Covid-19 sero survey: Over 9 crore infected, tenfold hike in infections among adult Indians in past 3.5 months

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September 30, 2020 8:30 AM

A sero survey is a scientifically chosen random sample — that represents the country’s entire population — whose blood is tested for Covid antibodies; since such antibodies last for a few months after the infection, the survey results give the number of those infected in the last few months.

The second all-India sero survey was done between August 17 and September 22 while the first was done between May 11 and June 4.

There has been a tenfold hike in the number of adult Indians infected with Covid-19 over the past three-and-a-half months (see graphic), from around 64 lakh in the first sero survey, to 6.3 crore in the second one; once you include those in the 10-18 age group, included in the second survey for the first time, the number of infected rises to 9.1 crore, or 6.6% of the above-10 population.

The second all-India sero survey was done between August 17 and September 22 while the first was done between May 11 and June 4.

A sero survey is a scientifically chosen random sample — that represents the country’s entire population — whose blood is tested for Covid antibodies; since such antibodies last for a few months after the infection, the survey results give the number of those infected in the last few months.

Infection levels are much higher in urban slums where 15.6% of those above the age of 10 are infected; the number is 8.2% for non-slums in urban areas and 4.4% for rural areas.

Infection levels are also much higher for select cities. The latest sero survey, in September, for Puducherry showed 22.7% of the population was infected, it was 7.8% for Indore (August), 29.1% for Delhi (August), 21.5% for Chennai (July), 17.6% for Ahmedabad (June-July) and 33% for Mumbai (57.8% for slums and 17.4% for non-slums) based on the survey done in June-July.

While India has the world’s second-highest number of infections based on the daily test data, the sero survey suggests India already has the most cases. On September 22, the day the sero survey ended, the daily test data suggested India had 5.6 million infections versus 6.9 million for the US. Based on the sero survey, that is far more reliable, India has 9 crore infected persons versus 3.1 crore for the US where the latest sero survey indicates 9.3% of the population is infected.

The good news here is that, with a lot more persons infected than the daily tests data show, fatality rates of the disease are far lower than what is commonly believed. Till September 22, India reported a total of 89,000 deaths, so with 5.6 million infections, this meant a case fatality rate of 1.6%. If, however, the 89,000 deaths is divided by the 9+ crore infections that India actually has – based on the sero survey data – the fatality rates are a much lower 0.1%.

Even at these death rates and infection rates, though, India is already experiencing a shortage of ICU beds and ventilators in many cities and also of oxygen as adequate health infrastructure was not built during the lockdown period – or the months after it – in most parts of the country.

While the government is celebrating the fact that India is reporting more ‘recoveries’ every day than the number of fresh infections – this was done at the same press conference where the sero-survey data was presented – a rapid jump in testing, using RT-PCR rather than the less effective RAT tests, will result in a massive jump in infection levels.

On September 22, the daily tests data showed India had 5.6 million infected as compared to the 9.1 crore revealed by the sero-survey; in other words, a ramp-up in testing could see the number of infections rising more than 16 times.

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