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Early evidence suggests that COVID-19 vaccines may reduce the chance that a vaccinated person will transmit the virus to others. Alexi Rosenfeld/Getty Images
  • Experts say it appears that COVID-19 vaccines can help reduce the transmission of the new coronavirus from person to person.
  • They say this is accomplished by reducing the viral load in a vaccinated person’s nose.
  • Experts say until further studies are done, vaccinated people should still abide by safety protocols such as mask wearing and physical distancing.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.

There’s strong evidence that the COVID-19 vaccines approved in the United States are highly effective at preventing severe illness, hospitalization, and death.

But one big question remains unanswered: Can the vaccines prevent fully vaccinated people from transmitting SARS-CoV-2 to others?

The answer to this has implications for what our lives will look like going forward, such as when it will be safe for vaccinated people to stop wearing masks in public spaces.

A growing body of evidence suggests that, yes, some of the vaccines cut down on viral transmission, at least to some extent. Exactly how much, though, still needs to be determined.

“We think it is likely that vaccination will reduce transmission, but the definitive studies to prove this are still ongoing,” said Dr. Sarah George, an associate professor of infectious diseases at Saint Louis University School of Medicine in Missouri.

In the meantime, public health officials are cautioning people to continue wearing masks and physical distancing in public, even after they’re vaccinated.

This will help protect those who don’t have immunity to the virus.

Most of the COVID-19 vaccines are injected into the muscle.

This generates an immune response throughout the body — without causing COVID-19 — that trains the immune system to recognize and fight the coronavirus if it later encounters it.

If a person inhales virus particles that someone else has breathed out, the virus first encounters the mucus-secreting surfaces of the nose and throat, where it can infect cells and replicate.

If that person has been vaccinated, the body will mount an immune response targeted at the virus.

Some research, though, suggests that it can take longer for the immune system to clear a coronavirus infection from the nose than from the lungs.

Until the body clears the infection from the nose, a person may still be capable of shedding virus particles that other people could potentially contract.

To reduce person-to-person transmission, a vaccine has to either block infection completely or reduce the number of virus particles shed from the nose.

Scientists are examining both these scenarios.

George points to a recent Israeli study involving people who received both doses of the Pfizer-BioNTech vaccine.

Researchers saw an 89 percent drop in asymptomatic infections among this group compared with an unvaccinated comparison group. These results haven’t been published yet in a peer-reviewed journal.

While asymptomatic infection doesn’t measure transmission directly, experts say that when there’s no virus present in the nose or back of the throat, it likely translates to no transmission.

“We think reduction in asymptomatic cases is key,” George told Healthline, “as preventing these cases will reduce or block transmission.”

Other studies have seen a similar drop in asymptomatic cases after vaccination.

In a preprint study in U.K. healthcare workers, researchers saw an 86 percent drop in coronavirus infections — both symptomatic and asymptomatic — after two doses of the Pfizer-BioNTech or AstraZeneca-Oxford vaccine.

In addition, data that vaccine-maker Johnson & Johnson submitted to the Food and Drug Administration (FDA) as part of its emergency approval application showed a 74 percent drop in asymptomatic cases.

This data is based on an interim analysis, but Johnson & Johnson Chairman and CEO Alex Gorsky told CNBC on March 1 that researchers will continue to follow people who participated in the clinical trial.

He said they expect to have a more definite answer “in the coming months.”

While injection of the COVID-19 vaccine into the muscle is the most common delivery method, some researchers are working on vaccines that take the fight right to the source of viral transmission: the nose.

A group of researchers from the Washington University School of Medicine in St. Louis developed an intranasal vaccine that generated a strong immune response in mice in both the cells lining the nose and upper airway as well as cells in the rest of the body.

A handful of other nasal COVID-19 vaccines are being developed around the world.

Some experts caution that just looking at asymptomatic infections doesn’t fully answer the question of whether vaccines block transmission.

Doug Reed, PhD, an associate professor of immunology at the University of Pittsburgh Center for Vaccine Research, said viral load studies can also shed some light on whether COVID-19 vaccines reduce transmission.

In this type of study, researchers measure the amount of virus in the body of a vaccinated person who contracts a coronavirus infection and compare it with the viral load of an unvaccinated person with an infection. This is typically done with a PCR nasal swab.

In one preprint study, Israeli researchers found that the viral loads in vaccinated people with an infection were 3 to 4.7 times lower than in unvaccinated people with an infection.

“If the viral load in [vaccinated] people is substantially reduced, it suggests that their ability to transmit the virus to other people would be also substantially reduced,” Reed said.

While this kind of study doesn’t fully answer the question of transmission, “there is good data to say that the higher the viral load, the more likely a person is to transmit the disease,” Reed told Healthline. “So, if the viral load is very low, the likelihood of transmission is also quite low.”

George said combining a viral load study with a contact tracing study can provide more “definitive proof” of reduced transmission.

In this scenario, contact tracing involves studying the close contacts of vaccinated and unvaccinated people to see whether vaccinated people who contract an infection are less likely to transmit the virus.

“That kind of study is more time and labor intensive,” she said, especially since you have to rule out close contacts contracting an infection who are outside the household.

Dr. Davey Smith, MD, MAS, an infectious disease researcher at the University of California, San Diego, is leading this type of study. It will involve collecting data on household contacts of people who have been vaccinated, reports The San Diego Union-Tribune.

Even if scientists determine how much a vaccine reduces transmission, those results may not apply to every situation or every vaccine.

All the approved vaccines protect against severe disease and death.

However, certain vaccines are less effective at preventing overall infection. So, people who receive one of these vaccines are more likely to contract a coronavirus infection. And even if their symptoms are milder, they may also be able to transmit the virus.

Coronavirus variants are another wild card. Some, like the B.1.351 variant first identified in South Africa, reduce the efficacy of certain vaccines. Other variants, such as B.1.1.7, which emerged in the United Kingdom, spread more easily.

This could increase the number of vaccinated people who contract an infection, or could lower the viral threshold for infection.

“If vaccinated people have more virus in their bodies and it takes less of that virus to infect another person, there will be higher probability a vaccinated person could transmit these new strains of the coronavirus,” Deborah Fuller, PhD, a professor of microbiology at the University of Washington School of Medicine, wrote for The Conversation.

Community factors can also affect transmission. When few people in a community are wearing masks or physical distancing, there are more opportunities for a vaccinated person with an infection to transmit the virus to another person.

These public health measures are also effective at lowering virus transmission in a community.

Likewise, when vaccination rates are low, there’s a greater chance that a vaccinated person with an infection will come into contact with an unvaccinated person.

That’s why public health officials are trying to vaccinate as many people as quickly as possible. Doing so will speed up the return to “normal.”

“I urge everyone to get vaccinated as soon as they are eligible with whatever vaccine they can get,” George said.

Reed cautioned that our understanding of transmission will change as more studies are completed. This is especially true when looking at the effect that coronavirus variants have on transmission.

“We are really learning as we go,” he said. “A lot of our initial information [about COVID-19] was based on what we knew from prior pandemics or other diseases.”

And while it may seem like we will be wearing masks forever, there are signs that life will soon shift for people who are fully vaccinated.

The Centers for Disease Control and Prevention (CDC) is expected to issue new public health guidance that will touch on small gatherings of vaccinated people.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, also addressed this at a White House COVID-19 briefing March 1.

“Small gatherings in the home [of vaccinated people], I think you can clearly feel that the relative risk is so low you wouldn’t have to wear the mask,” Fauci said. “You could have a good social gathering in the home.”

What the CDC will recommend for other scenarios, said Fauci, will be based on a combination of data, modeling, and “good clinical sense.”

Many of the studies that will generate this data are already in progress, with more to follow.