(CNN) – As the death toll mounts from the novel coronavirus, questions are arising about whether U.S. officials might have been wrong about how it spreads.
On March 1, Secretary of Health and Human Services Alex Azar said on ABC’s “This Week” that it’s mainly spread by people who are already sick.
"You really need to just focus on the individuals that are symptomatic,” Azar said. “It really does depend on symptomatic presentation.”
Regarding whether someone can spread the virus without being sick, the Centers for Disease Control and Prevention writes, in part: “Some spread might be possible before people show symptoms; there have been reports of this occurring with this new coronavirus, but this is not thought to be the main way the virus spreads.”
But in Massachusetts, more than 80 people contracted coronavirus at a conference held by the biotech company Biogen.
The state Department of Public Health told CNN that none of the people who attended displayed any symptoms during the conference.
"We now have conclusive evidence that this disease is also being transmitted through asymptomatic carriers, or people who show no symptoms, and trying to stop that transmission is like trying to stop the wind," said Dr. Michael Osterholm, an epidemiologist and director of the Center of Infectious Disease Research and Policy at the University of Minnesota.
A study, which has yet to be peer reviewed, found that in Singapore and in the Chinese city of Tianjin, infection was transmitted about two to three days before symptom onset.
That makes the outbreak much more difficult to control.
"Honestly, with this kind of transmission, we're never going to stop it,” Osterholm said. “What we best are able to do is slow it down."
None of the experts CNN spoke to could say what percentage of people are getting infected with the virus by people who don’t have symptoms.
CNN reached out to Azar and the CDC but did not receive responses.
On Saturday, Dr. Deborah Birx, the coordinator of the White House coronavirus response task force, suggested that people under age 20 could possibly be spreading the virus without having any symptoms.
"And until you really understand how many people are asymptomatic and asymptomatically passing the virus on, we think it's better for the entire American public to know that the risk of serious illness may be low, but they could be potentially spreading the virus to others," Birx said.
Some experts said the comment was a step in the right direction.
"This is a time for straight talk,” Osterholm said. “This is a time when we have to tell the public what we know and don't know."