A peer-reviewed study published this week that examined the cases of coronavirus patients in China during different points in their infections provides evidence that — as has been suspected — the virus is highly contagious in asymptomatic or presymptomatic people, moreso than after symptoms set in. The study's publication may have played a role in growing numbers of public-health orders mandating face masks for everyone in public.
The study, published earlier this week in the journal Nature, looked at 94 COVID-19 patients in China along with 77 separate "infectee-transmission pairs," in which one patient was known with certainty to have infected another. It found that people were most infectious beginning 2.3 days before the onset of symptoms, with infectiousness peaking right before symptoms began, or 0.7 days before onset. The person's documented viral load was then shown to decrease beginning at symptom onset throughout a 21-day period afterwards.
It is yet more evidence that the only way to reduce transmission of the virus is to assume everyone has it. And the study reinforces earlier guidance from the World Health Organization that suggested contagiousness was high between 1 and 3 days before symptom onset.
The researchers conclude that "Significant presymptomatic transmission would probably reduce the effectiveness of control measures that are initiated by symptom onset, such as isolation, contact tracing and enhanced hygiene or use of face masks for symptomatic persons."
Looking at the infectee-transmission pairs in the study, 44 percent of the second infections were believed to have occurred during the first infection's presymptomatic period.
The study further concludes that "More inclusive criteria for contact tracing to capture potential transmission events 2 to 3 days before symptom onset should be urgently considered" in order to more effectively contain outbreaks.
Such contact tracing is now launching in San Francisco, and will be key to the relaxing of strict social-distancing measures moving forward.
But studies like these should certainly make everyone more nervous about being on airplanes, being in crowds, or riding on subways for the foreseeable future.