The share of people who are infected with the coronavirus but never get sick varies widely from place to place, from less than 20% of cruise ship passengers in Japan to a whopping 95% of inmates at an Ohio prison, underscoring the challenge in weeding out infections and isolating the virus as parts of the world reopen.
During the mosquito-borne Zika outbreak in 2015 and 2016, scientists were confident that 75% of those infected would not develop symptoms.
But scientists are having a hard time pinpointing a global average for COVID-19, the disease caused by the new coronavirus, and are finding different rates in different places.
A study in Iceland found that half of those who tested positive for the coronavirus infection showed no signs of illness. Nearly 1 in 5, or 17.9%, of infected passengers on the Diamond Princess cruise ship off Japan were asymptomatic, according to a March study.
The Center for Evidence-Based Medicine at Oxford University said 50% to 70% of people in an Italian village west of Venice were asymptomatic, compared with 31% of Japanese nationals evacuated from Wuhan, China, where the outbreak began in December.
“We’re still in learning mode. The numbers are a bit all over the place,” said Peter J. Hotez, dean of the National School of Tropical Medicine at the Baylor College of Medicine.
The Trump administration said it has reached a point where states can test Americans who have symptoms and come forward, plus the contacts of those people, though it is starting to expand its reach into asymptomatic people. Some states offer drive-thru sites where anyone can be tested.
Collecting more data will help scientists understand the pattern of viral spread and who might have antibodies. Policymakers will be able to use that information to reinforce face mask policies and other safety measures to the public.
“It would give us a better sense of how penetrant the virus is in our communities,” said William Schaffner, an infectious disease specialist at Vanderbilt University. “Because we’ve been limited in our testing, we’ve kind of surmised there is more virus here in Nashville than in rural Tennessee, but we really don’t know that.”
Massachusetts Gov. Charlie Baker, a Republican, said the sneaky virus has made the situation “profoundly difficult” for long-term care providers such as nursing homes, where staffers are necessarily in regular close contact with the elderly.
“If you think back 60 days ago, most people didn’t even appreciate at that point in time that many of the people who carry this virus don’t even have any symptoms,” he said. “And I think in some ways it was perfectly designed to create far more havoc and sorrow in long-term care facilities than in almost any other place you can think of.”
The mere recognition that some proportion of people don’t show symptoms but can spread the virus was a key development in responses to the coronavirus, which has sickened 4.4 million people worldwide since December.
The federal government didn’t urge Americans to wear masks in public until the start of April, when the likelihood of asymptomatic spread became clear.
“One of the [pieces of] information that we have pretty much confirmed now is that a significant number of individuals that are infected actually remain asymptomatic,” Dr. Robert R. Redfield, director of the Centers for Disease Control and Prevention, told WABE 90.1 FM on March 30. “That may be as many as 25%. That’s important, because now you have individuals that may not have any symptoms that can contribute to transmission, and we have learned that in fact they do contribute to transmission.”
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It’s not clear why some people feel no symptoms from COVID-19 and some get sick and die. The reason might be genetics or the amount of virus that the immune system can handle.
“This is why we need good serologic testing in large numbers of representative people,” said Barry R. Bloom, a research professor at Harvard’s T.H. Chan School of Public Health, referring to tests that check for immune responses to COVID-19.
Learning the percentage of people who have antibodies would give researchers information about who had been infected and didn’t know it and might have transmitted the virus to others. The ability to transmit a pathogen without showing symptoms varies by disease.
During the outbreak of Zika, which was spread by mosquitoes, people who were asymptomatic could serve as links in transmission. People without symptoms also could transmit the viral disease sexually.
Also, “a person with Ebola cannot spread the virus while asymptomatic but those with influenza can,” said Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security.
Children rarely become severely ill from the flu but are big transmitters of the virus. Their potential role in transmitting the new coronavirus is one of the key questions about the pandemic.
New York Gov. Andrew Cuomo, a Democrat, said Wednesday that the state Department of Health is investigating more than 100 cases in which children who might have had COVID-19 show symptoms mirroring those in a serious inflammatory condition known as Kawasaki disease.
“Because it happened after the fact and does not present as a normal COVID case, it may not have been initially diagnosed as a COVID case,” he said.