Eight members of the New York Yankees — seven coaches and support staffers, plus shortstop Gleyber Torres — have tested positive for the coronavirus over the past three days, even though all of them were fully vaccinated. The news came as the Centers for Disease Control said Thursday that fully vaccinated Americans no longer have to wear masks indoors or outdoors in most cases.

“With regard to the Yankees, we obviously need to learn more about that situation. My understanding is that six of the seven infections were indeed asymptomatic infections, and we will look to more data from that report to understand what happened there,” CDC Director Rochelle P. Walensky said at a news conference. “All of the real-world data we’ve seen that’s been in the published literature, large studies, in many different settings, have demonstrated that those vaccines have a high effectiveness against disease.”

The CDC had previously warned that no vaccine is 100 percent foolproof and that “a small percentage of people who are fully vaccinated will still get covid-19 if they are exposed to the virus that causes it.” Such people are deemed “breakthrough” cases, and the Yankees are now the first professional sports team to have an outbreak of them.

According to the Yankees, all of the infected team members had been inoculated with the single-dose Johnson & Johnson vaccine, which was found to be 72 percent effective at preventing moderate to severe covid-19 and 74 percent effective at preventing asymptomatic infection. Other vaccines by Pfizer and Moderna, which require two doses, have been found to be more effective by the CDC at halting the disease.

“The Johnson & Johnson vaccine is very good at preventing what we’re most concerned about, which is hospitalizations and death due to covid; [it’s] as effective as mRNA vaccines from Pfizer and Moderna. But it is less effective at preventing mild disease, in this case preventing asymptomatic infection,” Costi Sifri, director of hospital epidemiology at UVA Health, said in a telephone interview Thursday. “Is [the Yankees’ outbreak] predictable? I think it is predictable if you have a vaccine that doesn’t prevent that particular outcome of covid and if you’re relaxing some of the infection-prevention practices that we’ve been practicing for a while now during the pandemic. So if you’re gathering indoors and taking off masks and being closer to one another and you have a vaccine that doesn’t prevent asymptomatic infection [as well as the others], then you’ll see these events if you’re testing for it.”

On April 30, the Yankees were allowed to relax their MLB-mandated coronavirus protocols after reaching an 85 percent vaccination rate among players, coaches and staff members. They no longer had to wear masks in dugouts and bullpens, and restrictions on mobility during road trips were loosened. But after the positive tests started coming in during a trip to Florida this week, players began taking precautions again. Staff ace Gerrit Cole, for instance, wore a mask during a pregame videoconference session with reporters Tuesday and in the dugout during the game against the Tampa Bay Rays later that night.

Such a high rate of breakthrough cases would seem to be a statistical anomaly. As of April 26, the CDC reported 9,245 breakthrough cases out of the more than 95 million people who had been fully vaccinated, an infinitesimal percentage. The Yankees have seen about 13 percent of their traveling party, which, per the New York Times, is “around 60 people,” test positive.

But quantifying how unusual this outbreak is would be difficult to determine at this stage, according to Angela Rasmussen, a virologist and research scientist for the Vaccine and Infectious Disease Organization at the University of Saskatchewan. For starters, it can take years to determine how effective vaccines are at preventing disease and infection. The coronavirus vaccines, on the other hand, were created via an expedited process and only began being administered a few months ago.

“If you were vaccinated and exposed to enough virus, it’s totally possible that you could get an infection that your immune system hadn’t caught up to,” Rasmussen said in a telephone interview. “It won’t be as severe as if you were completely unvaccinated ... but it’s still possible to get infected. But in general, even though we don’t know the true rate of breakthrough infections, one thing is quite clear, and that is the vaccines are very protective against disease. So even in fully vaccinated people who end up testing positive, they’re not likely to develop a rip-roaring case of covid-19.”

All but one of the coaches and staff members who tested positive are asymptomatic, Yankees Manager Aaron Boone told reporters this week.

As mandated by MLB’s coronavirus protocols, players and on-field personnel are tested for the coronavirus at least every other day. That makes the discovery of breakthrough infections among professional baseball players much more likely than it would be among the population as a whole.

“If somebody [in the general public] is vaccinated, they’re probably not going to go out and get tested all the time to see if they’ve had a breakthrough infection. They’re probably only going to be tested if they actually get sick,” Rasmussen said. “So as a result, if there are more asymptomatic breakthrough infections occurring, we wouldn’t be able to see any of those just because people aren’t seeking out testing and we’re not testing people who have been vaccinated.”

Said Sifri: “If you’re testing so frequently, you are going to be able to identify events of asymptomatic infection. Otherwise it would fly underneath the radar.”

Both Sifri and Rasmussen said that variants of the virus also could be a reason for the positive tests.

“They’re still protective. ... The vaccines still work,” Rasmussen said. “The variants have not figured out how to resist vaccine-induced immunity. But the vaccines do appear to be a little bit less effective against some of these variants, specifically ones that have mutations. ... Depending on what variant they were infected with, that also might have something to do with the breakthrough infections that were seen on the Yankees.”

In Torres’s case, he already had tested positive for the coronavirus in December, making his case — a vaccinated person who already had tested positive — even more peculiar.

With MLB’s frequent testing, “you also have the possibility to find people who have had past infection, lingering positive tests that reflect past infection, because we found that people in some instances can test positive for a significant period of time, sometimes a couple months after previous infection,” Sifri said, adding that breakthrough cases should be expected with any vaccination campaign.

“This is one of the potential outcomes of widespread vaccinations: Do we convert SARS-COV2 to a virus that’s transmitted that causes little in the way of symptoms, maybe mild or no infection?” he said. “Or do we actually get to the point where we have herd immunity. ... In either instance, it’s better than we were prior to having the vaccine.”