As states relax coronavirus mitigation measures and begin to reopen, people who don’t show symptoms of the coronavirus could still be spreading it.

Two studies published this week detail asymptomatic cases of the virus, with one finding that 81% of people who tested positive for COVID-19 on a cruise ship did not show symptoms. Passengers on the cruise ship, which left from Argentina in mid-March, were all screened for coronavirus symptoms before boarding for the planned 21-day Antarctic voyage.

The other found that 42% of a group of infected people in Wuhan, China, were asymptomatic. The patients were either exposed to someone who had the virus or a seafood market that some believe is the origin of the outbreak.

Taken together, the studies suggest that the number of people who have the virus without knowing it could be significant.

“As countries progress out of lockdown, a high proportion of infected, but asymptomatic, individuals may mean that a much higher percentage of the population than expected may have been infected with COVID,” Alan Smyth, the joint editor-in-chief of the journal Thorax, where the cruise ship study was published, said in a statement.

This week, the U.S. reached the grim milestone of 100,000 deaths from COVID-19. Health officials report that the case count is nearing 1.7 million, according to Johns Hopkins University. All 50 states have started reopening nonessential businesses in an effort to kickstart their struggling economies.

The Centers for Disease Control and Prevention has put the estimate of asymptomatic cases at 35%, lower than the two studies.

The CDC cautions that these cases are “challenging to identify because individuals do not know they are infected unless they are tested, typically as a part of a scientific study.”

If infected people don’t know they have the virus, they might not feel a need to follow all mitigation guidelines, like wearing a mask, and could then infect others.

“Although patients who were asymptomatic experienced less harm to themselves, they may have been unaware of their disease and therefore not isolated themselves or sought treatment, or they may have been overlooked by health care workers and thus unknowingly transmitted the virus to others,” according to the Wuhan study, which was published in the journal JAMA Network Open.

That study did, however, find some good news. Asymptomatic patients might emit the virus for a shorter period of time than those who do show symptoms of the coronavirus, according to the research.