Yes, vaccines block most transmission of COVID-19

The latest data show that getting a shot not only protects vaccinated individuals, it reduces the chance they can spread the virus to others.

COVID-19 vaccines have provided an opportunity to slow the spread of the virus and end the pandemic. Now scientists are trying to learn just how much the vaccines can prevent transmission from occurring at all. New data from the CDC shows that COVID-19 infections do occur in vaccinated people, but they appear exceptionally rare.

As of April 14, the Centers for Disease Control and Prevention had received reports that 5,814 fully vaccinated people had developed COVID-19 infections. Nearly half of these infections (45 percent) were in people at least 60 years old. Seven percent of people with breakthrough infections—infections that occur after complete vaccination—were hospitalized and one percent died.

With more than 85 million people in the United States fully vaccinated against COVID-19, the CDC has been cautiously expanding guidelines about what those fully vaccinated people can safely do. The expansion has been gradual as experts awaited data on not just how well the COVID-19 vaccines prevent disease, but also whether a fully vaccinated individual could develop an infection—without symptoms—and unknowingly pass the virus along to someone else.

The distinction is important because many people do not realize that vaccines primarily prevent the disease but not necessarily infection. That means not all vaccines block fully vaccinated people from transmitting the pathogen to others.

“The holy grail of vaccine development always is to stop people from ever getting infected, but it is monumentally difficult to get that,” says Jason Kindrachuk, an assistant professor of virology at the University of Manitoba in Winnipeg, Canada. That holy grail is called sterilizing immunity, completely protecting a person from disease as well as stopping the microbe from getting into cells in the first place, he says.

Four months after the Food and Drug Administration authorized the first vaccines against COVID-19, the CDC has enough data to suggest the vaccines substantially reduce infections—and therefore reduce the possibility of a vaccinated person infecting others.

How vaccines protect people

Vaccines work by mimicking an infection in the body to trick the immune system into mounting a defense against it—and then remembering what to do if they see the same pathogen again, explains Juliet Morrison, an assistant professor of microbiology at the University of California, Riverside.

After any infection, “you have white blood cells, specifically T and B cells, that hang around and remember that initial infection so that if you do become infected again, these memory cells respond by immediately multiplying their numbers,” she says. The B cells produce antibodies that bind to circulating viruses and infected cells while T cells “basically punch holes in the infected cell and pump them full of these toxins that tell the infected cell to commit suicide.”

A vaccine induces the same immune memory as an infection so if the real virus comes along, the immune system switches on immediately and produces T cells, B cells, and antibodies.

“That will allow you to clear the infection without you even recognizing that you’ve gotten sick,” Morrison says.

What’s key, however, is that you did actually have an infection. That is, the virus entered cells and began replicating. The immune system simply shut it all down before the virus or the immune system itself began damaging tissue—the disease process, explains Kindrachuk.

Asymptomatic infections can still transmit the virus

If the virus enters cells and begins replicating but never causes disease, that’s an asymptomatic infection. With presymptomatic infections, on the other hand, a person goes on to develop symptoms and is especially contagious in the days before symptoms appear, says Natalie Dean, an assistant professor of biostatistics at the University of Florida in Gainesville.

“We know from contact tracing data unrelated to vaccines that people who never develop symptoms tend to be less infectious,” Dean says.

Morrison adds that asymptomatic people probably have an excellent initial immune response to slow down how quickly the virus can copy itself, “but not enough that viral replication is completely shut off,” she says. “That’s why they could still shed virus but we’re not seeing any disease symptoms.”

Supporting that idea is the fact that the severity of COVID-19 disease tends to correlate with the total number of viruses in the body, called viral load, Kindrachuk says. Early research showed that people with lower viral loads transmit less virus, further suggesting that asymptomatic infections are less contagious than symptomatic ones. But less is not zero: People with asymptomatic infections still have replicating viruses in their system that they can transmit to others.

When the vaccines were authorized, experts did not yet know whether the shots could prevent infections entirely or whether vaccinated people could develop an asymptomatic—but still contagious—infection.

Why didn’t clinical trials track infections? 

The clinical trials testing vaccines from Moderna, Pfizer-BioNTech, and Johnson & Johnson measured each vaccine’s ability to prevent serious disease, not its ability to block transmission of the virus.

“Frankly, transmission wasn’t the primary concern at that point of the trials,” Kindrachuk says. “It was to make sure people weren’t getting sick.”

With thousands of people being hospitalized and dying every day, the first priority was to measure whether a vaccine prevented severe disease and death. While researchers recognized that it was important to measure whether vaccines prevented asymptomatic infection, doing that was very difficult and costly, Dean says. So researchers tracked symptomatic infections instead. That left unanswered the question of whether any vaccinated people without symptoms could have an asymptomatic infection.

“There were some questions about whether you could still have virus in your nose and still be infectious,” Dean says.

Even a tiny amount of virus in a vaccinated person might present a risk to others.

“We don’t have a good idea of what the infectious dose is for somebody—how much virus you have to be exposed to to get infected,” Kindrachuk says. “It’s not about the one dose you get in a single moment, but the accumulation over minutes to hours.”

Early data looked promising

Although the vaccine manufacturers did not track infections for all phase three trial participants, they did gather some data. Moderna tested all participants when they received their second dose and reported in December that fewer asymptomatic infections occurred in the vaccinated group than the placebo group after the first dose. Johnson & Johnson also reported data from nearly 3,000 phase three trial participants who were tested two months after vaccination to see if they had antibodies from a new infection since vaccination. That preliminary data suggested a 74 percent reduction in asymptomatic infection.

Those findings hinted that the vaccines had the ability to prevent infections. That development was followed by three preprints—not yet peer-reviewed—that suggested even more good news. One found that people vaccinated with one dose of the Pfizer-BioNTech vaccine had viral loads up to 20 times lower than viral loads in unvaccinated, infected people.

Two others, from the Mayo Clinic and the U.K., included more than 85,000 routinely tested healthcare workers who were fully vaccinated with the Pfizer-BioNTech vaccine. The vaccine reduced infection by 85 to 89 percent. All this evidence underscores all three vaccines’ ability to prevent infection in the majority of those vaccinated.

A consensus begins to emerge

More evidence accumulated in March with a slew of studies about the mRNA vaccines. One with 9,109 healthcare workers in Israel found infections cut by 75 percent after two doses of the Pfize-BioNTech vaccine. Another revealed that the viral load fell fourfold in those who received one dose and then developed an infection.

Among more than 39,000 people screened for infection at the Mayo Clinic, patients had a 72 percent lower risk of infection 10 days after the first dose of either mRNA vaccine and 80 percent lower after both doses. The New England Journal of Medicine published research letters showing reduced infections in fully vaccinated healthcare workers at the University of Texas Southwestern Medical Center, the Hadassah Hebrew University Medical Center in Jerusalem, and the University of California in Los Angeles and San Diego.

The most persuasive evidence, according to Dean, came from an early April CDC study of 3,950 healthcare workers who were tested weekly for three months after receiving both doses of either mRNA vaccine. Full vaccination reduced infection—regardless of symptoms—by 90 percent, and a single dose reduced infection by 80 percent.

Then there’s the evidence all around us, Kindrachuk says.

“We’ve seen a pretty drastic decrease of transmission in the country,” he says. “That suggests not only are the vaccines protecting against severe disease but it suggests there’s a reduction in transmission.”

Taken together, the evidence shows that full vaccination with either mRNA vaccine cuts risk of infection by at least half after one dose, and by 75 to 90 percent two weeks after the second dose. Though less research is available on the Johnson & Johnson vaccine, the trial data suggest an infection reduction of more than 70 percent is likely. With the vaccines preventing this much infection, they’re also stopping the majority of vaccinated people from passing along the virus.

Along come the variants

The concern now is how much the variants might change the game, Kindrachuk says. Several of the studies from England and Israel with the Pfizer-BioNTech vaccine occurred when the B.1.1.7 variant was dominant.

“The vaccines seem to be holding their own against the variants, but we also know that these variants tend to be more transmissible,” Kindrachuk says. One concern is that greater transmissibility could mean it takes a lower dose to get infected, he says.

Since the vaccines don’t block 100 percent of infections, it’s possible that vaccinated people who develop an asymptomatic infection from that variant could be more contagious than they would have been before with the strain dominant since early in the pandemic.

Further, there isn’t as much data for the Moderna or Johnson & Johnson vaccines against B.1.1.7 infections, and virtually no data on infections from the other two variants of concern, B.1.351 from South Africa and P.1 from Brazil, both of which have shown some ability to evade antibodies against other variants of the COVID-19 virus.

Scientists are also studying how well the variants replicate.

“If they’re replicating to higher levels, then there could be more viral shedding and more opportunity for transmission,” Morrison says.

The future still looks bright 

Despite the uncertainty posed by the variants, the overall picture right now is reassuring, Dean says.

“These vaccines have really exceeded expectations in so many ways, and it’s just an enormous value that they can keep you from getting sick but also keep you from transmitting to others,” she says. “Nothing is 100 percent, but I think people can understand the big reduction and the value of that. It changes how I think about what I want to do in a big way.”

But that doesn’t mean throwing caution to the wind, Morrison says.

“If you’re vaccinated, you can pretty much assume that you are protected against severe disease and very likely protected against enough infection to transmit, but because we have these variants emerging and the fact that we’re not even close to herd immunity, people should still be taking precautions,” Morrison says.

Interacting with other vaccinated people without masks makes sense, but she also agrees with the CDC recommendation for vaccinated people to visit without masks or social distancing only with low-risk unvaccinated people in a single household. With so many infections still occurring daily, that limitation further reduces the likelihood of vaccinated people picking up and spreading infections from an unvaccinated home.

“The real worry is for the unvaccinated people you come into contact with,” she adds. “Even if the potential for them to pass it on to you is low, it’s not zero.” Similarly, an infected vaccinated person has lower—but not a zero—likelihood of infecting others who aren’t vaccinated or have conditions or medications suppressing their immune systems.

The more vaccinations increase, the more everyone’s risk of infection drops, Dean says.

“I still think about how much transmission is ongoing in my community,” Dean says. “We’re starting to see the population level impact of vaccines, but every single person vaccinated adds up to feeling safer about getting together.”

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