A coronavirus outbreak among vaccinated players and staff has “rocked” the New York Yankees and attracted the attention of the CDC this week. So far, eight people, including shortstop Gleyber Torres, have tested positive more than 14 days after being fully vaccinated.

Because the team has a high level of vaccinated personnel, the Yankees have not missed any games while weathering this outbreak, and seven of the eight cases have been asymptomatic. Still, the accidental case study of post-vaccine positives could have implications for the remainder of the baseball season as well as the broader public.

Immunology and virology experts who spoke to Yahoo Sports on Thursday were not terribly surprised that fully vaccinated people were testing positive, particularly considering all the Yankees' affected personnel got the Johnson & Johnson vaccine. But they identified important questions for fully understanding the implications of the outbreak.

The Yankees cleared MLB’s 85 percent threshold for lowered protocols at the end of April, and manager Aaron Boone said that “very few” members of their traveling party are not vaccinated. But baseball players are not in a secure bubble, and the broader population is still vaccinated at a much lower overall rate.

“Problem is we treat this as if our family or work, or our social group is the herd,” explained Dr. Benjamin Neuman, a professor of biology and GHRC Chief Virologist at Texas A&M University. “But the herd is everybody, and herd immunity means everybody's gotta get [vaccinated].”

With cases and hospitalizations declining, it’s clear that vaccinations are working. But herd immunity remains far off, if it will ever be accessible. In the meantime, “A lot of little exposures can lead to a breakthrough,” Neuman said.

How a single breakthrough led to eight cases among a highly vaccinated population, however, is at the crux of a public health issue.

Dr. John P. Moore, a professor of microbiology and immunology at Weill Cornell Medical College, wondered whether all the breakthrough cases were the result of the same exposure to an unvaccinated infectious source. “Or did one Yankee get infected and then transmit to someone else?”

“It's not too unexpected that vaccine recipients will get asymptomatic or low-grade infection, particularly with J&J, because J&J is known to be 70 percent, approximately, effective at preventing mild disease,” he added. (The CDC says that, in clinical trials, J&J was 66.3 percent effective at preventing positive COVID-19 tests).

“But, what would be unexpected is an infected J&J vaccine recipient then passing the infection on to someone else," Moore said. "That would be surprising.”

Moore would also want to know precisely which sequence of the virus is circulating among the Yankees. Multiple more transmissible variants have circulated recently in New York City.

“And we know that these variants change the equation in terms of how well protected a person will be and how long that protection will last, because all the vaccines are made against the original 2019 version of the virus and the virus has been out there working hard and changing, improving itself out in the world,” said Neuman.

Both experts expressed concern that the Yankees received the Johnson & Johnson vaccine. For a baseball team whose operations depend on a clean slate of negative results, they would have recommended teams pursue one of the mRNA vaccines (Moderna and Pfizer), which have been shown to be 94.1 percent and 95 percent effective, according to the CDC.

Neuman stressed that in clinical trials even the J&J vaccine worked better with multiple doses and the decision to market a one-dose vaccine was more strategic (it adds a critical level of flexibility for vaccinating transient groups, rural populations, and homebound people.)

“It's more likely that you will run into this problem with J&J vaccinated people than with mRNA vaccinated people because that's what the vaccine efficacy trial results show,” Moore said. “Again, if your goal is to keep your players on the field of play, that means you have to have negative testing, and you are more likely to get negative testing with the more potent vaccines.”

In baseball, the case could be a preview of problems to come for other clubs that relaxed protocols after reaching the 85 percent threshold with J&J shots.

This is not to say that people who receive the J&J are at significantly more risk from COVID-19. Even in the case of the Yankees’ outbreak, the vaccine has proven valuable and effective at mitigating the severity. Seven of the eight Yankees who tested positive have not experienced any symptoms and third-base coach Phil Nevin, the first person to test positive and the only one to feel sick, is now asymptomatic as well.

For most people relying on vaccines to protect them from COVID-19 illness, hospitalization and death, the J&J is a perfectly viable option. In fact, as of Friday, the CDC will update its categorization to only count fully vaccinated patients who test positive for COVID as “breakthrough” cases if they’re hospitalized or die, which means the current Yankees cases wouldn’t even be tallied.

But because the Yankees have regimented, robust testing (Boone was tested at least three times Thursday), there’s a chance to better understand how COVID-19 spreads between vaccinated individuals. Which brings us back to that key question: Did all of the infected Yankees catch it from a shared, unvaccinated source, or did fully vaccinated individuals pass it to one another?

“I would want to know what happened,” Moore said, “because it's of interest – not just from a sports fan perspective, although I like sports – it's of interest from the wider public health perspective to figure out what happened.”

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