Eight people who work for the New York Yankees baseball team, including a player, have tested positive for Covid-19—and all of them were vaccinated against the virus a little more than a month ago.
That seems bad. But don’t panic. (Or, if you’re a Red Sox fan, stop smiling.) These “breakthrough” cases don’t mean that vaccines don’t work, or that some vicious new vaccine-proof variant went on a superspreading streak through the Yankees’ locker room. They do mean, though, that it might be a little early to take off those masks.
Manager Aaron Boone announced seven cases among team staff to the press on Wednesday; on Thursday the Yankees also put shortstop Gleyber Torres on the Covid-19 injured list—he had the disease in December, got vaccinated, and has tested positive again. Six of the seven staff cases were, thankfully, asymptomatic. The reason anybody found out about them was that the team regularly tests the staff and players.
That’s the curve ball here. “Breakthrough cases are underreported, since many will be asymptomatic,” says Ana Isabel Bento, a disease ecologist at the Indiana University School of Public Health. “And most people won’t get tested unless required to, or they’re experiencing symptoms.”
So it’s impossible to tell how weird this team outbreak really is, because nobody knows the denominator—the number of people who, after getting vaccinated, still get infected but never get sick. The Yankees got the single-dose Johnson & Johnson vaccine on April 7. That’s enough time for their immune systems to get completely spun up against the virus. But no vaccine is perfect. “Johnson & Johnson’s was 100 percent effective at preventing hospitalization and death, 85 percent effective against severe cases, and 72 percent effective at preventing moderate illness in various trials,” Bento says. “So we expected that if vaccinated individuals become infected, they will most likely be asymptomatic.”
In the interest of beating the raging pandemic, most of the Covid vaccines got tested against their ability to prevent severe illness and death. They all do that very well. “But to figure out if this outbreak is consistent with reported vaccine efficacy, which J&J did report against infection, you’d want to compare the rates,” says Sarah Cobey, an epidemiologist and evolutionary biologist at the University of Chicago.
Which we can do, almost. In trials of the vaccine, Johnson & Johnson researchers actually tested some subjects for asymptomatic infection: Did people in the trial who tested negative for the virus before the study then test positive for antibodies against the virus after, whether they got sick or not? About two and a half months after vaccination, 2.8 percent of the people who didn’t get the vaccine were sick without symptoms. But so were 0.7 percent of people who did get vaccinated. That’s an efficacy, a reduction in relative risk, of about 74.2 percent (with a pretty wide confidence interval, indicating that the statistical power here is only so-so). In a preprint posted earlier this week (so not yet peer reviewed), independent researchers looking at real-world results from the J&J vaccine at two weeks after the shot found that just three of 1,779 vaccinated people tested positive for the virus that causes Covid-19, compared to 128 people out of 17,744 unvaccinated folks. That’s a very, very good rate—but not 100 percent.
The problem is, the Yankees haven’t said how many people overall were vaccinated—a team spokesperson hasn’t responded to my questions—so it’s impossible to tell whether eight breakthrough cases is exceptionally bad or just par for the course (if you’ll pardon a metaphor from a whole other sport).
“We carefully review all reports of adverse events in individuals receiving our medicines and vaccines. This involves closely monitoring real-world data for vaccinated individuals who may experience breakthrough infections with Covid-19,” reads a statement that a J&J spokesperson sent to me. “No Covid-19 vaccine is 100 percent effective in preventing infection. However, our authorized Covid-19 vaccine has been shown to help prevent infection and reduce the severity of illness.” The company has also faced a pause in administration due to the vaccines’ association with rare, dangerous blood clots and manufacturing problems at one of the company’s major production facilities.
What’s really at stake here is whether the circumstances of this breakthrough outbreak are unique to a traveling professional sports team, or if they say something about how the vaccine will work for the rest of us. “I imagine the team spends a lot of time together in poorly ventilated spaces and probably relaxed a lot after getting vaccinated. I also suspect they have lots and lots of social connections,” says Sam Scarpino, a mathematical biologist and head of the Emergent Epidemics Lab at Northeastern University. “At this point, I’d put a strong prior on this being what otherwise would have been a superspreading event being reduced to a handful of asymptomatic cases.”
See? This is good news, in a way. Except for the bad news. “What this really highlights is the big gap in our surveillance systems for infectious diseases,” Scarpino says. “We should already have a better answer to the question of whether this is normal or abnormal.”
And what we don’t know may literally hurt us here. Because whether someone has symptoms or not, if they’re infected they can probably still transmit the virus, at least a little bit. No big whoop if you’re vaccinated; maybe a very big whoop if you aren’t. “If anything, these episodes serve as a ‘sentinel’ of what could be happening in the population at large. Meaning there are likely many more breakthrough infections, but the expectation is that, for the most part, people are less likely to get infected if vaccinated,” Bento says. “The chances of breakthrough infections will decrease as more and more people get vaccinated and people continue to practice social distancing and handwashing.”
But then the converse may also be true. On Thursday, Rochelle Walensky, director of the US Centers for Disease Control and Prevention, said at a briefing for reporters that fully vaccinated people could stop wearing masks, indoors and out. But even putting aside that there’s no way other than the honor system to know whether the people next to you in the beer line at Yankee Stadium are vaccinated or not, the team’s outbreak at least hints that even the most vaccinated people can still, under some conditions, transmit the disease, at least a little bit. “That’s what vaccines do. They prevent illness,” says David Boulware, an infectious disease physician and researcher at the University of Minnesota. “That’s why, when there’s a large unvaccinated population, people who’ve been vaccinated should still wear masks. It’s an incredibly low risk of transmission, but it’s not zero.” And as any obsessive Moneyballer or box-score-keeper knows, those statistical edge cases can really start to add up in the later innings.
Updated 5/14/21 9:20 AM to correct Sam Scarpino's title.