Question: Do you have any recent data to share about covid transmission by asymptomatic carriers? In looking at your daily qanda I did not see anything more recent that July 2020, unless I missed it.
Answer: There are a few more recent posts I recommend you check out– Q&A of 3/13/21 describes the risk of asymptomatic transmission and Q&A of 7/26/21 describes the risk of asymptomatic transmission among vaccinated individuals. Of course, the Delta variant upends a bit of what we’ve known about SARS-CoV-2. Here’s a bit more on what we know/don’t know:
- Disentangling asymptomatic and presymptomatic cases remains challenging. CDC’s current best estimate (from March 19, pre-Delta) is that 30% of infections are asymptomatic. When it comes to breakthrough infections, in its August 2nd update CDC reports, “1,816 (26%) of 7,101 hospitalizations reported as asymptomatic or not related to COVID-19; 316 (21%) of 1,507 fatal cases reported as asymptomatic or not related to COVID-19.”
- Asymptomatic infections contribute substantially to onward transmission, though to a lesser degree than symptomatic infections (ex: PNAS, Lancet: Regional Health).
- When it comes to the Delta variant, we now know that vaccinated individuals with breakthrough, symptomatic infections can transmit the virus to others, but we do *not* know whether vaccinated individuals who are asymptomatically infected can transmit the virus to others. CDC is exploring this question (among so many others!).
- Breakthrough infections (symptomatic or asymptomatic) remain rare (Figure 1).
Figure 1. State by State Trends in Percent of Fully Vaccinated Individuals Who Experience Breakthrough Infection (from July 30 Kaiser Family Foundation analysis)*note the X axis is 0%-5%. The highest proportion of fully vaccinated people with breakthrough infection is 0.54% in Arkansas (a state with very low vaccination coverage and very high levels of community transmission).
