https://www.scientificamerican.com/article/covert-coronavirus-infections-could-be-seeding-new-outbreaks/

As coronavirus outbreaks surge worldwide, research teams are racing to understand a crucial epidemiological puzzle — what proportion of infected people have mild or no symptoms and might be passing the virus on to others. Some of the first detailed estimates of these covert cases suggest that they could represent some 60% of all infections.

Many scientists have suspected that there is an undetected pool of covert cases showing limited to no symptoms because an increasing number of infected people cannot be linked to known COVID-19 cases or travel to epidemic hotspots. Most people with mild infections would not be ill enough to seek medical help, and would likely slip past screening methods such as temperature checks, so the extent of the phenomenon and its role in virus transmission has remained elusive.