Researchers have emphasised that the coronavirus is spread mainly by people who are already showing symptoms like fever, cough or difficulty in breathing.
However,a Massachusetts coronavirus cluster with at least 82 cases was started by people who were not yet showing symptoms, and more than half a dozen studies have shown that people without symptoms are causing substantial amounts of infection, reports the CNN.
US federal officials have warned for weeks that asymptomatic transmission can happen. However, they added that it is not a significant factor in the spread of the pandemic.
Speaking on a television show on March 1, US Secretary of Health and Human Services Alex Azar said that asymptomatic spread is "not the major driver" of the spread of the new coronavirus.
"You really need to just focus on the individuals that are symptomatic. It really does depend on symptomatic presentation."
US Centers for Disease Control and Prevention has been assertive of that assessment.
"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," says the US Centers for Disease Control and Prevention website.
The White House Coronavirus Response Coordinator Deborah Birx, during a press briefing on Saturday, expressed a different opinion about asymptomatic transmission.
According to Deborah Birx, the response unit is trying to understand people under the age of 20 who are not showing "significant symptoms."
"Are they a group that is potentially asymptomatic and spreading the virus?" she asked.
Brix warned: "Until you really understand how many people are asymptomatic and asymptomatically passing the virus on, we think it is 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. That is why we are asking every American to take personal responsibility to prevent that spread."
Significance of asymptomatic transmission
The exact percentage of the transmission in the outbreak that is fueled by people who are obviously sick versus those who have no symptoms or very mild symptoms remains unclear.
However, it has become clear that transmission by people who are asymptomatic or mildly symptomatic is responsible for more transmission than previously thought.
Director of the Center for Infectious Disease Research and Policy at the University of Minnesota, Michael Osterholm said: "We now know that asymptomatic transmission likely [plays] an important role in spreading this virus."
Urging public officials to be clearer about the way the virus is spread, Osterholm stated that it is "absolutely clear" that asymptomatic infection "surely can fuel a pandemic like this in a way that's going to make it very difficult to control."
"At the very beginning of the outbreak, we had many questions about how transmission of this virus occurred. And unfortunately, we saw a number of people taking very firm stances about how it was happening this way, or it wasn't happening this way. And as we have continued to learn how transmission occurs with this outbreak, it is clear that many of those early statements were not correct," he added.
Bill and Melinda Gates Foundation Co-Chair Bill Gates expressed concern about the spread of the disease by people who have not yet developed symptoms, or who are only a little sick.
"There is also strong evidence that it can be transmitted by people who are just mildly ill or even pre-symptomatic. That means COVID-19 will be much harder to contain than the Middle East respiratory syndrome (MERS) or severe acute respiratory syndrome (SARS), which were spread much less efficiently and only by symptomatic people," wrote Bill Gates in a recent article in the New England Journal of Medicine.
Others agree that people without serious symptoms play a substantial role in the spread of the new coronavirus.
"Asymptomatic or mildly symptomatic transmission is a major factor in transmission for Covid-19. They are going to be the drivers of spread in the community," said Dr William Schaffner, a professor at Vanderbilt University School of Medicine and longtime adviser to the US Centers for Disease Control.
"This is time for straight talk," he said. "This is time to tell the public what we know and don't know."
Employees of Biogen, a biotechnology company in Cambridge, Massachusetts attended a company meeting last February. After the meeting was over, three employees tested positive for the virus.
The three employees did not have symptoms during the meeting, according to Ann Scales, a spokeswoman for the Massachusetts Department of Public Health. She noted that an investigation into the outbreak is ongoing, and new information about cases and their symptom status may become available later.
International studies on the spread
Significant transmission by people who are asymptomatic or only mildly symptomatic have been reported in other countries of the world also.
Sandra Ciesek, director of the Institute of Medical Virology in Frankfurt, Germany tested 24 passengers on Tuesday. The passengers had just flown in from Israel.
Of the 24 passengers, seven tested positive for coronavirus. Four of those had no symptoms, and Ciesek was surprised to find that the viral load of the specimens from the asymptomatic patients was higher than the viral load of the specimens from the three patients who did have symptoms. Viral load is a measure of the concentration of the virus in someone's respiratory secretions. A higher load means that someone is more likely to spread the infection to other people.
While Ciesek has not yet published this finding, on February 18, she published a letter in the New England Journal of Medicine about two passengers who returned to Germany from Wuhan, China, and tested positive for coronavirus.
One of these positive passengers had no symptoms and the other had a faint rash and a mild sore throat. When she took their testing samples back to the lab, she successfully infected a cell culture with the patients' swabs.
Large-scale studies using mathematical modelling of outbreaks in Tianjin, China, and Singapore in January and February have also found significant amounts of spread by people who had not yet developed symptoms.
Both studies were posted on MedRxiv, a pre-print server founded by Yale University, the medical journal BMJ and Cold Spring Harbor Laboratory in New York. Articles on this server have not been reviewed by the authors' scientific peers.
Belgian and Dutch research show that between 48 percent and 66 percent of the 91 people in the Singapore cluster contracted the infection from someone who was pre-symptomatic. Of the 135 people in the Tianjin cluster, between 62 percent and 77 percent caught it from someone who was pre-symptomatic.
Canadian, Dutch and Singaporean researchers looked at the same outbreaks in Tianjin and in Singapore, and found that infection was transmitted on average 2.55 days and 2.89 days before symptom onset began respectively in each location.
"Our analysis would suggest that pre-symptomatic transmission is pretty commonplace," said the study's lead author, Caroline Colijn, who leads the mathematics, genomics and prediction in infection and evolution research group at Simon Fraser University in British Columbia.
Officials at the US Centers for Disease Control said that while it is clear asymptomatic spread does happen, it does not appear to be the driver of the outbreak ─ or as the organisation says on its website, asymptomatic transmission "is not thought to be the main way the virus spreads."