Nearly 60 percent of all Covid-19 spread may come from people with no symptoms, according to a report from the Centers for Disease Control and Prevention published Thursday in the journal JAMA Network Open.
Using mathematical modeling, CDC researchers estimated that 35 percent of Covid-19 spread is from people who are contagious before they develop symptoms, called presymptomatic transmission. Another 24 percent comes from people who are truly asymptomatic, and never go on to develop symptoms. 
Such widespread asymptomatic transmission means that simply identifying and isolating people who have symptoms of Covid-19 "will not control the ongoing spread of SARS-CoV-2," the virus that causes Covid-19, the study authors wrote. 
Rather, the findings mean that everyday behaviors to mitigate the spread of the virus are even more critical.
"Measures such as wearing masks, hand hygiene, social distancing and strategic testing of people who are not ill will be foundational to slowing the spread," the study authors wrote, "until safe and effective vaccines are available and widely used."