NEW YORK (CBSNewYork) — As you’ve heard, members of the New York Yankees organization, including infielder Gleyber Torres, have tested positive for COVID-19 even though they were reportedly vaccinated.

How, why, and to whom does this so-called “breakthrough” COVID happen?

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As CBS2’s Dr. Max Gomez reported Thursday, the first thing to know is that no vaccine is 100% effective and while all three COVID vaccines have proven to be extremely effective at preventing serious disease, hospitalization and especially death from the coronavirus, some breakthroughs can and have happened.

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There are so many possible variables that can contribute to a vaccine breakthrough. The individual’s own immune system may not respond well enough. He or she may have been exposed to a COVID variant that wasn’t fully prevented by the vaccine, or they may have tested positive before the vaccine had a chance to fully rev up immunity.

That said, Dr. Thomas Balcezak, the chief clinical officer for the Yale New Haven Health System, says you also have to understand how vaccines work.

“They either prevent the virus from entering the body or put a stop to it once it begins replicating. So with all vaccines for COVID-19, it’s possible that the virus can enter your cells, begin replicating and then be stopped after they begin replicating, therefore presenting a period of time when you may test positive and may be asymptomatic,” Balcezak said.

In other words, test three times a day as the Yankees do, and you’re likely to find a positive test in someone who isn’t sick. That’s all but one of the positive Yankees coaches, and who may never get sick thanks to the vaccine. Not to mention that the shots dramatically reduce an infected person’s ability to spread the virus to others.

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There have been a few thousand so-called breakthrough cases, but it’s hard to tell how many of those got truly sick and that’s compared to nearly 120 million Americans fully vaccinated. That works out to less than a thousandth of 1%.