In the fight against the coronavirus pandemic (Covid-19), policymakers are fighting blind, because we do not know the extent of the infections in the population or the mortality rate due to it. It is possible to track the number of deaths, the cases that required hospitalisation and those who have tested positive for the virus at hospitals, but those numbers present a limited picture of the pandemic. Our response to this crisis depends critically on figuring out how fast the disease is spreading outside of hospitals, in the community, and how likely the virus is to kill individuals who have been infected. The only way to obtain this information is through random testing in the population.

Existing methods of measuring the disease’s prevalence are likely to give inaccurate results. India, like most countries, has a shortage of testing kits. As a result, it prioritises the testing of the individuals who have severe symptoms and show up at the hospital. It misses those who have symptoms, but not severe enough to go to the hospital, and those who are asymptomatic — lack of cough, fever or trouble in breathing — but, nonetheless, are carriers who can infect others.

There are good reasons to think that many individuals are asymptomatic, which means that true caseload is under-measured. For example, the city of Vò (Italy) tested all its inhabitants, and found as many as 50% of individuals were asymptomatic. On the Diamond Princess cruise ship, 18% of the infected population showed no symptoms, while in Iceland, 50% of those who tested positive were asymptomatic. A recently released report from Maharashtra suggests that 85% of cases in the state were asymptomatic, similar to the recent findings from China, published in The British Medical Journal. They suggest that we will never be able to measure the true prevalence of Covid-19 by testing only high-risk populations.

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