Only one team member has symptoms, and all were diagnosed via routine testing.

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Seven people traveling with the New York Yankees, including three coaches, have tested positive for COVID-19—and all were fully vaccinated against the virus.

The team made the announcement about the breakthrough cases in a press conference on March 11, noting that four non-coach support staff members also tested positive for the virus. All seven of those who tested positive received the Johnson & Johnson vaccine, the team said, and just one of the seven had "some symptoms" of COVID-19. The positive cases were picked up through routine testing.

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Credit: Getty Images / Photo Illustration by Jo Imperio for Health

"We're learning as we go," team manager Aaron Boone said during the press conference. "We're getting informed on what we need to do exactly, and just trying to do the best we can to be able to make quick adjustments on the fly and pivot."

Each member of the Yankees traveling party has been given at least three COVID-19 tests—including PCR nasal swabs and saliva tests—on Tuesday.

Several players and coaches started wearing masks again in the dugout during the game on the night of the press conference. "One of the good things about being vaccinated is that we are blunting the effects of this virus," Boone said. "I feel like, in a lot of ways, because we're vaccinated, we're good and we're able to deal with this."

The term "breakthrough cases" was used a lot during the press conference, and you might have some questions about what, exactly, a breakthrough case is. Doctors explain.

What is a breakthrough case?

A breakthrough case happens when someone who has been fully vaccinated against an illness, like COVID-19, later develops that illness, according to the Centers for Disease Control and Prevention (CDC). With COVID-19, at least two weeks must have passed since a person's last shot in order for this be considered a breakthrough case, per the CDC. At that point, they're considered fully vaccinated against the illness.

It's important to get this out of the way upfront: Breakthrough cases aren't unique to COVID-19. They can also happen with other illnesses, like the flu, William Schaffner, MD, an infectious disease specialist and professor at the Vanderbilt University School of Medicine, tells Health.

Why do breakthrough cases happen?

The CDC specifically says that breakthrough cases can and do happen. "COVID-19 vaccines are effective. However, a small percentage of people who are fully vaccinated will still get COVID-19 if they are exposed to the virus that causes it," the agency states online.

On a basic level, the reason is because no vaccine is perfect. "Even at their best, these vaccines are 95% effective—not 100%," Dr. Schaffner says. "It was anticipated that breakthrough cases would occur."

According data from clinical trials, the Pfizer-BioNTech vaccine is 95% effective at preventing symptomatic COVID-19 infections, the Moderna vaccine is 94.1% effective, and the Johnson & Johnson vaccine is 66% effective.

While it's tempting to say that the Johnson & Johnson vaccine has the highest risk of breakthrough cases, that's not necessarily correct, infectious disease expert Amesh Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security in Maryland, tells Health. "You can't compare efficacy numbers like that, because the vaccines were not studied head to head, at the same time, or with the same prevalence of variants," he explains.

The CDC also says that a person could become infected just before or after they've been vaccinated and still get sick. The organization also mentions that some COVID-19 variants might cause illness in some people after they've been fully vaccinated.

How common are breakthrough cases?

As of April 26, a total of 9,245 breakthrough cases of COVID-19 have been reported to the CDC after more than 95 million Americans have been fully vaccinated. Of those 9,245 people, 27% of the infections were asymptomatic (meaning, the patients had no symptoms). The CDC also reported that 9% of people with breakthrough infections were hospitalized and 132 (or 1% of total cases) died.

The CDC is planning to transition to reporting only breakthrough cases in people who were hospitalized or died, starting May 14, "to help maximize the quality of the data collected on cases of greatest clinical and public health importance."

How are breakthrough cases treated?

Breakthrough cases don't necessarily require any treatment. "Many of these people are without symptoms, or have very mild symptoms," Dr. Schaffner says. "That shows the vaccines are working." If someone has a more severe case of a COVID-19 breakthrough case, they will be given the same treatment as someone who has not been vaccinated, Dr. Schaffner says.

Despite the fact that breakthrough cases happen, experts still urge people to get vaccinated against COVID-19. "The goal of the vaccines is to do three things: stop serious disease, stop hospitalizations, and stop deaths—and they are tremendously successful at that," Dr. Adalja says.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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