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How deadly is the coronavirus? Scientists are close to an answer
One of the most crucial questions about an emerging infectious disease such as the new coronavirus is how deadly it is. After months of collecting data, scientists are getting closer to an answer.
Researchers use a metric called infection fatality rate (IFR) to calculate how deadly a new disease is. It is the proportion of infected people who will die as a result, including those who don’t get tested or show symptoms.
“The IFR is one of the important numbers alongside the herd immunity threshold, and has implications for the scale of an epidemic and how seriously we should take a new disease,” says Robert Verity, an epidemiologist at Imperial College London.
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Calculating an accurate IFR is challenging in the midst of any outbreak because it relies on knowing the total number of people infected — not just those who are confirmed through testing. But the fatality rate is especially difficult to pin down for COVID-19, the disease caused by the SARS-CoV-2 virus, says Timothy Russell, a mathematical epidemiologist at the London School of Hygiene and Tropical Medicine. That’s partly because there are many people with mild or no symptoms, whose infection has gone undetected, and also because the time between infection and death can be as long as two months. Many countries are also struggling to count all their virus-related deaths, he says. Death records suggest that some of those are being missed in official counts.
Data from early in the pandemic overestimated how deadly the virus was, and then later analyses underestimated its lethality. Now, numerous studies — using a range of methods — estimate that in many countries some 5 to 10 people will die for every 1,000 people with COVID-19. “The studies I have any faith in are tending to converge around 0.5–1%,” says Russell.
But some researchers say that convergence between studies could just be coincidence. For a true understanding of how deadly the virus is, scientists need to know how readily it kills different groups of people. The risk of dying from COVID-19 can vary considerably depending on age, ethnicity, access to healthcare, socioeconomic status and underlying health conditions. More high-quality surveys of different groups are needed, these researchers say.
IFR is also specific to a population and changes over time as doctors get better at treating the disease, which can further complicate efforts to pin it down.
Getting the number right is important because it helps governments and individuals to determine appropriate responses. “Calculate too low an IFR, and a community could underreact, and be underprepared. Too high, and the overreaction could be at best expensive, and at worst [could] also add harms from the overuse of interventions like lockdowns,” says Hilda Bastian, who studies evidence-based medicine, and is a PhD candidate at Bond University in the Gold Coast, Australia.
Bridging the gap
Some of the first indications of the virus’s deadliness were gleaned from the total number of confirmed cases in China. In late February, the World Health Organization crudely estimated that 38 people had died for every 1,000 with confirmed COVID-19 diagnoses. The death rate among these people — known as the case fatality rate (CFR) — reached as high as 58 out of 1,000 in Wuhan, the city where the virus emerged. But such estimates exaggerated the disease’s deadliness because they did not account for the many people who had the virus but were not tested, obscuring the outbreak’s true spread.
Researchers tried to address this gap by estimating the IFR from models that projected the virus’s spread. The result from these early analyses hovered around 0.9% — 9 deaths for every 1,000 people infected — with a broader range of 0.4–3.6%, says Verity. His own modelling estimated an overall IFR for China of 7 deaths for every 1,000 people infected, increasing to 33 per thousand among those aged 60 or older1.
Sources: China*: T. W. Russell et al. Eurosurveillance 25, 2000256 (2020); China†: R. Verity et al. Lancet 20, 669–677 (2020); France: H. Salje et al. Science https://doi.org/10.1126/science.abc3517 (2020); Brazil: P. Hallal et al. Preprint at medRxiv https://doi.org/10.1101/2020.05.30.20117531 (2020); Spain: Spanish Ministry of Health, Consumer Affairs and Social Welfare 2020 report.
Russell’s team also used data gathered from a large COVID-19 outbreak on the Diamond Princess cruise ship in early February to estimate an IFR in China. Almost all of the 3,711 passengers and crew were tested, enabling researchers to count the total number of infections, including asymptomatic ones, and deaths in a known population. From this, his team estimated an IFR of 0.6%, or 6 deaths for every 1,000 infected people2.
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Nature 582, 467-468 (2020)
doi: https://doi.org/10.1038/d41586-020-01738-2
References
Verity, R. et al. Lancet 20, 669–677 (2020).
Russell, T. W. et al. Euro Surveill. 25, 2000256 (2020).
Streeck, H. et al. Preprint at medRxiv https://doi.org/10.1101/2020.05.04.20090076 (2020).
Hallal, P. C. et al. Preprint at medRxiv https://doi.org/10.1101/2020.05.30.20117531 (2020).
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