Whether vaccination can prevent transmission of the virus is “the looming question,” Fauci, director of the National Institute of Allergy and Infectious Diseases, said during a White House coronavirus response team briefing. “If a person gets infected despite being vaccinated — we refer to that as a ‘breakthrough’ infection — does that person have the capability of transmitting to another person?”
“There have been some studies that are pointing in a very favorable direction,” he said, adding that these studies will have to be corroborated by additional research.
Fauci highlighted two recent studies that looked at a person’s viral load — that is, how much virus he or she has in the body — and transmissibility.
One study from Spain, published Feb. 2 in The Lancet, found a direct correlation between viral load and transmissibility. The higher the viral load, the greater the transmissibility of the virus.
That’s in line with what years of research on HIV have shown: there’s a direct link between the viral load in someone's blood and the likelihood that individual will transmit HIV to a sexual partner, Fauci said.
For SARS-CoV-2, the virus that causes COVID-19, researchers are focused on how much virus is found the nasopharynx, the upper part of the throat behind the nose that's reached with a long, skinny swab.
The second study Fauci described — a paper that has not undergone peer review that was posted last week to the preprint server medRvix — looked at coronavirus infections in Israel, a country with very high rates of vaccination.
That paper found that individuals who were infected after receiving their first dose of the Pfizer-BioNTech vaccination had a “markedly diminished” viral load compared with unvaccinated people.
It’s another example of “scientific data starting to point to the fact that [the vaccine] … has very important implications from a public health standpoint for interfering and diminishing the dynamics of the outbreak,” Fauci said.
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Dr. John Anthony Vanchiere, chief of pediatric infectious diseases at Ochsner LSU Health Shreveport in Louisiana, said that the two studies "go nicely together hand in hand."
“We know that it is the case for flu and other respiratory viruses that higher viral loads are associated with increased transmission," he said. “The fact that the vaccine reduces the viral load, even shortly after getting your first dose, it's very important data to have."
Both Pfizer-BioNTech and Moderna are currently studying how vaccination affects transmissibility, Fauci said.
"The bottom line message," he said, is "when your turn to get vaccinated comes up, get vaccinated. It’s not only good for you and your family and community, it will have a very important impact on the dynamics of the outbreak in our country."
This story first appeared on NBCNews.com.