New study @NatureMedicine estimates that nearly half of #covid19 transmission occurs in the 2-3 days before any symptoms, & peak infectiousness may be close to the actual day that symptoms start.

https://www.nature.com/articles/s41591-020-0869-5

“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.”

This is why rapid diagnostics & wide community testing of people who are symptomatic AND those who are pre-symptomatic or asymptomatic w/ exposure to known cases- is critical. If we have to wait for symptoms to start, we may get too far behind. #covid19

Cc @RanuDhillon @sri_srikrishna @diegochowell @IngridKatzMD @thehowie