This study investigates the temporal dynamics of viral shedding and transmissibility of COVID-19. Researchers analyzed 94 patients with laboratory-confirmed COVID-19 and modeled infectiousness profiles from 77 infection-infectee transmission pairs. They found that the highest viral load was observed at the time of symptom onset, suggesting that infectiousness peaked on or before this point. The study estimated that 44% (95% CI, 30–57%) of secondary cases were infected during the presymptomatic stage, particularly in settings with substantial household clustering, active case finding, and quarantine outside the home. The results indicate that disease control measures should account for significant presymptomatic transmission. The study also compared the infectiousness profile of COVID-19 with that of SARS and influenza, noting that the infectiousness profile of COVID-19 may more closely resemble that of influenza. The findings suggest that more inclusive criteria for contact tracing are needed to effectively control the outbreak, especially in rapidly expanding epidemics where contact tracing and isolation may not be feasible.This study investigates the temporal dynamics of viral shedding and transmissibility of COVID-19. Researchers analyzed 94 patients with laboratory-confirmed COVID-19 and modeled infectiousness profiles from 77 infection-infectee transmission pairs. They found that the highest viral load was observed at the time of symptom onset, suggesting that infectiousness peaked on or before this point. The study estimated that 44% (95% CI, 30–57%) of secondary cases were infected during the presymptomatic stage, particularly in settings with substantial household clustering, active case finding, and quarantine outside the home. The results indicate that disease control measures should account for significant presymptomatic transmission. The study also compared the infectiousness profile of COVID-19 with that of SARS and influenza, noting that the infectiousness profile of COVID-19 may more closely resemble that of influenza. The findings suggest that more inclusive criteria for contact tracing are needed to effectively control the outbreak, especially in rapidly expanding epidemics where contact tracing and isolation may not be feasible.