Coronavirus study points to vast number of cases under the radar in China
- Researchers in Hong Kong find that Covid-19 definitions make big differences to the pandemic’s bigger picture
- Mainland China might have had four times as many infections as official total if broader criteria used, team says
In a study published in the medical journal The Lancet on Tuesday, the researchers said China might have had 232,000 confirmed cases – rather than the official total of about 55,000 – by February 20 if a revised definition adopted earlier in the month had been applied throughout.
“We estimated that there were at least 232,000 infections in the first epidemic wave of Covid-19 in mainland China,” they said, referring to the disease caused by the coronavirus.
“The true number of infections could still be higher than that currently estimated considering the possibility of under-detection of some infections, particularly those that were mild and asymptomatic, even under the broadest case definitions.”
Unconfirmed cases may be behind rapid virus spread in China, researchers say
The researchers – led by Peng Wu from the University of Hong Kong’s school of public health – looked at the various classification systems used by the government after the epidemic erupted in the central Chinese city of Wuhan in late December.
China has published seven editions of diagnosis and treatment guidelines, changing the classification system as understanding of the disease developed.
The Hong Kong team found that different definitions made a big difference to the number of cases.
“We estimated that when the case definitions were changed from version 1 to 2, version 2 to 4, and version 4 to 5, the proportion of infections being identified as Covid-19 cases was increased by 7.1 times from version 1 to 2, 2.8 times from version 2 to 4, and 4.2 times from version 4 to 5,” the paper, co-authored by Peng’s HKU colleagues epidemiologist Benjamin Cowling and medical faculty dean Gabriel Leung, said.
The calculations were based on epidemiological curves included in a World Health Organisation report published on February 28 after a joint mission to Wuhan. The curves show the number of cases based on symptom onset dates and reporting dates.
The inclusion of these cases led to a sudden jump in confirmed cases before the authorities reversed the decision more than a week later on February 17.
The HKU team found that if the fifth definition were used throughout the epidemic, the total number of confirmed cases could have reached 232,000 by February 20, instead of the 55,508 confirmed cases throughout the country. The official tally in the HKU paper is slightly higher than the National Health Commission’s 54,965 due to reporting technicalities.
The impact of later versions of diagnosis guidelines cannot be gauged because no data based on symptom onset dated after February 20 has been released yet.
The team found that if the revisions of case classifications were taken into account, the original growth rate estimate would be an overestimate, and epidemiological studies should take into account changes in case definitions.
It also suggested that countries that did not have enough test kits for the disease should include clinical diagnosis in their classification guidelines to better gauge the number of infections and determine the public health response.
