Question: A bit confused as to the risk of passing on infection to others, in spite of being vaccinated ourselves.  For those who are vulnerable (because they have not had the vaccination), could we unintentionally be carriers — I think of it as being the vehicle to transmit, even though we aren’t likely to “injest” the virus personally, as though the virus rides on us without penetrating our systems.

Answer: Put in public health terms, I think you are asking whether vaccinated people who are asymptomatically infected can transmit the virus to others.  The short answer is: Transmission from asymptomatically infected vaccinated individuals is theoretically possible, but it is exceedingly rare (so much so that scientists haven’t been able to measure it).  First off, vaccination is extremely protective, not just from symptomatic infection, but from asymptomatic infection as well. When it comes to asymptomatic infection, CDC keeps a compilation of real-world data on vaccine effectiveness (Table), which shows that the vaccines are also effective at preventing asymptomatic infection.  Other accumulating data show that in the event that vaccinated people become infected, they are far less likely to transmit the virus on to others (see Q&A of 7/14).  Risk of breakthrough infection is low and risk of onward transmission after breakthrough infection is low; low risk + low risk = extremely low risk.      

*Notes: We do not have data on transmission from vaccinated individuals further disaggregated by asymptomatic/symptomatic status, but we would expect even lower risk of transmission among asymptomatically infected vaccinated individuals. Additionally, we have less information about the Delta variant, but we do have solid evidence that 2 doses of Pfizer and AstraZeneca vaccines remain effective against symptomatic infection caused by the Delta variant: full vaccination with Pfizer is 88.0% effective (95% CI, 85.3 to 90.1), and full vaccination with AstraZeneca is 67.0% effective (95% CI, 61.3 to 71.8). 

Table. Real-world risk of asymptomatic infection among vaccinated individuals (from CDC)