Question: I just want to be sure I’m understanding the transmission potential of the virus…..isn’t it possible that a fully vaccinated person can “carry” the infection and transmit it to an unvaccinated person, without actually being infected themselves?  Is that the definition of asymptomatic?  I just don’t want to be THAT person who infects someone who’s not as protected as I am.

Answer: Your description of what it means to be asymptomatic is close, but not quite right.  To clarify– “asymptomatic” means that a person is infected with the virus, but is not experiencing any symptoms (Box). Your concern that you could be infected without knowing it and then spread the infection to others, even if you’re vaccinated is valid; but if you are vaccinated, and especially if you are boosted, the risk is low.  Specifically, if you are boosted, your risk of breakthrough infection is very low; the risk that the infection will present as “asymptomatic” is even lower (see below bullets).  To further reduce risk, I recommend that you get boosted if you haven’t yet; and if you’re around someone who is not as protected (e.g. an immuno-compromised person or a young child), you could wear a mask and/or give yourself a rapid test pre-visit to alleviate any lingering concern.  Many parents of young children and caretakers of immunocompromised individuals have this concern and work to reduce risk by trying to avoid situations that put them into close contact with unvaccinated individuals.  With that in mind, I’ll take the opportunity to nudge folks to get themselves vaccinated/boosted and their eligible children vaccinated.  The more of us who are vaccinated, the lower the risks to those of us all, especially those who are less protected.  

• Frequency of asymptomatic infection: Scientists estimate that about one-third of all COVID cases are asymptomatic with higher rates of asymptomaticity among children and individuals with no underlying conditions.  When it comes to breakthrough infections (e.g. infections among fully vaccinated individuals), results from two studies— a point prevalence survey during the Delta surge in England this summer and a randomized control trial comparing outcomes among mRNA vaccine vs. placebo recipients– both found that the risk of asymptomatic infection is two-thirds lower among vaccinated individuals as compared to unvaccinated individuals. Vaccination seems to greatly reduce the risk of asymptomatic infection, which is important for controlling the epidemic.  
• Onward transmission from asymptomatic infection: Onward transmission from asymptomatic infection does occur, but at a lower rate than symptomatic infections; one systematic review found, “the relative risk of asymptomatic transmission was 42% lower than that for symptomatic transmission.”  Among vaccinated individuals, we know that the infectious period is shorter than among unvaccinated individuals, and as a recent JAMA editorial explains, “vaccinated people are less likely to transmit SARS-CoV-2 to close contacts compared with unvaccinated people.”  One small study found that vaccinated individuals were 65% less likely than unvaccinated individuals to infect someone else.   
• Frequency of breakthrough infection: Breakthrough infections are much less frequent than infections among unvaccinated individuals.  In October in the US, the rate of new cases per 100,000 individuals was 91/100,000 among vaccinated individuals and 452/100,000 among unvaccinated individuals.  Furthermore, the COVID case rate is much, much lower among individuals who are boosted. 

Box. Definitions of asymptomatic and pre-symptomatic (from The BMJ)