People infected with the novel coronavirus can transmit the infection one-to-three days before symptoms start to appear, according to a study published by the US Centers for Disease Control and Prevention (CDC). This means infections are being spread by people who have no clear symptoms, complicating efforts to gain control of the pandemic. A study conducted by researchers in Singapore and published by the CDC Wednesday is 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 agencys 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. That reinforces the importance of social distancing and other measures designed to stop the spread, experts said. “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 hadnt 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. CDC officials say they have been researching asymptomatic and pre-symptomatic infections, but the studies are not complete. 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. Meanwhile, the head of the World Health Organization (WHO) voiced deep concern on Wednesday about "the rapid escalation and global spread" of COVID-19 infections with the new coronavirus, which has now reached 205 countries and territories. "In the past five weeks there has been a near exponential growth in the number of new cases and the number of deaths has more than doubled in past week," Tedros Adhanom Ghebreyesus, WHO director-general, told a virtual news conference in Geneva. "In the next few days we will reach 1 million confirmed cases and 50,000 deaths worldwide," Tedros said.