A couple of large papers on important topics. The first is a meta-review of papers on asymptomatic and presymptomatic transmission. (PLOS Paper) The researchers sought to ascertain what prior research said about the percent of people who were asymptomatic at the time of diagnosis, how many stayed asymptomatic and how much transmission might be caused by persons who were asymptomatic, initially or throughout their infection. A total of 79 studies were included, but the last search was in June. This is supposedly a living review, meaning it will be regularly updated. The overall estimate of people who are asymptomatic and stay that way was 20%, with a wide range across studies. The studies which carefully tracked people for symptoms had an overall estimate of 31%. Because the studies were so variable the authors were unable to assess what percent of those intially testing positive but who were asymptomatic at the time of test, were merely presymptomatic. People who were asymptomatic were only 35% as likely as those who were symptomatic to transmit the disease to another person. Estimates for pre-symptomatic transmission were not found to be reliable.
The second review focused on viral load and when it peaked. (Medrxiv Paper) This was also a meta-review and included 66 studies. Viral load was generally found to peak before symptom onset, decline somewhat rapidly, and median viral shedding duration was around 5 days. Disease severity and age were associated with higher viral load. Test specimen type also was associated with viral load. Since cycle number was most often reported and not actual load by number of copies, the authors often had to infer copy number.
Together the papers suggest that asymptomatic and certainly pre-symptomatic transmission does occur, but for those who stay asymptomatic, transmission is likely limited. While virus shedding can occur for an extended period, the viral load results would also indicate that there is only a limited period of likely infectiousness. The presence of large numbers of asymptomatic cases suggests even more undetected ones, hindering efforts at contact tracing. In fact, the ability to suppress transmission by contact tracing and isolation is probably very limited given the very large number of undetected cases.