GTA5

Test everyone once a week

The current Covid-19 situation in India is disastrous, with thousands of deaths reported per day despite recent reductions in case numbers. We believe there’s a way out that the Indian government has thus far not made use of – universal weekly testing of asymptomatic individuals using at-home rapid antigen tests.

Self-administered rapid-antigen tests are used around the world, notably in great quantities in the UK and the US, and are based on the same lateral flow principle as pregnancy tests; they are paper strip-based, and directly detect the proteins of the Covid-19 virus. To use them, one swabs the nose with a provided cotton swab, which can be a shallow Anterior Nares (AN) swab from the front part of the interior of the nose, as opposed to the deep and painful Nasopharyngeal (NP) swab that is currently standard in India. One then mixes the swab into a small tube pre-filled with a buffer solution, and drips the mixture onto the paper strip. If one line appears after 15-30 minutes, the user is negative for Covid-19, and if two lines appear, the user is positive for Covid-19.

These tests are fast, easy to use, and new evidence suggests that they are very accurate (80%+ sensitivity) at detecting the presence of Covid-19 in people who are actively spreading the virus (which corresponds to detectability by RT-PCR with a cycle threshold – CT value – of 30 or less). Sensitivity approaches 100% when people are near their peak viral loads – the time when they are most likely to transmit or be superspreaders. These tests can be manufactured in India using primarily domestic raw materials for as little as Rs. 50 a test, and potentially less at greater scale.

ICMR recently approved one at-home rapid antigen test, but their advisory suggests that it should only be used for individuals showing Covid-19 symptoms or close contacts of such symptomatic individuals, and caution that it should not be applied ‘indiscriminately’. But this misses the most important feature of how rapid tests can be used – for frequent screening of asymptomatic people to help curb spread. We recommend that the Government of India deploy these tests in major metropolitan areas such as Mumbai, Delhi, Pune, Bangalore, and Ahmedabad, as well as emerging hotspots across rural India, to test every individual in those areas at least once per week. If you test a large fraction of the population of an area with these tests regularly, asymptomatic and presymptomatic people who have Covid-19 but would be otherwise unaware can self-isolate instead of spreading the disease, dramatically reducing the incidence of new cases. This would not, however, be possible if each test were required to be conducted by a healthcare worker: there will never be enough trained personnel to carry out the ~100 million tests a day that testing a significant fraction of India each week would require.

Large-scale trials of population-scale rapid testing have shown it to be a remarkably fast and effective tool for curbing Covid-19 spread, including in Slovakia, where testing the whole country using rapid antigen tests cut new cases by an estimated 70% in just three weeks (peer-reviewed publication in Science). The UK government is also pursuing this strategy by providing everyone in the country with a free at-home rapid antigen test kit every week. The US National Institutes of Health is now also pursuing this as a strategy across multiple US cities.

There has been a general reluctance to allow largescale rapid antigen testing due to concerns about the false negative rate of these tests – while quite sensitive to detect individuals who are actively spreading the virus, they are still not as sensitive as RT-PCR tests overall. For public health however, this is a good thing. PCR detects people during the initial span of ~7 days (in most cases) that they are infectious but also during the following 20 days that they are no longer infectious. Detecting people and isolating them after they are no longer infectious is not good for society, potentially taking up precious medical resources and isolation capacity. Unlike PCR, rapid antigen tests are highly specific to the period of time when people are transmitting but quickly turn negative afterwards, catching infections in the act of transmitting and preventing unnecessary isolations.

Epidemiological models from a number of different public health researchers and economists at Harvard and elsewhere demonstrate, however, that even a relatively insensitive rapid antigen test with a false negative rate against PCR for the detection of any RNA of as much as 60% would, if applied once a week universally, make a sufficiently large dent in the spread of Covid-19 to cause outbreaks to collapse. This is the case because, in effect, most Covid-19 cases in India today are false negatives – most people who have the disease don’t know that they do because they have no symptoms and are never tested. Adding to the percentage of Covid-19 carriers who know they have it thus produces an improved situation even if the false negative rate of the tests is high.

You might ask why this is necessary if we have vaccines. Vaccination is the long-term solution to Covid-19, but we won’t have the manufacturing capacity in India to achieve immunity via vaccination for at least several months, and potentially longer – more than four months after the start of vaccination in India, only about 4% of the country’s adult population has been fully vaccinated. We can’t afford to wait for vaccination to catch up; every day we do, thousands of people die.

Population-scale rapid testing of asymptomatics is an extremely effective public health intervention which has the potential to save hundreds of thousands of lives now but has so far been ignored in India. We hope that spreading this message far and wide will motivate the national government to adopt this as an important part of their pandemic response toolkit.



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Views expressed above are the author’s own.



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