New York Yankees' Gleyber Torres (center) is congratulated by teammates after they beat the Washington Nationals in a May 8 game. Torres is one of eight members of the organization who were fully vaccinated against Covid-19 and tested positive for the coronavirus this week.

ight members of the New York Yankees organization who were fully vaccinated against Covid-19 tested positive for the coronavirus this week. And that news has led to a lot of people saying, wait, what?

Below, STAT outlines what we know about the cases and some of the factors that might have contributed to the cluster.

What do we know so far?

On Sunday, third base coach Phil Nevin reported feeling some symptoms and tested positive for Covid-19. (He had recovered as of Thursday, General Manager Brian Cashman said.)

The team quarantined people who’d had close contact with Nevin and expanded its testing program, which turned up another seven asymptomatic cases by Thursday, when shortstop Gleyber Torres became the eighth person — and first player (the other seven were coaches and support staff) — to test positive.

All eight people had received the Johnson & Johnson Covid-19 vaccine and were considered fully vaccinated.

What does this mean about breakthrough infections and vaccines?

The Covid-19 vaccines have been shown to be overall highly effective at preventing illness, and even more so at staving off the worst outcomes, like serious disease and death. But even the best vaccines can’t block all infections, and those that still occur after immunization are called breakthroughs.

The clinical trials that led to the authorization of the different Covid-19 vaccines generally measured how effective they were at preventing symptomatic cases, and in its trial, the one-dose J&J shot proved to be 66% efficacious at blocking moderate and severe Covid-19. And while studies have shown the vaccines also prevent asymptomatic infections in many cases, researchers don’t have firm conclusions at just how effectively they do so. The assumption is that many asymptomatic cases go unreported in the absence of broad and frequent testing.

Which brings us to the Yankees. Eight breakthrough infections seem like a lot, and something odd could be at play here. But it’s notable that only Nevin had some symptoms. It’s possible that at least some, if not all, of the other seven other infections would have been missed if they didn’t occur on a team that’s undergoing regular testing.

And while it can’t be known for sure, it’s possible that without vaccines, this cluster could have been worse, both in terms of the number of infections and how sick certain people got. The Yankees travel with some 50 people.

Overall, the Centers for Disease Control and Prevention has received reports of 9,245 breakthrough cases, a tiny fraction of which resulted in hospitalization or death. More than 118 million people in the United States have been fully vaccinated. The low number of breakthrough infections has been touted as a testament to the power of vaccines, which are providing both protection for individuals and helping drive down transmission.

But experts also point out two caveats with the number of breakthrough infections. For one, some large number of the people who’ve been fully vaccinated haven’t since been exposed to the coronavirus, so it’s not known how many infections there would have been without that vaccine coverage. The tally of breakthroughs is also certainly an undercount because many asymptomatic cases go undetected — unless, of course, they occur in a setting with widespread and frequent testing, like on a professional sports team.

Two extra points: Starting Friday, the CDC will only report breakthrough infections that lead to hospitalization or death because those have the “greatest clinical and public health importance.” The Yankees cases, of course, didn’t rise to that level.

And Torres, the shortstop, had Covid-19 last year, indicating that his case is both a breakthrough infection and a reinfection. Scientists think reinfections remain rare, though given that second infections are generally thought to be mild or asymptomatic (akin to breakthrough infections), experts similarly don’t have great estimates for how often they occur. But considering Torres was asymptomatic, his case fit with what experts expect from most reinfections.

How did this happen?

The team has said it’s conducting contact tracing, and officials from both the New York state health department and CDC are in touch with the team. So far, it’s not clear how this transmission occurred.

Two of the most pressing questions we can think of: