Question: What’s the difference between asymptomatic and pre-symptomatic?

Answer: I’m glad you asked because I see the word “asymptomatic” used far more than “pre-symptomatic” when it comes to characterizing folks who test positive for COVID-19 but do not have symptoms. And there is an important difference. Asymptomatic folks are those who test positive for the virus but never exhibit or experience symptoms. Meanwhile, pre-symptomatic folks are folks who exhibit symptoms some time after they test positive. A== without Pre==before. Just today, WHO epidemiologist, Maria Van Kerkhove, said that data out of China suggest that 75% of patients originally listed as asymptomatic go on to develop symptoms. ProPublica reports that “This matches up with the CDC’s findings at the nursing facility in Washington. Of the 13 positive patients who initially reported no symptoms during testing,10 later developed symptoms.” For the CDC analysis itself, see this fascinating MMWR report that CDC just published today. All this to say, when Iceland, for example, finds that 50% of those tested via population-based testing report being asymptomatic, it does not mean that 50% of people who contract COVID-19 never have symptoms. Instead, what scientists are finding is that the overwhelming majority of people who contract COVID-19 go on to develop symptoms. But one of the extra-challenging components of COVID-19 is that there seems to be a pre-symptomatic phase of infection where a person does not feel sick, but is able to pass the virus on to others. The ProPublica article I cited above gives a nice run-down of the issues, so check it out if you want to know more!