During the mosquito-borne Zika outbreak in 2015 and 2016, scientists were confident that 75% of those infected would not develop symptoms.

But scientists are having a hard time pinpointing a global average for COVID-19, the disease caused by the new coronavirus, and are finding different rates in different places.

A study in Iceland found that half of those who tested positive for the coronavirus infection showed no signs of illness. Nearly 1 in 5, or 17.9%, of infected passengers on the Diamond Princess cruise ship off Japan were asymptomatic, according to a March study.

The Center for Evidence-Based Medicine at Oxford University said 50% to 70% of people in an Italian village west of Venice were asymptomatic, compared with 31% of Japanese nationals evacuated from Wuhan, China, where the outbreak began in December.