The coronavirus can spread about one to three days before symptoms start, a top official of the World Health Organization said Monday.
Whether the person is symptomatic or presymptomatic, the disease is still spread in the same way — through droplets from the nose and mouth, said the official, Dr. Maria Van Kerkhove, the WHO's team lead on the coronavirus.
"What we know from reports, what we know from published literature, is that the predominant way in which transmission is occurring is among symptomatic individuals. These are people who can be symptomatic early on in symptoms, even when they start to feel a little bit unwell, "Kerkhove said at a news conference at WHO headquarters in Geneva. "We also know it's possible that people can transmit in the few days before they become symptomatic in their presymptomatic phase."
She said some people can "shed [the] virus one to three days before having symptoms." She added that models of the outbreak suggest there are "large numbers of unrecognized transmission," and she emphasized those are unrecognized patients, not asymptomatic.
"I'm saying we may be missing people who are out there who are infected but we're missing them because of certain surveillance strategies," she said.
Dr. Mike Ryan, the WHO's executive director for emergencies program, agreed that more people are "infected unknowingly."
He said part of understanding the way the virus transmits is not only finding how many people have been infected, but whether more severe cases are linked to people being infected with higher levels of the disease from other individuals or surfaces.
"If somebody's exposed to a high dose from an individual directly or a surface that's heavily contaminated, you can imagine a large dose," Ryan said. "In other circumstances, individual virus particles potentially floating through the air, it can be demonstrated that that may happen in certain circumstances — certainly in health care settings — but is that significant in terms of driving infection? These are all very important things that still need to be studied," Ryan said.
A study from Singapore released last week by the Centers for Disease Control and Prevention examined how presymptomatic or asymptomatic coronavirus carriers could spread the deadly virus. Researchers concluded that people who spread the virus, despite feeling healthy, could pose a threat to disease control.
"These findings also suggest that to control the pandemic it might not be enough for only persons with symptoms to limit their contact with others because persons without symptoms might transmit infection," the CDC researchers said.
The researchers examined all 243 coronavirus cases reported in Singapore from Jan. 23 to March 16. They identified seven clusters of cases where presymptomatic transmission most likely occurred.
The seven clusters occurred between Jan. 19 and March 12 and involved two to five patients each, according to the report. Ten cases within the clusters could be attributed to presymptomatic transmission, and accounted for 6.4% of the 157 locally acquired cases reported as of March 16, the researchers added.
"The possibility of presymptomatic transmission of [the coronavirus] increases the challenges of COVID-19 containment measures, which are predicated on early detection and isolation of symptomatic persons," the researchers said in their findings. "The magnitude of this impact is dependent upon the extent and duration of transmissibility while a patient is presymptomatic, which, to date, have not been clearly established."
— CNBC's Jessica Bursztynsky contributed to this article.