COVID Vaccines May Prevent Spread but Inoculated People Should Still Be Careful

Whether or not COVID-19 vaccines reduce transmission of the SARS-CoV-2 virus has been one of the key questions of the pandemic. Vaccines that could significantly lower the number of infections would make a huge difference in bringing the pandemic under control.

While it is clear that the three COVID-19 vaccines authorized for emergency use in the United States are highly effective at preventing disease—particularly serious cases and deaths—the science is not yet settled on the question of transmission.

Nevertheless, experts say there is evidence to suggest that these vaccines do reduce infections. Scott Halpern, an intensive care physician and epidemiologist at the University of Pennsylvania, told Newsweek, this evidence comes in three main forms.

Firstly, while the initial clinical trials for the Pfizer and Moderna vaccines did not systematically study whether the shots prevented asymptomatic infection, the Moderna study did actually provide "direct evidence of reduced" infection because at the time of the second shot, all participants were tested for the virus, according to Halpern.

"Even at that time, before the second shot, the vaccine had reduced all infections by two thirds," Halpern said.

Marc Lipsitch, a professor of epidemiology at the Harvard T.H. Chan School of Public Health, told Newsweek that he suspects the reduction may be "considerably larger" with two doses, further out in time.

Secondly, Halpern said there is "high-quality" evidence from Israel—the country with the most comprehensive vaccine rollout to date—suggesting the Pfizer vaccine reduces transmission in the real-world, although some of this evidence should be taken with a "grain of salt" because it has yet to undergo scientific peer review.

Thirdly according to Halpern, there are a number of country-level analyses that compare infection rates among groups of people who were eligible to be vaccinated versus groups of people who were not.

"The greater reductions in infection seen since the start of vaccination campaigns among groups of people eligible to receive the vaccine (e.g., older persons) relative to people ineligible to receive vaccines (e.g., younger persons) is rather suggestive, and in fact more suggestive than similar analyses comparing people who were vaccinated versus those who were not," Halpern said.

"This is because comparisons of vaccinated versus unvaccinated people could be biased by differences in risk-taking or testing behaviors in those groups—limitations that are less likely to apply to comparisons of groups of people who are or are not eligible for vaccination."

Peter Chin-Hong from the University of California San Francisco points towards several pieces of real-world data that supports this hypothesis.

"For the Pfizer vaccine, investigators from the Israel Heath Ministry and Pfizer demonstrated a reduction in infection by 89 percent in asymptomatic cases and 94 percent in symptomatic cases," according to one pre-print—i.e. non-peer-reviewed—study, Chin-Hong told Newsweek.

"In another study, vaccination with the Pfizer in a single center also showed an 85 percent and 75 percent reduction in symptomatic and asymptomatic infections respectively. And in a Lancet pre-print publication examining U.K. health care workers, Pfizer vaccine effectiveness—including asymptomatic infections—was 86 percent, seven days after the second dose," Chin-Hong said.

There is also a pre-print from Israel which showed that vaccinated people who had been infected had lower viral loads than unvaccinated, infected individuals. In fact, viral load was reduced four-fold in infections that occurred 12-28 days after the first dose of the Pfizer vaccine.

Chin-Hong believes that these kinds of results would also apply to the Moderna vaccine, which is based on similar mRNA technology.

"Although most of the 'real world' studies so far has been in the Pfizer vaccines, most believe that the Moderna vaccine would yield similar benefits," he said.

As for the Johnson & Johnson vaccine, which was authorized for emergency use last week by the U.S. Food and Drug Administration, there is preliminary data to suggest that asymptomatic transmission is decreased by 74 percent, indicating that this phenomenon is not just a characteristic of the two mRNA vaccines.

The evidence for reduced transmission is currently limited and further research involving more rigorous studies is needed to confirm the extent of this reduction. But experts say the available evidence that have at least some impact on infections is strong.

"So far the evidence is high to moderate quality and very limited in quantity. We can definitely say that the Moderna vaccine reduces transmission, but not that it eliminates it. Pfizer is almost certainly similar. J&J has different kinds of evidence but also very strong suggestion of reduced transmission because of reduced infection," Lipsitch said.

Meanwhile, Halpern said evidence that the vaccines reduce transmission is "very strong." However, "it also clear that the vaccines do not prevent transmission entirely—in other words, whereas the vaccines are almost perfectly successful in eliminating deaths due to COVID, the reductions in infections are unlikely to be so uniform."

"Right now we can only say confidently that the vaccines reduce transmission, we can't say by exactly how much. Putting all the evidence together, I'd guess that in the coming months we'll learn that the Moderna and Pfizer vaccines reduce transmission by about 75 percent. But if further evidence suggests the number is really as low as, say, 60 percent, or really as high as 90 percent, I wouldn't be shocked."

The expert added that it is "essential" people realize that the vaccines don't completely eliminate the risk of transmission, so he recommends that inoculated people shouldn't significantly change their behavior just yet in most cases.

"So what you should or shouldn't do after getting vaccinated depends on exactly how effective the vaccines are, and also on what the infection rates are in your communities. Right now, cases are falling across the U.S., but they are not yet low enough for people to safely go out in public without a mask."

"Masks and vaccines together work better than either one alone. But if everyone gets vaccinated and wears a mask for now, it won't be too long before we get to a point where infection rates are low enough that we can ditch the masks."

Some experts think that it may be OK for vaccinated people to be less cautious in some circumstances.

"I think it is reasonable to be slightly less cautious if vaccinated, but would strongly recommend continuing to mask and reduce contacts until the data are clearer," Lipsitch said.

Meanwhile, Chin-Hong suggested that fully vaccinated individuals should be able to get together without protection in small settings—what some have referred to as "immunity bubbles."

"Our messaging needs to be more nuanced than the current black and white 'just say no' warnings," he said.