Joe Biden overstates how well vaccines prevent person-to-person virus spread

During a visit to a construction site near Chicago, President Joe Biden promoted his policies to increase vaccination rates, arguing that beating back the coronavirus is the best way to boost the nation’s economy.

"My administration is now requiring federal workers to be vaccinated," Biden said on Oct. 7. "We’ve also required federal contractors to be vaccinated. ... We’re requiring active duty military to be vaccinated."

Biden then addressed vaccine requirements for medical workers: "We’re making sure health care workers are vaccinated, because if you seek care at a health care facility, you should have the certainty that the people providing that care are protected from COVID and cannot spread it to you."

Some critics of Biden suggested that he had left an inaccurate impression — that if you're vaccinated, you can't spread the virus.

Those critics had a point: Biden did overstate the degree to which vaccination curbs an individual’s ability to spread the virus to someone else. (The White House did not respond to inquiries for this article.)

In fact, this is the stated position of the Centers for Disease Control and Prevention. In a discussion of the more readily transmissible delta variant of the coronavirus published on Aug. 26, the CDC wrote that "fully vaccinated people with delta variant breakthrough infections can spread the virus to others." 

Experts contacted by PolitiFact agreed. 

Biden "should not be so firm" in his phrasing, said Tara C. Smith, a Kent State University epidemiologist. "Vaccination does significantly reduce transmission from vaccinated breakthrough cases but does not completely eliminate it."

Babak Javid, a professor of medicine at the University of California-San Francisco, said there are "many examples of onward transmission from vaccines," even beyond the most well-known example, an outbreak in the summer of 2021 that included many vaccinated people in Provincetown, Mass.

So Biden is exaggerating when he suggests that someone who is vaccinated "cannot spread" the virus. But scientists agree that vaccination does cut transmission of the coronavirus significantly, even for the delta variant.

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For starters, vaccination makes it less likely that someone will be infected with the virus in the first place. But even beyond that, vaccinated people who do contract the virus have been found to be infectious for a shorter time than those who aren’t vaccinated. For instance, a study from Singapore found that virus levels dropped quickly after infection among people who had been vaccinated.

"This makes sense, since the immune response from the vaccination is coming in to fight the virus," said Monica Gandhi, a professor of medicine at the University of California-San Francisco.

And the shorter someone is at peak infectiousness, the less likely they are to transmit the virus to others.

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Here are some studies that show a reduction in transmission by vaccinated individuals:

• An Oxford University study of contract-tracing data from 100,000 initial cases in the United Kingdom found that vaccination reduced transmission of the delta variant, despite similar viral loads between vaccinated and unvaccinated subjects. Contacts who had received the Pfizer vaccine, which is also used in the United States, were 65% less likely to test positive than people who had not been vaccinated. The AstraZeneca vaccine, which is not approved for use in the United States, reduced positive tests by 36%. (The study did find that the effectiveness in preventing transmission waned with time.)

• A study from Umeå University in Sweden found that people without immunity were at a significantly reduced risk of being infected if family members were vaccinated or had experienced a previous coronavirus infection. This reduction ranged from 45% to 97%, depending on how many other family members were infected. 

• A study from the Netherlands looked at infections among more than 24,000 health care workers. It found that infectious virus shedding was lower among vaccinated individuals who had been infected than it was among those who were unvaccinated. 

Biden said that people who are vaccinated for the coronavirus "cannot spread it to you."

That’s overstating the case. Multiple studies show that a vaccinated person is significantly less likely to pass the coronavirus to someone else than an unvaccinated person is, but the transmission risk is not reduced to zero.

We rate the statement Half True.