Although it’s widely assumed that virus particles carried by the vaccinated and unvaccinated are the same, basic principles of immunology actually predict otherwise, Kedl told me. One reason for this may be that vaccinated people carry less infectious virus particles, as (not-yet-peer-reviewed) research from the Netherlands has recently illustrated. In other words, even if vaccinated and unvaccinated people have the same viral load, it may not necessarily mean they are just as likely to spread the virus. But the researchers concluded that viral load is just one of many factors correlated to transmission reduction. Much of the original Delta concern was based on something called “viral load”-the amount of virus a person carries while infected. ![]() In the other study, researchers in the United Kingdom found that the Pfizer and AstraZeneca vaccines consistently reduced transmission downstream of breakthrough cases. One shows that although transmission did occur among the vaccinated in Provincetown, those cases represent what Kedl calls a “very limited” proportion out of the total number of infections that occurred as part of that outbreak. He pointed me to two studies, neither of which has been peer-reviewed, to make his point. “We’re back in this category of Yeah, it can happen, but it seems to be a very rare event,” Ross Kedl, an immunology professor at the University of Colorado School of Medicine, told me. Some recent research shows that even once they’ve been infected, the vaccinated are less likely to spread the coronavirus than the unvaccinated. But there are plenty of reasons to be optimistic beyond that. In early September, the CDC found that six unvaccinated people were testing COVID-positive for every one vaccinated person. Vaccinated people spread the virus less overall because they are significantly less likely to get infected in the first place. Read: No, vaccinated people are not ‘just as likely’ to spread the coronavirus as unvaccinated people ![]() But we still don’t know just how widely that spread actually happens. ![]() Breakthrough infections are still happening, and they can lead to transmission. But that doesn’t mean that the vaccinated are in the clear, either. Three months later, we have fortunately not seen this doomsday scenario come to pass-the fears raised by the Provincetown report were largely overblown. The CDC quickly went back to recommending that vaccinated people wear masks indoors while news outlets ran headlines such as “ Vaccinated People With Breakthrough Infections Can Spread the Delta Variant, CDC Says.” The worst-case scenario-that vaccinated people might be going about their lives only to be seeding tons of new coronavirus cases-all of a sudden seemed possible. The supposed implication of that finding was even more ominous: Vaccinated people were just as likely to spread the virus as the unvaccinated. In early August, the CDC published its findings on a huge cluster of COVID cases in Provincetown, Massachusetts, concluding that 74 percent of cases had occurred in vaccinated people. ![]() And it also spurred a full-on freak-out that our understanding of who could spread the virus was all wrong. The hyper-contagious variant sent cases skyrocketing and led ICUs to yet again fill up with COVID patients. Interim guidance shared by the CDC in March stated that these cases “ likely pose little risk of transmission,” and a few weeks later, CDC Director Rochelle Walensky said that “ vaccinated people do not carry the virus.”Īnd then came Delta. In the early days of vaccine bliss, many Americans had thought that the shots were a ticket to normalcy-and at least for a while, that’s precisely what public-health experts were telling us: Sure, it was still possible for vaccinated people to get COVID-19, but you wouldn’t have to worry much about spreading it to anyone else. The fear of breakthrough COVID-19 infections spoiled the summer.
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