The potential for asymptomatic transmission, when a person is infected and infectious, but has no symptoms, is constantly used by some public health officials as a justification for mitigation measures like social distancing, mask wearing, school closures and stay-at-home orders. This paper is a meta-review of studies on asymptomatic transmission. (Medrxiv Paper) Asymptomatic transmission can be confused with pre-symptomatic transmission, because most people will have some period of time, a few days, between infection and development of obvious symptoms, so it could take some time to determine who was truly asymptomatic. The authors found and extracted data from 19 studies, most of which were limited to contact tracing research, as that has a higher level of accuracy. Across all studies, asymptomatic patients had secondary attack rates of 0% to 2.8%, meaning that of all those patients, this is the percent of contacts of these persons who became infected. It isn’t always completely clear that the first person was responsible for the infection in a second person. By comparison in the same studies, symptomatic persons had secondary attack rates of .7% to 16%. For persons who turned out to be pre-symptomatic, the secondary attack rates varied from .7% to 32%. The most common situation for transmission was in a household setting, with some other group settings playing a role.
As the authors note, there are some definitional issues in determining the role of asymptomatic transmission. Some people may not recognize a symptom as related to CV-19, and some symptoms for CV-19 are common to other diseases. Behaviorally, we would hope that anyone who has a potential CV-19 symptom would get tested immediately and would limit their contacts until they know the test outcome. But these definitional issues make it hard to determine which people are truly asymptomatic at the time of potential transmission to a second person, and then to ascertain the actual rate of asymptomatic transmission. The other underlying issue is some continuing controversy over relative viral shedding by asymptomatic or symptomatic patients and the length of any period of infectiousness. My assessment is that truly asymptomatic patients have lower viral shedding and shorter periods of infectiousness, which accounts for their limited role in transmission.
While asymptomatic and pre-symptomatic transmission can occur, truly asymptomatic patients appear to be much less likely to spread the virus to others. This has clear implications for schools and for other settings with large numbers of young people in particular, who are often not symptomatic. And public health officials should stop using this as a justification for actions unless and until they release results of contact tracing studies that prove such transmission is occurring at a significant rate.