Why COVID-19 is different: People are contagious before they show symptoms. By Helen Branswell.
When the world saw a SARS outbreak in 2002-2003, one of the reasons it could be contained was because people were most infectious about seven days after they started to be sick — by which point they were generally already in isolation and their contacts were in quarantine. The same has been true in the case of some other related viruses.
But [Maria Van Kerkhove, who heads the World Health Organization’s emerging diseases and zoonoses unit] said early studies on Covid-19 suggest people who have contracted the coronavirus are emitting, or “shedding,” infectious viruses very early on — in fact sometimes even before they develop symptoms.
“We do know from shedding studies that people can shed in the pre-symptomatic phase,” Van Kerkhove said, adding that while the data are still preliminary “it seems that people shed more in the early phases rather than the late phases of disease.” …
“If you are feeling a little bit unwell and you’re in your early stage of disease, you’re not necessarily in hospital. It takes a few days for you to develop more severe disease and you wouldn’t necessarily seek health care. So it does make sense in terms of what we’re seeing with the epidemiology” of the outbreak, Van Kerkhove said.