The Crucial Vaccine Benefit We’re Not Talking about Enough

COVID vaccines have proved to be magnificent successes, dramatically decreasing the number of cases, hospitalizations and deaths. However, there has been uncertainty about whether vaccinated people who still get infected—perhaps with very mild symptoms, or none at all—might pass on the virus to others. Such silent spread could complicate efforts to control the pandemic.

In recent months, there has been a deluge of data on the risk of transmission after vaccination. These findings have important implications for how quickly we can get the pandemic under control, and for what we say to those who are hesitant about getting vaccinated.

Vaccine trials are typically designed to determine whether an immunization prevents people from getting sick. These are the efficacy numbers in the headlines—as high as a 95 percent reduction in symptomatic COVID cases for the two FDA-authorized mRNA-based vaccines. But the trials provide little or no data on whether the vaccines can entirely block infection, which is the surest way of minimizing spread of the virus.

Considering that at least one third of COVID infections are entirely symptom-free, yet still potentially contagious, it is important to know whether vaccinated people are likely to become carriers of the virus. Not every carrier spreads the virus, though. If a carrier’s viral load is relatively low—meaning that fewer viral particles are shed while breathing and speaking—the risk of transmission is substantially reduced. One possible indirect benefit of a COVID vaccine, then, may be to reduce the viral load in so-called breakthrough cases, or vaccinated people who get infected.

Because many health care workers are now routinely tested for COVID, regardless of whether they have symptoms, much of the early real-world data on vaccine effectiveness in blocking infection has come from this population. In several studies of fully vaccinated health care workers—those more than two weeks past their second dose of either mRNA-based vaccine—the likelihood of having a symptomatic or asymptomatic infection was reduced by 80 to 90 percent, compared with those who were unvaccinated.