Data from antibody tests in 10 different cities and states indicate that many people with no symptoms may be spreading the virus.
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The number of people infected with the coronavirus in different parts of the United States was anywhere from two to 13 times higher than the reported rates for those regions, according to data released Tuesday by the Centers for Disease Control and Prevention.
The findings suggest that large numbers of people who did not have symptoms or did not seek medical care may have kept the virus circulating in their communities.
The study indicates that even the hardest-hit area in the study — New York City, where nearly one in four people has been exposed to the virus — is nowhere near achieving herd immunity, the level of exposure at which the virus would stop spreading in a particular city or region. Experts believe 60 percent of people in an area would need to have been exposed to the coronavirus to reach herd immunity.
The analysis, based on antibody tests, is the largest of its kind to date; a study of a subset of cities and states was released last month.
About 40 percent of infected people do not develop symptoms, but they may still pass the virus on to others. The United States now tests roughly 700,000 people a day. The new results highlight the need for much more testing to detect infection levels and contain the viral spread in various parts of the country.
For example, in Missouri, the prevalence of infections is 13 times the reported rate, suggesting that the state missed most people with the virus who may have contributed to its outsized outbreak.
Dr. Havers emphasized that even those who do not know their infection status should wear cloth face coverings, practice social distancing and wash their hands frequently.
The results indicate that in vast swaths of the country, the coronavirus still has touched only a small fraction of the population. In Utah, for example, just over one percent of people had been exposed to the virus by early June. The rate was 2.2 percent for Minneapolis-St. Paul as of the first week of June, 3.6 percent for the Philadelphia metropolitan region as of May 30 and 1 percent for the San Francisco Bay Area as of April 30.
In some regions, the gap between estimated infections and reported cases decreased as testing capacity and reporting improved. New York City, for example, showed a 12-fold difference between actual infections and the reported rate in early April, and a 10-fold difference in early May.
“This is not coming as a shock or surprise to epidemiologists,” Carl Bergstrom, an infectious diseases expert at the University of Washington in Seattle, said in an email. “All along, we have expected that only about 10 percent of the cases will be reported.”
Tracking the numbers over time can provide useful insights into the virus’ spread and about a region’s capacity to cope with the epidemic, other experts said.
“The fact that they’re sort of marking it out over time and looking at it over a longer duration will actually be super-informative,” said Dr. Rochelle Walensky, a researcher at Harvard University who wrote an editorial accompanying the JAMA paper.
For example, South Florida ticked up to 2.9 percent as of April 24 from 1.9 percent just two weeks earlier. Missouri’s numbers barely budged from 2.7 percent as of April 26 to 2.8 percent as of May 30. Numbers for both regions are likely to be much higher in the next round of analyses because of the surge of infections in those regions since those dates.
New York City showed the biggest leap in its rate, from 6.9 percent as of April 1 to 23.3 percent as of May 6, consistent with its outbreak.
The city’s estimate closely matches the 22.7 percent prevalence found by a state survey, which tested patrons in supermarkets from April 19-28.
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Frequently Asked Questions
Updated July 27, 2020
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- It could be a good idea, because mortgage rates have never been lower. Refinancing requests have pushed mortgage applications to some of the highest levels since 2008, so be prepared to get in line. But defaults are also up, so if you’re thinking about buying a home, be aware that some lenders have tightened their standards.
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- The coronavirus can stay aloft for hours in tiny droplets in stagnant air, infecting people as they inhale, mounting scientific evidence suggests. This risk is highest in crowded indoor spaces with poor ventilation, and may help explain super-spreading events reported in meatpacking plants, churches and restaurants. It’s unclear how often the virus is spread via these tiny droplets, or aerosols, compared with larger droplets that are expelled when a sick person coughs or sneezes, or transmitted through contact with contaminated surfaces, said Linsey Marr, an aerosol expert at Virginia Tech. Aerosols are released even when a person without symptoms exhales, talks or sings, according to Dr. Marr and more than 200 other experts, who have outlined the evidence in an open letter to the World Health Organization.
What are the symptoms of coronavirus?
- Common symptoms include fever, a dry cough, fatigue and difficulty breathing or shortness of breath. Some of these symptoms overlap with those of the flu, making detection difficult, but runny noses and stuffy sinuses are less common. The C.D.C. has also added chills, muscle pain, sore throat, headache and a new loss of the sense of taste or smell as symptoms to look out for. Most people fall ill five to seven days after exposure, but symptoms may appear in as few as two days or as many as 14 days.
Does asymptomatic transmission of Covid-19 happen?
- So far, the evidence seems to show it does. A widely cited paper published in April suggests that people are most infectious about two days before the onset of coronavirus symptoms and estimated that 44 percent of new infections were a result of transmission from people who were not yet showing symptoms. Recently, a top expert at the World Health Organization stated that transmission of the coronavirus by people who did not have symptoms was “very rare,” but she later walked back that statement.
Some experts criticized the state survey at the time because people shopping during the lockdown were more likely to be young, or might have recovered from illness and felt safe.
“These consistent results offer mutual support to two very different methods used,” Eli Rosenberg, an epidemiologist at the State University of New York at Albany and lead author of the state study.
The C.D.C. study also has limitations, Dr. Walensky said, because many of the people who ventured out during the lockdowns for tests or were hospitalized would have been severely ill, and might not have been representative of the general population.
Each region also varied “in terms of where they were on their own epidemic curve and varied in terms of the amount of testing that they did,” she said.
The study also did not collect data on race, ethnicity, diagnostic and symptom history or prevention behaviors, Dr. Rosenberg said. “The approach used in the grocery store study allows for these data collections by pairing the specimen collection with a survey,” he said.
Still, experts said the findings were valuable, despite limitations.
“This population may not be exactly representative of the population as a whole, but the hope is that it is close enough to allow us to draw meaningful conclusions,” Dr. Bergstrom said.
Several recent studies have suggested that antibody levels, especially in people with mild or no symptoms, may quickly decline. If that’s true, surveys like the C.D.C.’s might reflect only people who were infected within the previous two to three months, Dr. Rosenberg said, “and complicate interpretation of results over time.”
An earlier version of this article misstated the journal in which the new study was published. It is JAMA Internal Medicine, not JAMA.
Apoorva Mandavilli is a reporter for The Times, focusing on science and global health. She is the 2019 winner of the Victor Cohn Prize for Excellence in Medical Science Reporting. @apoorva_nyc
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