Scientists tracking the spread of the coronavirus reported on Monday that, for every confirmed case, there are most likely another five to 10 people in the community with undetected infections. These often-milder cases are, on average, about half as infectious as confirmed ones, but are responsible for nearly 80 percent of new cases, according to the report, which was based on data from China.The researchers modeled the virus’s natural spread in China before the government instituted a travel ban and an aggressive testing policy. During that time, from December of last year through late January, about 6 in 7 cases went undetected. That situation is analogous to the current state of affairs in the United States and other Western countries, where tests are not widely available, the researchers said.“If we have 3,500 confirmed cases in the U.S., you might be looking at 35,000 in reality,” said Jeffrey Shaman, an epidemiologist at Columbia University and the senior author of the new report, which was posted by the journal Science.The report is among the first to address two of the most pressing questions about the pandemic: How many people are walking around with unrecognized infections, and how infectious are they? As American policymakers have begun taking more aggressive measures to slow transmission, such as canceling events and closing restaurants, access to tests for the virus has been difficult or nonexistent in much of the country.In a conference call with reporters on Monday, scientists representing more than a dozen institutions tracking the disease stressed the importance of increased testing.“It is crucial to implement wide-scale testing,” said Dr. Elizabeth Halloran, a professor of biostatistics at the University of Washington and a senior researcher at the Fred Hutchinson Cancer Research Center. “And it’s important to develop inexpensive tests so people can get tested whenever they need to be.”The new analysis drew on documented infections in China and mobility data, as well as a model of social interaction across the population, to estimate the numbers of undocumented cases, as well as infection rates. It found that, after the Chinese government locked down the center of the outbreak on Jan. 23 and began widespread testing, the picture changed drastically. In time, testing identified some 60 percent of positive cases, up from 14 percent.The paper included authors from five institutions, including Imperial College London, Tsinghua University in Beijing and the University of Hong Kong.The scientists said the number of undetected cases for every confirmed one could vary twofold from country to country. The unidentified cases in China that proliferated before the lockdown, although less contagious on average than the confirmed ones, did not necessarily cause milder cases in the newly infected, the researchers said.“Just because you get the disease from someone with mild symptoms does not mean yours are going to be mild,” Dr. Shaman said. “You could still end up in the I.C.U.”
Updated March 16, 2020
What is a coronavirus? It is a novel virus named for the crownlike spikes that protrude from its surface. The coronavirus can infect both animals and people and can cause a range of respiratory illnesses from the common cold to lung lesions and pneumonia.
How contagious is the virus? It seems to spread very easily from person to person, especially in homes, hospitals and other confined spaces. The pathogen can travel through the air, enveloped in tiny respiratory droplets that are produced when a sick person breathes, talks, coughs or sneezes.
Where has the virus spread? The virus, which originated in Wuhan, China, has sickened more than 170,400 in at least 140 countries and more than 6,600 have died. The spread has slowed in China but is gaining speed in Europe and the United States. World Health Organization officials said the outbreak qualifies as a pandemic.
What symptoms should I look out for? Symptoms, which can take between two to 14 days to appear, include fever, a dry cough, fatigue and difficulty breathing or shortness of breath. Milder cases may resemble the flu or a bad cold, but people may be able to pass on the virus even before they develop symptoms.
How do I keep myself and others safe?Washing your hands frequently is the most important thing you can do, along with staying at home when you’re sick and avoiding touching your face.
How can I prepare for a possible outbreak?Keep a 30-day supply of essential medicines. Get a flu shot. Have essential household items on hand. Have a support system in place for elderly family members.
What if I’m traveling? The State Department has issued a global Level 3 health advisory telling United States citizens to “reconsider travel” to all countries because of the worldwide effects of the coronavirus. This is the department’s second-highest advisory.
How long will it take to develop a treatment or vaccine? Several drugs are being tested, and some initial findings are expected soon. A vaccine to stop the spread is still at least a year away.
