Nine members of MLB's New York Yankees who are vaccinated have tested positive for Covid with "breakthrough" infections.

The people who have tested positive include second baseman Gleyber Torres, pitching coach Matt Blake, third base coach Phil Nevin, first base coach Reggie Willits and five members of the Yankees' traveling staff, the team said in a release.

"Fortunately, they're all you know doing quite well," Aaron Boone, Yankees manager, said Sunday.

Seven of the infected members are asymptomatic and two have mild symptoms.

"This is the vaccine working," CDC director Dr. Rochelle Walensky told ABC's "This Week with George Stephanopoulos" Sunday. The fact that those who tested positive didn't get a severe infection is a good sign, she said.

The CDC is still investigating the cases as the infected people follow quarantine protocol in Tampa, Florida. But here's what this news means for you.

How did so many people in a group get Covid despite being vaccinated?

All of the members who tested positive received the Johnson & Johnson single-dose Covid vaccine on April 7. Part of the explanation may be that the J&J vaccine was 100% effective in preventing severe illness, hospitalization and death but 66% to 72% effective at reducing all Covid infection.

Additionally, breakthrough cases, though rare, are to be expected with any vaccine. The CDC defines a "breakthrough" infection as "the detection of SARS-CoV-2 RNA or antigen in a respiratory specimen collected from a person 14 days [or more] after they have completed all recommended doses of a U.S. Food and Drug Administration (FDA)-authorized COVID-19 vaccine."

In the general population, as of April 26, more than 95 million people in the United States had been fully vaccinated against Covid, and the CDC reported 9,245 breakthrough cases.

And while the Yankees situation seems like a higher percentage than the general population, it's actually likely that the CDC's numbers are missing a lot of breakthrough cases happening in the country. That's because testing is way down, especially for vaccinated people, says Zach Binney, an epidemiologist at Oxford College of Emory University. The CDC says that fully vaccinated people do not need to get tested following a known exposure.

The Yankees, on the other hand, "are a bit of a unique atmosphere, because all of their personnel were still being tested regularly, so that's why we uncovered each and every case that occurred, whereas in another environment we probably wouldn't have done that," Binney says. (All the cases were discovered during routine daily testing.)

The nature of how a baseball team spends time together could also have something to do with it: "A baseball team, like many workplaces, spends a lot of time indoors together, including in cramped and poorly ventilated spaces," Binney wrote on Twitter on May 14. "If transmission is going to happen, there's a lot of chances for it to here."

Back in April, Major League Baseball allowed teams who had 85% vaccination rate to drop safety protocols, such as wearing masks in the dugout and bullpens. The New York Yankees were among the nine teams permitted to ease up on restrictions.

If they were all unvaccinated, there likely would be many more cases.

How could vaccinated people transmit the virus to so many others?

With breakthrough infections people are "almost always" asymptomatic, Dr. Anthony Fauci, the nation's leading infectious disease expert told CBS "Face the Nation" Sunday.

When a vaccinated person gets infected, they typically have a much lower level of viral load in their nasal pharynx. "The level of virus is so low, it makes it extremely unlikely, not impossible, but very, very low likelihood that they are going to transmit it," Fauci said.

"Even if they test positive, the likelihood of their transmitting to someone else is really very, very low," Fauci said.

With the Yankees, since the first person who was infected (third base coach Nevin) exhibited symptoms, it's possible that he had enough of a viral load in his system to pass it to the eight other members, Binney says. Still, lots of questions remain about the details, because the CDC and New York Department of Health are investigating. But "the data that we're seeing from the Yankees is consistent with this idea," he says.

What this means for you

"If you are vaccinated, the evidence is very strong that you are extremely unlikely to get severely ill," Binney says. "There is nothing in the Yankees outbreak that should make you think different."

It's important to take into considerations your own risk profile even if you're fully vaccinated. For example, do you live with a person who's immunocompromised and can't get the vaccine and is at high risk for the virus?

"Assess your own personal risk profile and take appropriate precautions, which would include things like limiting the time indoors that you spend with these folks and masking up as much as you can," Binney says.

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