensitivity’,or their ability to identify those with antibodies to SARS-CoV-2 (true positive rate), and their ‘specificity’, or their ability to identify those without antibodies to SARS-CoV-2 (true negative rate)”.

The result can either be read as ‘reactive’, that is, IgG antibodies were detected, or ‘non-reactive’, which means antibodies were not found.

What Are The Antibodies That The Test Is Looking For?

The Ig in IgG stands for immunoglobulin, which are a class of proteins that function as antibodies and can be found in the blood and the immune system. Thus, the serosurveys being done in India are looking for the immunoglobulin G. Experts say that about “70-80% of the immunoglobulins in the blood are IgG”.

The body can produce specific IgG antibodies during an initial infection, which form “the basis of long-term protection against microorganisms”.

Reports say the fourth sero survey found that antibodies were present in 62.3 per cent of those who had not been vaccinated while for those with one dose, it was 81 per cent. Antibody prevalence was 89.8 per cent for people who had received both shots.

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