Covid-19: Getting testing right vital, do regular sero-surveys

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Published: July 27, 2020 6:15 AM

Can’t have a repeat of the Delhi where, after sero-survey said 24% people are infected, test data showed just 6% daily rate

Ideally, a sero-survey should help the government fine tune its testing strategy.Ideally, a sero-survey should help the government fine tune its testing strategy.

It has to strain credulity that, within a few days of the sero-survey in Delhi reporting that 24% of the capital’s population has been exposed to the novel coronavirus, the city’s daily testing data should indicate that just 6.1% of those tested on July 22 were infected. It is true that not all the 43 lakh persons the sero-survey indicated had been infected will be active cases—many will have recovered—but while the sero-survey indicates a high level of infection in the city, the daily testing data suggests Delhi has turned the corner; keep in mind, a month ago, the daily infection levels in Delhi were just under 20%. Ideally, a sero-survey should help the government fine tune its testing strategy. Since Delhi has a total of 1,27,364 infections while it has done 8.9 lakh tests so far, the case infection rate is around 14.3%; ideally, the daily tests and the sero-survey should both yield roughly similar results.

The reason why case infection rates are falling, and rapidly, is because Delhi’s testing strategy is flawed. While the capital city needed to ramp up its testing levels from the 5,000 or so per day in May, this was done by a rapid increase in rapid antigen tests (RAT) and, as it happens, the gold-standard test—the RT-PCR—has actually fallen. Between June 29 and July 22, where there is data on the types of tests conducted, the share of RT-PCR tests in the total fell from 60% to 26%. The Delhi government does not, unfortunately, give data every day on the infections caught by the two types of tests, but it did give such data to the Delhi High Court on a few occasions. That shows RAT tests yielded a 6.9% rate between June 18 and July 15; that is, 6.9 persons tested positive out of every 100. And, this fell to 4.3% between July 15 and 21. During the same period, however, the RT-PCR tests yielded infection rates of around three times as much. So, if the Delhi government is going to base its testing strategy on RAT rather than RT-PCR, it is consistently going to show lower infection levels.

Ideally, since this problem is well-known, the protocol should be that anyone who tests negative using RAT must be tested using RT-PCR. While the Delhi government says it does this—it reported a 15% infection level among those tested using RT-PCR after being found negative using RAT—this is not done for everyone, but just for those who are symptomatic. This has to be a learning for all states that are ramping up testing using RAT. Indeed, like Delhi, other states should also plan to have a monthly sero-survey to know just how widespread the infection is, so that they can strategise the rollout of their health facilities; indeed, sero-surveys can even help in deciding where to locate those facilities in large states. But, none of this is going to work if the government refuses to tweak the testing strategy to ensure it delivers the correct results. What is especially galling is that everyone knows the RAT has serious limitations, but the government—including the central one—continues to pretend there is no issue at all. Indeed, using RAT only adds to the problem since the infected persons it fails to identify are free to go and infect others; it is in everyone’s interests to get the tests right even though, in the short run, this may end up showing infection levels are either rising or not falling fast enough.

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