New study casts more doubt on true scale of China’s coronavirus outbreak

April 24, 2020, 8:15 AM UTC

Coronavirus cases in China may have been four times higher than officially reported numbers, according to a study published in the Lancet medical journal.

Infections would have been 232,000 in China as of Feb. 20 if the calculation of cases confirmed by “clinical diagnosis” had been applied throughout the outbreak, the Lancet reported. That compares to the 55,508 cases announced at the time by the country’s National Health Commission, according to the report.

Measurement of infections can change significantly as a virus spreads, more information becomes available and testing evolves and increases. The study’s authors said seven versions of the coronavirus case definition were used by the NHC in China from Jan. 15 to March 3 and changes should be considered when estimating growth rates.

change of methodology that included cases diagnosed with CT imaging scans, alongside the previous method of nucleic acid testing kits, led to the addition of nearly 15,000 virus cases in a single day. Previously, patients with pneumonia-like symptoms found only via CT scans weren’t confirmed as positive cases without the other test, which was short of supply back then.

“The case definition was initially narrow and was gradually broadened to allow detection of more cases as knowledge increased, particularly milder cases and those without epidemiological links to Wuhan, China,” according to the study published April 21 and led by Tim K Tsang from the Hong Kong University School of Public Health.

While the study doesn’t project the current total of cases using the different methodology, it casts further doubt on the true scale of the outbreak in China, where data and death tolls have been repeatedly revised in the past three months. There have been 83,884 infections and 4,636 deaths in China, according to Johns Hopkins University data.

With the virus infecting more than 2.7 million people globally, and killing 190,000, countries are having issues working out accurate numbers of cases, especially when it comes to asymptomatic infections.

In China, serological surveys have been started to detect how many people overall have been infected, while the government urged mass testing of both the virus and its antibodies nationwide.

Amid accusations it’s under-reported numbers and concealed the true extent of its outbreak, China added confirmed infections and an extra 1,290 fatalities to its tally last week. The revision was aimed at including cases where people died without being treated or tested and from overwhelmed health institutions that failed to report them in time.

The Lancet article says among the 232,000 estimated infections nationwide by Feb. 20, 127,000 cases would have been from the city of Wuhan where the pathogen was first identified in December. This includes some 11,000 infections that met the fifth version of case definition with illness onset by the start of the year.

“Estimates of key epidemiological parameters using epidemic curves could be biased if they do not account for such changes in case definitions,” Tsang and other scientists wrote.

They added that when the case definition changed, there could be a backfill of cases that met the new definition around the change time.

Still, the findings suggest the lockdown measures in Hubei province turned out to be very effective in curbing growth as Wuhan implemented unprecedented control measures on Jan. 23. Before the lockdown, 92% of cases were undetected, the study said.

More coronavirus coverage from Fortune:

Bill Gates explains how to beat the coronavirus pandemic
—Elective surgeries—mostly paused due to the pandemic—are critical to hospital finances
—World Health Organization says 3 more coronavirus vaccines are in human trials
—Late payments soar, revealing extent of coronavirus pain on European companies
14% of women considered quitting their jobs because of the coronavirus pandemic
—Forget “wet markets” and bats: For scientists, failing environmental policies have created a boom time for outbreaks
—Out of work, but not unemployed: How much Europe is paying its idled workers
—PODCAST: COVID-19 might have upended the concept of the best companies of the year
—VIDEO: 401(k) withdrawal penalties waived for anyone hurt by COVID-19

Subscribe to Fortune’s forthcoming Eastworld newsletter for expert insight on what’s dominating business in Asia.

Subscribe to Well Adjusted, our newsletter full of simple strategies to work smarter and live better, from the Fortune Well team. Sign up today.