At least half of coronavirus disease 2019 (Covid-19) transmissions globally may have been caused by symptom-free infected people unknowingly spreading the virus to others, a study published Thursday by JAMA Network Open found.

In addition, nearly one in four cases of Covid-19 spread involves infected people who remain asymptomatic, the researchers estimated.

The findings highlight the importance of public health measures such as social distancing, mask-wearing and hand-washing – even for people who don’t feel sick – in preventing the spread of the virus, they said.

“To control the pandemic, we must address the “silent pandemic” of spread from persons without symptoms,” study co-author Dr. Jay Butler said.

“Community mitigation measures will continue to be important for the time being to control Covid-19 spread as vaccine uptake increases and we continue to work together to return life to normal,” said Butler, deputy director for infectious diseases at the US Centers for Disease Control and Prevention.

Through Thursday morning, there have been more than 21 million confirmed cases of Covid-19 in the United States, and more than 360,000 deaths attributed to the virus, according to Johns Hopkins University.

As cases surge in many parts of the country, so-called “super spread” events have been reported, but the origins of many outbreaks remain unknown.

This could be due to higher-than-expected asymptomatic transmission, or spread by people who don’t feel sick, Butler and his colleagues said.

To derive their estimates of asymptomatic transmission, the CDC researchers analyzed data from 10 studies and meta-analyses, or research papers that crunch data from multiple sources, they said.

As a result, their estimates are based on data from thousands of cases globally, covering outbreaks through July, the researchers said.

After accounting for a number of factors, including the time it takes for infected people to become contagious, at least 50 percent of new Covid-19 cases globally may have originated from exposure to infected individuals without symptoms, they said.

Up to 59 percent of all disease transmission came from asymptomatic people, with 35 percent from those who later developed symptoms and 24 percent from those who never did, according to the researchers.

“More than half of new Covid-19 cases were estimated to come from infected people without symptoms,” Butler said.

“Strategically planned testing of persons who are not ill, such as those known to have been exposed to Covid-19 or those with frequent unavoidable contact with the public, can likely also reduce the spread of Covid-19,” he said. CURRENTPH