UW Researcher: COVID-19's Infection-Fatality Rate 10 Times Larger Than Flu's

May 08, 2020

The infection-fatality rate for symptomatic COVID-19 cases is 1.3%, which is more than ten times the rate for seasonal flu, according to calculations by a University of Washington researcher that were published in an ahead-of-print article in Health Affairs this week.

Assuming that 35.5 million Americans contract COVID-19, an assumption that is based on the number that contract seasonal flu, the researcher, Anirban Basu, says the 1.3% infection-fatality rate would mean nearly 500,000 COVID-19-related deaths in the U.S. this year.

“To the extent that COVID-19 is more infectious than flu and does not have the protection from a vaccine or treatment, the number of deaths would be higher,” writes Basu, although he adds that “certainly with mitigation strategies, the death toll will be lower.”

The infection-fatality rate is the ratio of deaths caused by COVID-19 (the numerator)  to the total number of people in the population “who are genuinely infected by the virus,” (the denominator), explains Basu, Ph.D., M.S., director of the Comparative Health Outcomes, Policy, and Economics (CHOICE) Institute at the University of Washington in Seattle.

The case-fatality rate is talked about more often, but Basu says it “provides a biased estimate of the IFR [infection-fatality rate].” His modeling (it’s based on some assumptions and so a model) to arrive at an infection-fatality rate makes adjustments to account for the improving accuracy in both the tally of the deaths in the numerator and the number of people infected in the denominator.

Basu fed data in Github from Johns Hopkins and the New York Times into this model. make his calculations. After various exclusions and winnowings for statistical reasons, Basu wound up with a sample of 116 counties and 40,835 confirmed cases of COVID-19 and 1,620 confirmed deaths.

Basu notes that a “true IFR (infection fatality rate)” would also include people who recover from an infection without any symptoms, thereby increasing the count of cases that go into denominator of the ratio. But those asymptomatic cases do not “contribute” to the current statistics on COVID-10, so he qualifies his infection fatality rate as “IFR-S,” which stands for infection-fatality rate, symptomatic. 

How many people with COVID-19 don’t have symptoms or only very mild ones is an open question. Basu says that serological testing of the passengers of the Diamond Princess cruise ship showed 17.9% of infected people never developed symptoms. If that’s true for the entire population, then his infection fatality rate would need to be adjusted downward by about 20%, he says (from 1.3% to just a little over 1%). Even with that adjustment the infection-fatality rate for COVID-19 would be 10 times greater than rate for flu, which according to Basu is 0.1%.