In late April, the New York Yankees reported that at least 85 percent of their traveling party was vaccinated, the percentage at which Major League Baseball says teams can relax many of their most restrictive protocols meant to combat the spread of the coronavirus.

On Sunday, the Yankees announced a positive test in their traveling party. By Thursday, they had eight positive tests among their staff, players and coaches.

A post-vaccination outbreak at any workplace would probably raise questions, particularly after the Centers for Disease Control and Prevention announced Thursday that fully vaccinated people no longer need to wear masks inside or outside — a major rhetorical and symbolic shift as the country pushes toward a return to pre-pandemic activity levels. That it happened with the Yankees, baseball’s highest-profile team and trailblazers in MLB’s vaccination push, is sparking nationwide conversations about what the vaccine can and can’t do to eradicate the pandemic.

But according to multiple public health experts, the main takeaway from the Yankees’ outbreak should be that all but one of the affected individuals was asymptomatic, meaning the vaccine is working. The CDC consistently has warned that some “breakthrough cases” will emerge and that “there will be a small percentage of people who are fully vaccinated who still get sick, are hospitalized, or die from covid-19.” The promise, rather, was a lower likelihood of infection and a much lower likelihood of severe outcomes.

According to the CDC, the Johnson & Johnson vaccine, which the Yankees received, “was 66.3% effective in clinical trials at preventing laboratory-confirmed covid-19 illness” but “had high efficacy at preventing hospitalization and death in people who did get sick.” A recent study published in the New England Journal of Medicine found that the Johnson & Johnson vaccine becomes far more effective after four weeks than two. Though most of the Yankees’ traveling party was vaccinated in the first week of April, the team reached the 85 percent vaccination threshold about two weeks ago.

As of this week, the CDC changed its policy in reporting on breakthrough cases to include only those cases “that were hospitalized or died to help maximize the quality of the data collected on cases of greatest clinical and public health importance.” Exactly how widespread these asymptomatic breakthrough cases may be, therefore, remains unclear.

Also unclear, according to Monica Gandhi, a professor of medicine specializing in infectious diseases at the University of California San Francisco, is how dangerous breakthrough cases might be to the population at large.

“The key with those asymptomatic infections is should we be calling those asymptomatic infections or is that all of your immunity in your nose fighting it and keeping the viral loads low?” Gandhi said. “The transmission to others has not yet been shown. There’s not a lot of cases yet where we’ve seen transmission from vaccinated individuals who aren’t sick.”

MLB and the players’ union announced Friday that 12 teams have reached that 85-percent vaccination threshold and another four have crossed that threshold but are not considered fully vaccinated until they are two weeks from their final dose. That’s more than half the teams in a sport whose union received so much concern about the vaccine that it told the league that under no circumstances would players agree to make vaccination mandatory.

The part of the Yankees’ outbreak that seems most puzzling is that eight people out of roughly 50 in one specific workplace tested positive — a seemingly high number for a vaccinated population, one that could raise concerns about employees returning to work in close quarters even after vaccination.

Zachary Binney, a sports epidemiologist at Emory University, alluded in a lengthy Twitter thread to a familiar baseball caveat — small sample size — as a reason concern should be minimal. Fifty or so people in the Yankees’ traveling party, he argued, represent a very small number in comparison to the thousands of people tested in trials and the millions more who have been vaccinated since.

Binney also pointed out that baseball teams work in what amounts to “a perfect storm” for coronavirus spreading: They spend a lot of time in close quarters in the dugout, on planes and in the clubhouse (particularly during a rain delay last Saturday).

“It may be nothing short of a vaccine miracle there were *only* 8 cases!” Binney wrote in that thread.

“How many Yankees *would* have gotten infected *without* the vaccine? Difficult to estimate know without a thorough epidemiologic investigation. But say it’s 24 (a bit over half the travel party, plausible based on other sports outbreaks). That would translate to 8 cases instead of 24, or a vaccine that reduces infections/transmission by 67%. That’s within the realm of prior estimates — in other words, what we expected.”

Another reason the case feels like a particularly jarring outlier is the frequency with which the Yankees, like all MLB teams, are tested: at least twice a week after the 85-percent threshold is reached, more frequently beforehand.

The CDC’s latest guidance suggests vaccinated people may “refrain from testing following a known exposure, if asymptomatic” and “refrain from routine screening testing if feasible.” That guidance also suggests that vaccinated people need not isolate after exposure to a case if they are asymptomatic, something MLB has accounted for in its protocol, too.

The eight Yankees affected were all isolating as of Friday afternoon, though MLB and the players union’s joint committee on the issue has the discretion to clear vaccinated, asymptomatic people if they continue to test negative. MLB declined to comment on the Yankees’ outbreak.

In a briefing Wednesday, CDC Director Rochelle Walensky said, “We obviously need to learn more about that situation.

“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 are effective, have a high effectiveness against disease,” she said.