Do the Vaccines Prevent Transmission?

We should probably know that before we start mandating them.

It’s crazy to me that it’s almost Year 3 of the pandemic, and almost Year 2 of having the vaccines, and we still don’t have a clear answer to this most basic of questions: do the vaccines prevent transmission of SARS CoV-2?

It’s especially crazy because the entire rationale for vaccine mandates — the issue that’s tearing the country apart right now — hinges on its answer.

This has come up a few times in arguments I’ve gotten into on Twitter, so I’ll take a beat to elaborate on that last point: the only justification for forcing people to get the vaccines is if it helps stop the spread of the disease.¹ That’s my opinion, at least.

Vaccine mandates may save lives of people who are currently unvaccinated, that’s true. But that’s not a credible rationale for mandating it; the unvaccinated, like all of us, are entitled to make their own decisions about what risks they’re willing to take. However, if by refusing to take the shot they are increasing transmission to others, then that’s everyone’s problem. In that case, the decision to forego the shot is no longer merely a personal decision; it impacts all of us.

So the answer to this basic question, one would think, would be at the center of all of our debates over vaccine mandate rules. But I barely hear it come up. People I’ve talked to just have their own preconceived ideas as to whether the vaccines do or do not reduce transmission, but when pressed aren’t really sure. Neither am I.

So I contacted two doctors to get their perspectives. Both of these are doctors I’ve spoken to before for this Substack. One is Katie Hisert, MD, PhD, a Denver pulmonologist who I interviewed on my podcast. The other is Steven Rosen, MD, a California oncologist, who I interviewed for my story on vaccines for kids. Dr. Hisert and Dr. Rosen come from two fairly divergent points of view on the vaccines in general — Dr. Hisert is more bullish and Dr. Rosen more skeptical — so I thought they’d provide a nice range of opinion.

Dr. Hisert pointed me to a few studies. One is an August study out of the UK that compared household transmission rates from vaccinated people who were infected with “breakthrough” cases of Covid-19 versus unvaccinated people with infections. That study concluded that the likelihood of transmitting the virus to others is 40 to 50 percent smaller if you’re vaccinated (for the vaccinated population in this study, some had mRNA shots and others had adenovirus shots). Another study she pointed me to was from October, also out of the UK. That study showed that only one in four vaccinated people living with someone infected with Covid caught the virus themselves, versus 38 percent for those who were unvaccinated. Finally, she sent me an article on an Israeli study that showed that people who got the third, booster shot who were infected had a far smaller viral load than those who never got the vaccine. A smaller load would tend to reduce transmission.

(The reason, by the way, that these are all household studies is because that’s the only way you can really test it, according to Dr. Hisert. You can’t ethically expose people to the virus deliberately, and for people who caught it from strangers, it’s very difficult to pinpoint who they caught it from. So researchers tend to look at households instead, where the transmission chain is much clearer. So it’s important to note that we’re talking about patients with prolonged exposure to each other. That might not carry over to sitting on the bus with a stranger for 15 minutes.)

Dr. Hisert also noted that the window for spreading the virus is way smaller for those who are vaccinated. If you’ve had at least two shots, then you’ll mount an immune response within just a few days of catching the virus, versus 10 days to two weeks for the unvaccinated. That means vaccinated people clear the virus out of their systems much faster than unvaccinated people, and are thus shedding virus for a much shorter period of time. So that also reduces transmission for the vaccinated.

“So to summarize,” Dr. Hisert wrote, “I 100 percent think that vaccination decreases transmission, and probably significantly so. I think two shots of the mRNA vaccines likely don't eliminate transmission, but we don't know what three doses does yet.”

Dr. Rosen wasn’t as expansive in his response, but he was clearly less convinced that the vaccines make a big difference with transmission. “What is clear,” he wrote, “is that both vaccinated and unvaccinated individuals can become infected and potentially transmit the virus, that immunity wanes and new variants will change infectivity and morbidity in both groups.” He added that “definitive studies concerning transmission risk from vaccinated versus unvaccinated individuals do not exist.”

As if to underscore that last point, I also happened to come across this recent study of prison inmates in Texas. Its results were very different than the ones that Dr. Hisert sent me (caveat: the paper is still a preprint and not yet peer-reviewed). It found no significant differences between vaccinated and unvaccinated subjects with regard to transmission, and concluded, “As this field continues to develop, clinicians and public health practitioners should consider vaccinated persons who become infected with SARS-CoV-2 to be no less infectious than unvaccinated persons.”

I sent the study to Dr. Hisert, who found it unconvincing. “They’re under powered to detect a signal,” she wrote, “so they really can't draw any conclusions from their negative result.”²

What’s abundantly clear is that the vaccinations only reduce transmission; they don’t eliminate it. So if we double-vaccinated everyone on earth, the virus would still circulate, though catching it would be far less serious (we don’t yet know what would happen if we triple-shotted everyone). Is it worth infringing on people’s freedoms by forcing them to get vaccinated against their will, for, say, a 20 percent reduction in transmission? What about 35 percent? What about 50 percent?

Unlike the question headlining this post, those aren’t scientific questions; they’re political ones. We don’t need to defer to experts to answer them. But until the science is clearer on just how much the vaccines reduce spread, we don’t yet have the empirical information we need to make those fraught policy decisions. As with so many other things related to this pandemic, we’re just shooting in the dark.

1. Other rationales for vaccine mandates include:

• ICUs being overrun and

• decreasing the ability for the virus to mutate.

I don’t think either meet the standard.

In the first instance, we could expand our country’s already shamefully low ICU capacity but have not done so in over two years of a global pandemic; we should at least take that step before resorting to coercion. Some have pointed to the difficulty in doing this because of a shortage of qualified healthcare workers, but a vaccine mandate would only make that problem worse; New York has lost 34,000 healthcare workers to its vaccine mandate, for instance.

In the second instance, while I understand the benefit, to me it’s just way too hypothetical to justify administering tangible punitive measures for non-compliance. We can’t be sure that new, more lethal or transmissible strains will emerge among the unvaccinated, and we can’t be sure that they won’t emerge among the vaccinated. The argument works only at the level of abstraction, and falls apart when you weigh it against an individual’s specific, concrete rights.

2. Dr. Hisert’s more detailed explanation is: "I am not a biostatistician, but it appears that the study is underpowered to detect a significant difference between the two cohorts, and thus the negative result does not necessarily reflect that there is no actual difference in viral shedding between vaccinated and unvaccinated individuals. Furthermore, the authors also neglect to explain some odd findings for subgroups in the cohorts, that put their aggregate data into question."