World Health Organization officials clarified Tuesday that it is not yet known how frequently asymptomatic individuals may spread coronavirus, after making a statement just the day before indicating that asymptomatic spread is “very rare.” The announcement on Tuesday came after other public health organizations pushed back on the statement. Studies have shown that patients who are presymptomatic, or have been infected with the virus but are not yet showing symptoms, may significantly contribute to the spread of the virus.

“The majority of transmission that we know about is that people who have symptoms transmit the virus to other people through infectious droplets,” WHO official Maria Van Kerkhove said. “But there are a subset of people who don’t develop symptoms, and to truly understand how many people don’t have symptoms, we don’t actually have that answer yet.”