“The possibility of presymptomatic transmission increases the challenges of containment measures,” CDC said in an early release from its “Morbidity and Mortality Weekly Report.”

Detailing a review of cases in Singapore, the officials said researchers had found cases “in which presymptomatic transmission is the most likely explanation” for people becoming infected. The new study bolstered preliminary evidence from studies in China.

The US Centers for Disease Control and Prevention warned in a study released Wednesday that people who are infected but have not developed symptoms may still transmit coronavirus, raising new concerns about how infectious the disease is.

The officials recommended that public health officials who are tracing the contacts of people who have been infected by the coronavirus “strongly consider” looking back in time a little further, to see who the infected person was in contact with before they developed symptoms.


Finding out who is infected and tracing who they’ve been in contact with to advise those contacts to self-quarantine is a key strategy in trying to stamp out the pandemic.

“Containment measures should account for the possibility of presymptomatic transmission by including the period before symptom onset when conducting contact tracing. 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 study said.

“Finally, these findings underscore the importance of social distancing in the public health response ... including the avoidance of congregate settings,” the report said.

Evidence that people who do not have symptoms can unwittingly transmit the virus comes at time of debate over whether the public should be advised to wear masks to slow transmission from the infected to the uninfected.

Dr. Robert Redfield, the director of the CDC, confirmed in an interview with WABE in Atlanta, a National Public Radio member station, on Monday that the agency was reviewing its guidelines on who should wear masks.


He cited data that showed transmission from people who are infected but have no symptoms.

He said some people are transmitting the virus probably as long as two days before showing any symptoms. “This helps explain how rapidly this virus continues to spread across the country, because we have asymptomatic transmitters and we have individuals who are transmitting 48 hours before they become symptomatic,” he said.

The study was the latest to estimate that around 10% of new coronavirus infections may be sparked by people who were infected with the virus but not experiencing symptoms.

In response to recent studies, the CDC changed how it was defining the risk of infection for Americans. The agency’s new guidance, also released Wednesday, targets people who have no symptoms but were exposed to persons with known or suspected infections. It essentially says that anyone may be a considered a carrier, whether they have symptoms or not, The Associated Press reported.

“You have to really be proactive about reducing contacts between people who seem perfectly healthy,” said Lauren Ancel Meyers, a University of Texas at Austin researcher who has studied coronavirus transmission in different countries.

The new study focused on 243 cases of coronavirus reported in Singapore from mid-January through mid-March, including 157 among people who hadn't traveled.

Researchers found that so-called pre-symptomatic people triggered infections in seven different clusters of disease, accounting for about 6% of the locally-acquired cases.


An earlier study in Hubei province, China, where the virus was first identified, suggested that more than 10% of transmissions could have occurred before patients spreading the virus ever exhibited symptoms.

Researchers are also looking into the possibility that additional cases are triggered by “asymptomatic” people who are infected but never develop clear-cut symptoms, and “post-symptomatic” people who got sick, appear to be recovered, but may still be contagious.

It remains unclear how many new infections are caused by each type of these potential spreaders, said Meyers, who was not involved in the Singapore study but was part of an earlier one focused on China.

In the initial months of the pandemic, health officials based their response on the belief that most of the spread came from people who were sneezing or coughing droplets that contained the virus.

Material from Globe wire services was used in this report.

Martin finucane can be reached at martin.finucane@globe.com