Happy 4th of July!

Question: In follow up to yesterday’s Q&A, one piece that I’m still at a loss around: if she‘s infected but asymptomatic, then regardless of a 14 day quarantine without symptoms, she can still pass the virus along, right? That’s why I was thinking quarantine plus a test would be more confirmation that she’s not infected. Do you know of any research on detecting infection in asymptomatic carriers? (I imagine this sort of data would be hard to get!)

Answer: Good question! And you’re right, because of the threat of asymptomatic transmission, self-isolation + test would be better confirmation than self-isolation alone. Here’s an update on what we currently know about asymptomatic transmission (not enough, but more than last week!):

What proportion of infected individuals remain asymptomatic?

CDC estimates that 35% of infected individuals are asymptomatic (range: 20%-50%). The proportion could very well depend on the characteristics of the population (like age, underlying conditions, previous exposure to other coronaviruses). For example,in the nursing home study we discussed in our Q&A of 4/3, only 13% of those infected (3 of 23 individuals) remained asymptomatic. Meanwhile, a study published by Nature just 4 days ago presented fascinating results from testing nearly everyone in a small Italian town at the beginning of lockdown and two weeks after lock down. The scientists found “42.5% (95% CI 31.5–54.6%) of the confirmed SARS-CoV-2 infections detected across the two surveys were asymptomatic (i.e. did not have symptoms at the time of swab testing and did not develop symptoms afterwards).” This paper from mid-June, COVID-19: asymptomatic carrier transmission is an underestimated problem, and this paper published in early June, “Prevalence of Asymptomatic SARS-CoV-2 Infection” both provide helpful syntheses of evidence to date.

Is the virus able to spread as easily from asymptomatic individuals as symptomatic individuals?

CDC estimates that asymptomatic individuals are as infectious as symptomatic individuals (range: half as infectious to as infectious). In line with CDC’s estimate, this literature review published by Journal of Infection last week found that viral loads are similar between symptomatic and asymptomatic individuals.

What is the duration of infectivity among asymptomatic individuals?

Our current best guess at how long asymptomatic people may be infectious is 10 days after testing positive. WHO recommends asymptomatic individuals leave self-isolation/quarantine 10 days after testing positive. This recommendation aligns with findings from studies that measure the duration of virus detection in asymptomatic individuals. For example, this study of 24 individuals with asymptomatic infection in Nanjing found the median duration of infectivity as measured by time from positive test two consecutive negative tests was 9.5 days while this study from Jiangsu Province found the median duration to be 7 days.

For more background, see Q&As of 6/30, 6/9. 5/28, 4/18, and 4/3.