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
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.
The agency now says that as of May 10, 1,359 breakthrough cases that resulted in hospitalization or death have been reported to CDC. At that time, about 115 million people in the United States had been fully vaccinated.
The definition of a breakthrough case — a confirmed Covid-19 case at least two weeks after a second dose of Pfizer or Moderna’s vaccines or a single dose of Johnson & Johnson vaccine — hasn’t changed. What is being updated is the data CDC collects and provides online.
“This was done in order to focus on the cases of greatest clinical and public health importance,” a CDC spokesperson told CNN.
The spokesperson said the change in breakthrough case reporting would not affect the understanding of vaccine efficacy.
“CDC is still doing studies on mild breakthrough infections, so people should not be worried that we are going to miss important data related to mild vaccine breakthrough infections,” the spokesperson said.
Sifri says the CDC’s shift in tracking breakthrough cases signals they will be a common part of the post-pandemic world, and mild to asymptomatic breakthrough cases will be a part of the “new normal.”
“What they’re in fact doing is they’re de-emphasizing and normalizing the fact that we will see breakthrough infections,” Sifri told CNN in an interview. “That if there are breakthrough infections that are mild or breakthrough infections that are asymptomatic, they really don’t matter that much.”
Vaccines provide protection
Experts have long predicted the occurrence of breakthrough infections, and real world data has shown vaccine still protects people, even if they do get infected.
“We know that in the rare event that people get infected after a vaccine, the resulting infection is more likely to have a lower viral load, be shorter in duration, and likely less risky of transmission to others,” CDC Director Dr. Rochelle Walensky said at a White House Covid-19 briefing last week.
That also means that people are unlikely to get very sick. In fact, there was so little virus present in many of the breakthrough cases, the CDC had trouble analyzing them.
“What we were starting to find was … a large portion of them were fully asymptomatic. And in fact when we went to study them and even sequence them, there was inadequate virus to even do so,” Walensky said in a White House Covid-19 briefing Tuesday.
With most breakthrough cases have not caused severe disease or death, experts want to focus on understanding those breakthrough infections that do.
But a breakthrough case can still put unvaccinated people at risk, Sifri told CNN.
“The place that they matter is if those mild cases or asymptomatic cases lead to transmissions to people who are not vaccinated and cause serious consequences,” he said, “which just underscores the importance of getting vaccinated.”