Evidence suggest 40+% of COVID cases can be traced to a presymptomatic individual, with high rates of transmission occurring two to three days prior to symptom onset.

The work by Xi He, et al appearing in the research paper titled “Temporal dynamics in viral shedding and transmissibility of COVID-19”, which appeared in Nature Medicine (published April 15, 2020), speaks to the priority of identifying individuals while they are in a presymptomatic state. The significant findings from the research include

1. 44% of individuals were infected by someone who was in a presymptomatic state, and

2. transmission is proportionately high 2 to 3 days prior to symptom onset.

The quote below is taken from the research paper. For the full research paper, go to https://www.nature.com/articles/s41591-020-0869-5.

“We report temporal patterns of viral shedding in 94 patients with laboratory-confirmed COVID-19 and modeled COVID-19 infectiousness profiles from a separate sample of 77 infector–infectee transmission pairs. We observed the highest viral load in throat swabs at the time of symptom onset, and inferred that infectiousness peaked on or before symptom onset. We estimated that 44% (95% confidence interval, 30–57%) of secondary cases were infected during the index cases’ presymptomatic stage, in settings with substantial household clustering, active case finding and quarantine outside the home. Disease control measures should be adjusted to account for probable substantial presymptomatic transmission.In conclusion, we have estimated that viral shedding of patients with laboratory-confirmed COVID-19 peaked on or before symptom onset, and a substantial proportion of transmission probably occurred before first symptoms in the index case. More inclusive criteria for contact tracing to capture potential transmission events 2 to 3 days before symptom onset should be urgently considered for effective control of the outbreak.”