A new study from China found that many people probably had COVID-19 and didn’t even know it.

Researchers looked at 30 million people around the world and found about 40% of COVID-19 cases are asymptomatic.

Scientists say asymptomatic infections are most common in pregnant women, air or cruise travelers and nursing home residents and staff.

The Centers for Disease of Control and Prevention believes the actual number of COVID-19 cases in the United States is four times what has been reported.