There is increasing research and evidence for asymptomatic transmission of COVID-19. Based on analysis using transmission models of confirmed patients.

SARS-CoV-2 viral shedding may begin 2 to 3 days before the appearance of the first symptoms. Study analysing the 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.

Schematic of the relation between different time periods in the transmission of infectious disease. From He et al. (2020)

For Full Article: He, X., Lau, E.H.Y., Wu, P. et al. Temporal dynamics in viral shedding and transmissibility of COVID-19. Nat Med (2020).

Analysis Excerpt:

"Based on 77 transmission pairs obtained from publicly available sources within and outside mainland China the serial interval was estimated to have a mean of 5.8 days (95% confidence interval (CI), 4.8–6.8 days) and a median of 5.2 days (95% CI, 4.1–6.4 days) based on a fitted gamma distribution, with 7.6% negative serial intervals (Fig. 1c). Assuming an incubation period distribution of mean 5.2 days from a separate study of early COVID-19 cases1, we inferred that infectiousness started from 2.3 days (95% CI, 0.8–3.0 days) before symptom onset and peaked at 0.7 days (95% CI, −0.2–2.0 days) before symptom onset (Fig. 1c).

...Infectiousness was estimated to decline quickly within 7 days. "