This study makes the case that a lot of disease transmission occurs before symptoms occur in index cases, and disease control measures need to be adjusted accordingly.
Researchers examined viral shedding in 94 patients with COVID-19. They also modeled COVID-19 infectiousness profiles from a different sample of 77 transmission pairs.
They found that the highest viral load in throat swabs occurred at the time of symptom onset. They inferred from this that infectiousness likely peaked at or before symptom onset. They estimated that 44% of secondary cases were infected while an index case was asymptomatic, likely in household clustering. This will make contact tracing and isolation more difficult.