Scientists on Wednesday provided more evidence that coronavirus is spread by apparently healthy people who have no clear symptoms, and the U.S. government has released new guidelines warning that anyone exposed to the disease can be considered a carrier.
A study by researchers in Singapore has become the latest to estimate that around 10% of new infections could be caused by people with the virus but who have not yet suffered from flu-like symptoms.
In response to this and other studies, the Centers for Disease Control and Prevention have changed the way they define the risk of infection for Americans. The agency’s new guidelines have targeted people who have no symptoms but who have been exposed to other people with or suspected of having infections. It basically says that anyone can be a carrier, whether or not they have symptoms.
The results complicate efforts to contain the pandemic and reinforce the importance of social isolation and other measures to stem the spread, experts said.
“You really have to be proactive to reduce contact between people who appear to be in perfect health,” said Lauren Ancel Meyers, a researcher at the University of Texas at Austin, who has studied the transmission of coronaviruses in different countries.
The latest research was published online by the CDC. It focused on 243 cases of coronavirus reported in Singapore from mid-January to mid-March, including 157 infections in people who have not traveled recently. Scientists have discovered that so-called pre-symptomatic people trigger infections in seven different disease groups, accounting for around 6% of locally acquired cases.
One of these infections was particularly striking. The infection of a 52-year-old woman was related to the fact that she was sitting on a seat in a church which had been occupied earlier in the day by two tourists who showed no symptoms but fell ill later, Investigators said after examining closed circuit camera recordings of religious services.
A previous study in China, where the virus was first identified, suggested that more than 10% of the transmissions came from people who were infected but did not yet feel sick.
It is believed that apparently healthy people who can transmit the virus fall into three categories: pre-symptomatic, who have no symptoms when they spread but develop disease a few days later; asymptomatic, which never develops symptoms; and post-symptomatic, who fall ill and recover but remain contagious. Studies in Singapore and China have focused on pre-symptomatic infections.
It is not known how many new infections are caused by each type of potential diffuser, said Meyers, who was not involved in the Singapore study but was part of the previous one on China.
CDC officials say they have researched asymptomatic and pre-symptomatic infections, but the studies are not finished.
In an interview with a radio station in Atlanta on Tuesday, CDC director Dr Robert Redfield cited an estimate that 25% of those infected may be asymptomatic. It was unclear what this estimate was based on, or whether it included pre-symptomatic or post-symptomatic people. The PA requested more information from the CDC, but the agency did not provide these details.
Redfield’s comment was in response to a question about whether the agency will recommend that people who appear to be in good health wear face masks or covers when they go out. He said the agency is reviewing its guidelines, examining research in Singapore, China and elsewhere to make the decision.
California Governor Gavin Newsom this week announced plans to announce new national wearing guidelines.
Wearing scarves or bandanas over your nose and mouth “is not necessarily going to protect you, but if you are a carrier of the disease, it can reduce the amount you transmit,” said Carl Bergstrom, evolutionary biologist at the University of Washington which studies emerging infectious diseases.
In the early months of the pandemic, health officials based their belief that most of the spread came from people who sneezed or coughed droplets containing the virus.
Another type of coronavirus caused fatal acute severe respiratory syndrome, or SARS, which was first identified in Asia in 2003 and caused a frightening but short-lived international epidemic that has never spread as widely as the new virus.
Although some asymptomatic infections have been discovered, none have been shown to have spread the disease. Because the symptomatic people were the spreaders, health officials could focus on them to see an epidemic occur and could better isolate the infected and stop the spread.
“It was much, much easier” to contain, said Bergstrom. With the new coronavirus, “we clearly have asymptomatic and pre-symptomatic transmission,” he added.
The Associated Press’s Department of Health and Science is supported by the Department of Science Education at the Howard Hughes Medical Institute. The AP is solely responsible for all content.