The Back-Story on Positive Tests in Vaccinated People

There was alarm last week in the US when it emerged that seven New York Yankees baseball players, all of whom were fully vaccinated, tested positive for SARS-CoV-2. The event tied into growing concerns about the capacity of mass vaccination to actually end the pandemic.

The players’ infections were confirmed during routine testing. “Fortunately, they’re all, you know, doing quite well,” Aaron Boone, Yankees manager, told CNBC last Sunday. Two of the seven players have mild symptoms, while the other five are asymptomatic.

The Yankees were vaccinated with the Johnson & Johnson single-dose vaccine in early April. Like the mRNA vaccines developed by Pfizer and Moderna, J&J’s vaccine has proven 100% effective at preventing severe illness and death in clinical trials, but its efficacy in preventing mild to moderate disease has been 61 – 72 percent across studies around the world, according to the University of California.

An additional factor, however, is the nature of PCR tests, which is how the Yankees players were tested. These are the tests with which we’re most familiar in Australia, which involve a nasal swab sample being sent to a laboratory and results returned in 12-48 hours. 

Speaking with NYMag, Harvard epidemiologist Michael Mina said PCR tests are a tool “just like a forensics detective wants…where you want to make sure you find the tiniest bit of virus.”

“This is a technology that can catch just ten molecules of virus,” says Mina. “But this is a virus that when it is contagious, there are billions of molecules.”

Public health authorities have so far focused on a yes-no binary of infection. Vaccines and the incredible accuracy of PCR tests are drawing out this simplification. 

Of course, we want as many people as possible to be vaccinated. But because the vaccines we have are not “fully-sterilizing,” vaccinated individuals can still have some presence of the virus, even if it’s such a small amount as to not pose a threat to the health of the carrying person or others.

Image courtesy of NYMag (available:

In a close to fully-vaccinated world, Mina supports rapid antigen tests, which take a throat swab and return a result in around an hour, because these are most likely to only turn positive when you are infectious and becoming ill. Heathrow Airport in London has been using these tests to screen passengers before flights since late last year.

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