The Chinese Center for Disease Control and Prevention has published the most comprehensive epidemiological analysis of the COVID-19 epidemic to date, revealing that less than 5% of infections in the central Hubei province were detected and reported during the early stages of the outbreak.
The study — titled “The Epidemiological Characteristics of an Outbreak of 2019 Novel Coronavirus Diseases (COVID-19)” — analyzed all 72,314 cases reported by Feb. 11, including over 44,000 confirmed cases, 16,000 suspected cases, 10,000 clinically diagnosed cases, and around 900 cases of positive tests without symptoms. The new research was published Monday in the Chinese Journal of Epidemiology (in Chinese) and China CDC Weekly (in English).
According to the National Health Commission’s COVID-19 treatment guidelines, a “confirmed case” requires a positive nucleic acid test, while a “clinically diagnosed case” is a special classification that was added to the fifth edition of the guidelines and applies only to Hubei. This category is defined as a symptomatic patient whose radiographic imaging indicates pneumonia, without a positive nucleic acid test.
Some of the findings from the study are consistent with what is already known about the novel coronavirus. The research team determined that the virus is highly contagious, having spread from a single city to the entire country in just one month, and despite authorities’ extreme containment efforts, including citywide lockdowns. In addition, they found that COVID-19 is not life-threatening for over 80% of patients, and that older patients with preexisting health conditions are more likely to die.
The retrospective study also reveals significant delays by Wuhan and Hubei officials in identifying and reporting cases, especially during the early stages of the outbreak.
For example, the analysis found that over 5,500 of Hubei’s confirmed cases might have fallen ill before Jan. 20. Officially, however, the provincial health commission had reported just 270 confirmed cases by this date, suggesting a diagnosis rate of less than 5%.
The study also said that 105 infections had occurred before Dec. 31 — the date Wuhan’s health commission first acknowledged an “unexplained illness” in just 27 patients.
With the large amount of data, researchers for the first time were able to construct epidemic curves, useful tools in understanding how the virus spread.
According to the curves, the cases from December all seemed to share a common source: the South China Seafood Wholesale Market in Wuhan. But from early January, the virus became a propagated epidemic, meaning the first wave of infected people had gone on to infect others.
A previous epidemiological study also led by the China CDC had analyzed just the first 425 confirmed COVID-19 cases. That research — which suggested that the virus was being passed between people as early as mid-December — was published by the New England Journal of Medicine on Jan. 29, just over a week after Chinese health officials finally admitted the reality of human-to-human transmission on Jan. 20.
The curves also showed that the number of confirmed infections increased dramatically in mid-January and peaked sometime between Jan. 24 and Jan. 28. Since the beginning of this month, the number of new cases identified each day has been steadily decreasing.
However, the authors warned of a possible resurgence in COVID-19 cases as hundreds of millions of people return to work and school in the coming weeks.
“We must remain vigilant, hone our skills, fund our defenses, and practice our responses,” the authors wrote.