Researchers did a systemic review and meta-analysis of nearly 100 studies to examine when the SARS-CoV-2 virus is most contagious. They included studies of other coronaviruses as well (MERS and SARS). Although patients with COVID-19 shed virus RNA in both their respiratory tract and stool for 17 days, the mere presence of the virus does not mean that it is able to infect others. Rather, results showed that the virus was viably infectious (able to grow on culture) when collected from patients in the first week of their illness, but not thereafter. Eight studies that used respiratory samples from patients in their first week of illness successfully cultured live virus, but none of the samples collected after 9 days of symptoms were able to produce live virus - even though there was a lot of virus in those later samples. The viral load of SARS-CoV-2 peaked in the patients' upper respiratory tract in the first five days after they started feeling sick. (MERS and SARS viruses peaked later, which may be the reason those diseases spread slower than COVID-19.)
The amount of virus in a patient did not seem to change whether they felt sick or had asymptomatic infections.
The lead author said, "Our findings are in line with contact tracing studies which suggest the majority of viral transmission events occur very early, and especially within the first 5 days after symptom onset, indicating the importance of self-isolation immediately after symptoms start.