https://www.medscape.com/viewarticle/926893
A Million COVID-19 Cases Already, Global Estimates Suggest
Ricki Lewis, PhD
March 16, 2020
Spread of the novel coronavirus SARS-CoV-2 from Hubei Province, China prior to travel restrictions imposed on January 23 likely seeded a million cases worldwide so far, many so mild that people might not have realized they were infected, according to findings published online today in the journal Science.
Researchers discussed the findings of the new study and a previously posted study on the effect of travel restrictions during a news conference today.
In the Science article, Ruiyun Li, PhD, from the MRC Centre for Global Infectious Disease Analysis, Imperial College London, UK, and colleagues used mathematical modeling that combined data on people's movements among 375 Chinese cities with reported infection data, using statistical inference techniques.
The analysis suggests that before travel restrictions were instituted in Wuhan, 86% of cases were undocumented. The model also estimates that these undetected cases were 55% as contagious as were documented infections.
"Six out of seven cases are undocumented. The majority of cases are mild, with few or no symptoms, so most people may think they have a cold or not recognize that they're ill. If globally 150,000 are confirmed, then it's not unreasonable to project that we're approaching close to a million infections globally," said senior author Jeffrey Shaman, PhD, from the Mailman School of Public Health at Columbia University, New York City.
That also means that the 3500 or so documented cases in the United States are likely the tip of an iceberg.
The travel restrictions imposed in late January might have had only a limited effect, said Elizabeth Halloran, MD, DSc, director of the Center for Inference and Dynamics of Infectious Diseases at the Fred Hutchinson Cancer Research Center in Seattle, Washington and a coauthor of the earlier study, posted on medRxiv preprint server. (Articles posted on the medRxiv server, including the one on travel restrictions, have not been peer-reviewed.)
"Travel restrictions alone do not do much to delay the spread of the disease. Reducing transmissibility is really the key," she said.
Other speakers concurred.
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