Vaccinated people generally spread the virus less because they are significantly less likely to get infected. In early September, the CDC found that six unvaccinated people tested COVID positive for every vaccinated person. But otherwise there are plenty of reasons to be optimistic. Some recent studies show that even once infected, the vaccinated people are less likely to spread the coronavirus than the unvaccinated. “We’re back in this category of yes, it can happen, but it seems to be a very rare event,” Ross Kedl, a professor of immunology at the University of Colorado School of Medicine, told me.
He pointed me to two studies, neither of which have been peer-reviewed, to make his point. One shows that although transmission took place among the vaccinees in Provincetown [Mass.], those cases represent what Kedl calls a “very limited” portion of the total number of infections that occurred as part of that outbreak. In the other study, researchers in the United Kingdom found that the Pfizer and AstraZeneca vaccines consistently reduced downstream transmission of breakthrough cases. Much of the original Delta concern was based on something called “viral load” — the amount of virus a person carries while infected. But the researchers concluded that viral load is just one of many factors associated with transmission reduction. In other words, even if vaccinated and unvaccinated people have the same viral load, it doesn’t necessarily mean they are equally likely to spread the virus.