Kerala with lowest exposure to Covid-19: Sero-survey

Updated on Jul 29, 2021 04:33 AM IST

The overall sero survey results were released last week by the Union government and suggested two out of every three Indians may have been exposed to the virus.

A health worker collects a swab sample of a passenger for a Covid-19 test at a railway station in Kerala. (PTI PHOTO.)(HT_PRINT)
A health worker collects a swab sample of a passenger for a Covid-19 test at a railway station in Kerala. (PTI PHOTO.)(HT_PRINT)
By, New Delhi

Kerala has had the lowest exposure to Sars-Cov-2, according to true infection estimates based on serological studies, new data by the Union government showed on Wednesday, with only 44% of the population projected to have been infected till early July compared to nearly 67% across the country as a whole.

The results, based on the study of how many people had antibodies in a survey involving nearly 29,000 people, appear to explain Kerala’s current high numbers and suggest the state was able to detect true infections better than any other region. The study was carried out by the Indian Council of Medical Research (ICMR).

The overall sero survey results were released last week by the Union government and suggested two out of every three Indians may have been exposed to the virus. If extrapolated, this would mean roughly 900 million people have been infected by the virus; India’s official Covid-19 count on July 10 (when the survey ended) was 30 million.


Including Kerala, the states with the three lowest estimated population exposure were Assam (50%) and Maharashtra (58%). The states with the highest exposure were Madhya Pradesh (79%), Rajasthan (76.2%) and Bihar (75.9%).

“(The findings) reinforces my point that case numbers mean very little, without sero-surveys to inform what fraction is being detected. This data fits better the severity of the second wave ,” said Anurag Agarwal, director of the Institute of Genomics and Integrative Biology (IGIB).

He added that this also means the second wave will “run longer for Kerala and Maharashtra because of less steep rise”.

To be sure, the fourth round of the sero study also included those who received vaccines. While the data released on Wednesday did not give a split between those with natural exposure and those with vaccine-mediated antibodies, the information released last week suggests the latter category accounts for a very small proportion: Of the 67.6% who had antibodies overall, 62% were not vaccinated, officials said at the time.

According to experts, current infection trends and the new data by the Union health ministry offer two important insights: Kerala may have the least amount of under-reporting, and it has the most number of vulnerable people at present.

“India was able to detect only 1 in 33 cases as of May 31st as estimated from the 4th ICMR serosurvey -- Bihar: 1/134; UP; 1/100; MP: 1/86. Detection was much better in Kerala (1/6) and Maharashtra 1/12). Unfortunately, the fingers keep pointing to MH & KL. For detecting cases better?” wrote Rijo M John, a heath economist, in a post on Twitter.

Having antibodies reduces the chances of being infected almost completely, and experts said last week that it could explain why cases have been falling, although they urged caution, pointing to the nearly 400 million people who remain vulnerable to the virus.

Kerala’s outbreak followed a similar trajectory as the nationwide second wave. But since around mid-June, the test positivity rate – a key metric that is often a proxy to estimate the size of an outbreak – levelled out at around the 10-12% from the third week of June. At the same time, this metric fell drastically across the country.

Taken together with the serology estimates of the true spread, this also means that the virus spread more gradually within Kerala, helping flatten the curve of infections, while it spread rapidly in other parts of the country, where testing as well as treatment facilities where overwhelmed.

A second expert said the study findings were not surprising: “Kerala implemented good Covid-appropriate behaviour and strict lockdown since the first wave. The cases went down in places such as Bihar, UP, and Delhi not because people started following Covid-appropriate behaviour but because a significant proportion was already exposed to the infection. What this means is that the second wave will continue in Kerala for a while and high number of new cases will be reported,” said Dr Sanjay Rai, head of the department of community medicine at the All India Institute of Medical Sciences (AIIMS).

“Delhi saw over 28,000 cases a day despite a sero-positivity of over 50% during April and May. Just imagine how many cases Kerala may see with such a high proportion of people being susceptible,” he added.

In the last two weeks, the number of new cases in the state has risen by 28%, which could partly be due to increased testing – over the same duration, the number of daily tests has gone up by an average of 10%, while the positivity rate has risen by 1.8%. It could also be on account of Covid-inappropriate behaviour, with the state government rashly relaxing restrictions on account of the festival of Bakrid.

On Wednesday, the state recorded 22,056 new cases ; on average, the state has added roughly 17,000 new cases over the last seven-day period.

The health ministry said on Wednesday that the national survey by ICMR “was designed to capture the extent of the spread of Covid infection at the national level” and urged states to conduct their own sero-prevalence studies in consultation with the Indian Council of Medical Research.

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  • ABOUT THE AUTHOR

    Anonna Dutt is a health reporter at Hindustan Times. She reports on Delhi government’s health policies, hospitals in Delhi, and health-related feature stories.

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