CNN  — 

In one week, there were nine cases of Covid-19 in fully vaccinated members of the New York Yankees organization: one player, three coaches and five team staff. The revelation has led to questions about vaccine efficacy and fears of super-strength variant strains.

But experts say these cases show the vaccine is working, and testing remains a useful tool.

A Major League Baseball spokesperson confirmed to CNN Wednesday that eight of those nine cases were asymptomatic. Each of the nine were fully vaccinated – it had been at least 14 days since their single dose of the Johnson & Johnson Covid-19 vaccine.

Dr. Costi Sifri, an infectious disease physician and hospital epidemiologist at the University of Virginia, told CNN that the news of the ballplayers is evidence that the Johnson & Johnson vaccine works.

“It’s preventing serious infections in those staff and players with the Yankees,” Sifri told CNN. “Those infections that occurred, the so-called breakthrough infections, importantly were for the most part mild to moderate infections.”

No one in this group has ended up severely ill or in need of hospitalization. However, finding so many breakthrough cases in one spot does say something about testing.

Covid-19 testing for MLB

CNN has previously reported on MLB’s testing protocol that requires all players and staff to be frequently and routinely tested for Covid-19 to ensure safe play. That testing infrastructure is a surveillance tool that can catch asymptomatic cases that would otherwise go unnoticed in the rest of the population.

In addition to their rigorous Covid-19 testing, the MLB has the unique ability to screen what variants are causing any positive cases.

That’s because the Utah lab was once used to test for doping by MLB players now only tests for Covid-19, and can also identify which variants are actually causing those infections.

It’s not clear what strain might be behind the MLB’s breakthrough cases, but it’s one area of investigation.

“Ultimately I believe the variant that we’re dealing with has been pretty aggressive,” Yankees Senior Vice President and General Manager Brian Cashman said in a recent news briefing.

Expected, but still uncommon

In the general population, breakthrough Covid-19 infections are rare. As of April 26, 9,245 breakthrough cases were identified out of some 95 million fully vaccinated people at the time, the CDC said.

Many of these breakthrough cases are so mild that as of last week, the CDC is no longer tracking them.

“We are shifting from tracking all breakthroughs – inclusive of breakthroughs for people who had mild disease – to instead focusing on tracking breakthroughs involving people who were hospitalized or who died,” a CDC spokesperson told CNN.

The Yankees cases, eight asymptomatic and one mild, would not meet the new criteria.