Asymptomatic transmission of covid-19 – BMJ Dec 21
Earlier estimates that 80% of infections are asymptomatic were too high and have since been revised down to between 17% and 20%.
It’s also unclear to what extent people with no symptoms transmit SARS-CoV-2. The only test for live virus is viral culture. PCR and lateral flow tests do not distinguish live virus. No test of infection or infectiousness is currently available for routine use.6 -8 As things stand, a person who tests positive with any kind of test may or may not have an active infection with live virus, and may or may not be infectious.
In a recent systematic review, no study was able to culture live virus from symptomatic participants after the ninth day of illness, despite persistently high viral loads in quantitative PCR diagnostic tests. However, cycle threshold (Ct) values from PCR tests are not direct measures of viral load and are subject to error. While viral load seems to be similar in people with and without symptoms, the presence of RNA does not necessarily represent transmissible live virus.
The transmission rates to contacts within a specific group (secondary attack rate) may be 3-25 times lower for people who are asymptomatic than for those with symptoms.1 12 14 15 A city-wide prevalence study of almost 10 million people in Wuhan found no evidence of asymptomatic transmission.16 Coughing, which is a prominent symptom of covid-19, may result in far more viral particles being shed than talking and breathing, so people with symptomatic infections are more contagious, irrespective of close contact.17 On the other hand, asymptomatic and presymptomatic people may have more contacts than symptomatic people (who are isolating), underlining the importance of hand washing and social distancing measures for everyone.
Searching for people who are asymptomatic yet infectious is like searching for needles that appear and reappear transiently in haystacks, particularly when rates are falling.19 Mass testing risks the harmful diversion of scarce resources.