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Eight 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? 

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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.

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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:

Where did the spread initiate?

Essentially, was this a superspreading event during which someone — likely an unvaccinated person — passed the virus to the eight people? That is one probable explanation. The SARS-CoV-2 coronavirus is particularly adept at transmitting in crowded, indoor settings, so perhaps someone who was shedding lots of virus came into contact with the team, and that’s how this outbreak occurred. The Yankees have surpassed the threshold of 85% players and personnel vaccinated that allowed them under league and union rules to relax some distancing and masking requirements.

It’s also possible that some of the transmission occurred among the eight people who’ve tested positive, though that would seem to be a rarer event. Even if a vaccinated person contracts the coronavirus, it seems they’re generally less infectious to others than an unvaccinated person with Covid-19 — especially if they’re asymptomatic. That might be because the vaccine reduces the amount of virus these people can shed, and also shrinks the window during which they’re contagious.

Was there a variant involved?

Certain coronavirus variants — most notably B.1.351, which first emerged in South Africa but has circulated at low levels in the United States — cause breakthrough infections at higher rates than other forms of the pathogen. The vaccines still prevent many cases, and retain their overall protection against severe illness and death. But the shots do seem to lose some of their potency in staving off mild or asymptomatic infections, and may not be as powerful at preventing people who have breakthrough infections from passing on the virus.

Major League Baseball is working with a Salt Lake City lab for its testing and sequencing efforts, so we could eventually get more information about this question. The sequencing data could also provide insights into whether the eight people were infected during the same event or independently of each other.

Does this mean the CDC was shortsighted in its recommendation Thursday that fully vaccinated people don’t have to wear masks in most settings?

Depending on the type of mask, face coverings have generally been more about protecting others than yourselves, meaning they’re more of a tool to stop you from spewing virus out into the world than blocking virus headed your way.

There was also some nuance in the CDC’s updated recommendations. The data indicate that the chances of an asymptomatically infected person who’s been vaccinated spreading the virus are extremely low. But CDC Director Rochelle Walensky noted Thursday that if someone starts feeling sick, they should start wearing a mask again and get tested.

Still, critics of the CDC decision could point to the Yankees cluster as a sign that prudence is merited. While many experts celebrated the CDC announcement, others argued that the agency was premature or that it should have tied the guidance to hitting thresholds like a further reduction in new cases per capita or reaching a certain level of vaccination coverage. (Given the variability of infection and vaccination rates around the country, some places have achieved these milestones.) The concern is that with transmission still at the rate it is, and with vaccination rates as low as they still are, people who are unvaccinated or who haven’t mounted a strong immune response to vaccines (like some people with cancer are experiencing) will be more vulnerable to contracting the coronavirus than if the CDC had just waited a few more weeks.

Was this a fluke?

Not every member of the Yankees organization who tested positive this week was vaccinated at the same time, so the outbreak can’t be attributed to one bad batch of vaccine or an error in keeping the shots at the proper temperature. Still, in a pandemic in which hundreds of millions of people have been infected and amid a global vaccine campaign, there are going to be some weird outlier situations. Simply put, perhaps something bizarre happened with the Yankees.

It underscores how important it is to avoid drawing conclusions from one case involving fewer than a dozen people. The clinical trials that established the vaccines’ efficacy involved tens of thousands of people, and real-world data that have backed up what the clinical trials concluded: the vaccines are immensely powerful against Covid-19. 

  • “Simply put, perhaps something bizarre happened with the Yankees.”
    -Payed for by the global pandemic big pharma alliance

  • You missed a question: are these all true positives? Were these guys tested orthogonally with different test to confirm? Low positive predictive value to test asymptomatic people in population with low prevalence of disease (vaxxed). Most positives will be false. It’s a statistical fact.

    • Isn’t this based on the assumption that the prevalence will be low? I agree that confirmatory testing would be useful. The article could also have stated which tests were used (PCR/antigen) and if all the tested people were vaccinated and if they all received the J&J vaccine.

  • The Janssen vaccine has always had it’s efficacy listed in terms of “severe disease and hospitalization”. I’ve never seen any study that said it prevented asymptomatic or mild disease, although that’s now being shown in the mRNA vaccines. The Yankee cases aren’t breakthrough cases, because they’re within the limits of the initial studies. Each member has a mild or asymptomatic case.

  • It seems at best short sighted for the CDC to quickly turnaround and say no masks are needed when indoors for fully vaccinated people when people are still getting infected with the virus. None of the vaccines are 100% effective in preventing symptomatic COVID19 infections, especially the J&J vaccine which in clinical trials had varying effectiveness between 57-72% (depending on the prevalence of variants at the test sites) in preventing moderate to serious COVID19 infections. The effectiveness was lower at 42.3% for persons over age 60 years with certain comorbidities. With mask requirements mostly gone and no verification of full vaccination to be required, even the unvaccinated and those still shedding virus can go indoors without masks. So will the breakthrough cases rise as a result, and what is the social stigma for persons wearing masks because they are immunocompromised or just want to be extra safe?
    For me, this is another flip flop of the CDC, from masks are not needed, masks are needed, two layer masks are needed, and even Dr. Fauci on the Today show recommending wearing two masks. This has now gone full circle. Single layer cloth masks were never effective against aerosol transmission of the virus indoors with infectious cases and poor air circulation.
    While the infection attack rate has dropped significantly with about 47% of the US population having been administered at least one dose, the vaccination rate has dropped significantly as well. There are still 43% of the US population unvaccinated, and still many symptomatic and asymptomatic cases out there with the US 7 day case rate at about 72.5 per 100,000 and much higher rates in certain local regions.
    So, my question is the new CDC guidance on masks more of a device to encourage people sitting on the fence to get vaccinated? If so, this is bad public policy, like telling people in the beginning that masks were not necessary, so people would not try to buy N95 masks which is one of the few masks works when worn properly.

  • The most important lesson here is that vaccinated people can still catch and transmit COVID-19 unless proven otherwise – at least for the J&J. This means that even if vaccinated, it is still important to wear a mask so you don’t infect an unvaccinated person, or infect a vaccinated person who could go on to infect an unvaxed person. We should open up, go about our business but continue to get vaccinated and continue to wear masks until herd immunity kicks in within our community.

  • The point of the vaccine is to prevent severe morbidity and mortality. So the J and J vaccine did just that with the Yankees. The reason for wanting to achieve herd immunity is to protect those that can’t be vaccinated ( at this point, kids under 12) , and to protect the 5-10 % who don’t develop immunity from the vaccine, many of whom have serious underlying immune problems ( extreme old age, cancer, etc). No one should interpret this story as a failure of the vaccine.

  • Hopefully, whomever the responsible agency is won’t squander this opportunity to learn about transmission and infectivity. I think this would be a perfect “real life” group to study and the CDC should be held accountable if they pass on it. Gained information would be valuable to professional, collegiate, and community sports teams. I would take an educated guess that a majority of athletes have opted for the J & J vaccine because of the single dose and lower risk of severe disease in their population. Let’s not forget it looks like the Tokyo Olympics is going forward so far and they too might be more likely to get the J & J vaccine.

  • “The Yankees travel with some 50 people” but the only people infected are those who received the Johnson & Johnson vaccine. I think this positively speaks to the efficacy of the Moderna and Pfizer/BioNTech vaccines. If there is a mix of vaccination choices in the Yankees clubhouse, then this shows that the mRNA vaccines had no recorded breakthroughs while exposed to the infected individuals.

    • I received my 2nd pfizer vaccine 2/22 and tested positive today and many others that I seen get reinfected and also test positive months later have received pfizer too

  • This Yankees Breakthrough Infection Outbreak highlights a number of non-science missteps on the part of the CDC. First, as mentioned, the mask proclamation that was more political and arm twisting in its intent than science. Second the decision not to investigate all breakthrough infections. To only look at those that result in hospitalizations and deaths means the Yankees breakthrough infection outbreak will not be investigated by the CDC but will be by the state of NY. That creates a piecemeal public health approach to a complex and concerning development after vaccination. The data collected and analysis needs to be standardized. Siloed approaches have been the Achilles heal of the pandemic.
    The CDC is on a slippery slope here, and its recent decisions contrary to what some media outletds have said is not based upon science.

    • The article says that the CDC will stop “reporting” breakthrough infections other than those that result in hospitalizations or death (nothing about investigating). In any case, the decision seems odd. Why is the public going to be better off with less information?

      As for masks, the CDC has been waffling; it seems like the finally got it right, but the process does not inspire confidence.

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