For every person who tested positive for the coronavirus in China, there were likely another five to eight asymptomatic people who didn't know they had the infection, according to a new study. What's more, these undocumented cases likely infected the majority of known — and thus likely more severe — cases.
As the world continues to battle the coronavirus known as SARS-CoV-2, the case counts continue to tick upward, rapidly filling online virus-spread maps with an unsettling coat of red. But underlying this very visible threat is an entirely different map outlining the movement of the coronavirus's invisible vehicles: people who have very mild or no coronavirus symptoms at all.
To figure out just how many COVID-19 cases went undocumented, a group of researchers from five institutions across the world crunched the early numbers from China, where the outbreak first began.
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The researchers created a mathematical model that analyzed the number of infections in 375 Chinese cities. Their model included travel time and distance traveled for people who participated in the country's Chunyan, or Spring Festival Period — which began Jan. 10 — by analyzing data from 2018. They then simulated observations between Jan. 10 and Jan. 23 from this year before China implemented travel restrictions.
Based on the model, during this time period, China should have had 86% more cases of COVID-19 than what the country had reported. And those undocumented cases were about half as likely as the documented cases to infect another person.
Part of the reason is that people with mild cases or asymptomatic cases likely have lower amounts of virus in their systems that they could shed, co-author Sen Pei, an associate research scientist at Columbia University's Mailman School of Public Health. However another study, though small in size, found that the coronavirus might be most infectious when symptoms are mildest, according to a Live Science report. The other reason is that these people sneeze and cough less than people with more severe diseases, Pei added.
Yet, their modeling showed that because there were so many undocumented cases, these people were the source of infection for 79% of all documented cases before Jan. 23. The virus has the potential to spread through the air and so could be spread just by speaking to an infected person; it can also spread by touching a surface that was contaminated by an infected individual, Pei told Live Science.
That means that even health care professionals who are caring for these patients carry the risk of spreading the coronavirus, especially without having access to proper gear. "Asymptomatic cases, no matter whether they are health care workers or not, can be contagious," Pei said.
These numbers will differ from country to country, as each will have different testing capacities. "Our estimate is specifically for China, so it's not necessarily transferable to the U.S," Pei said. Still, if mild or asymptomatic infections were counted, the mortality rate would be lower, he added.
"There's certainly transmission by less symptomatic people in the U.S., too," said Dr. Robert Horsburgh, a professor of epidemiology and medicine at Boston University School of Public Health and School of Medicine, who wasn't part of the study. "As we get more tests and better tests, I think we'll be able to identify more of those people."
However, the authors "overstated their case somewhat," Horsburgh said. There have been a number of studies that came out analyzing the data from China that have found lower percentages of asymptomatic people, and the reason is likely because this new study lumped together people who didn't get tested as being in the asymptomatic category, he added. But "in the early epidemic in China, just like in the U.S., they didn't have enough tests."
Recently, Dr. Robert Redfield, director of the Centers for Disease Control and Prevention (CDC), told NPR that 1 in 4 people with COVID-19 could be asymptomatic in the U.S. So while the exact numbers in the U.S. aren't yet clear, social distancing "is working, and we need to stick with it," Horsburgh said. Once testing becomes more widespread, "we can switch to the other strategy of isolating the infected people," rather than everybody, he said.
The findings were published March 16 in the journal Science.
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Originally published on Live Science.
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Yasemin is a staff writer at Live Science, covering health, neuroscience and biology. Her work has appeared in Scientific American, Science and the San Jose Mercury News. She has a bachelor's degree in biomedical engineering from the University of Connecticut and a graduate certificate in science communication from the University of California, Santa Cruz.