| In this week's edition of the Covid Q&A, we look at when a person who has been fully vaccinated still develops Covid-19, or what's known among epidemiologists as a "breakthrough" infection. In hopes of making this very confusing time just a little less so, each week Bloomberg Prognosis is picking one question sent in by readers and putting it to an expert in the field. This week's question is: Why do some people get Covid-19 infections even though they were recently vaccinated? It's important to acknowledge why this subject has been the source of so much concern and public fascination. After all, if coronavirus shots can't fully stop you from getting Covid-19, what's the point of getting vaccinated? But that's not the right way to think about it, because of one simple fact: No vaccine or medicine works 100% of the time. But the good news is that the best Covid-19 vaccines come pretty close. Real-world studies found them to be 97% effective against symptomatic illness, 86% against asymptomatic sickness and 94% against hospitalization. Tom Clark, who heads a Covid vaccine evaluation unit for the Centers for Disease Control and Prevention, says those are "the kind of numbers you want to see when you want to end a pandemic."  Getting a Covid infection after being fully vaccinated is rare, but it does happen. Photographer: Stephen Zenner/Getty Images North America Statistically speaking, though, that means some fully vaccinated people will still fall ill and may even get hospitalized and die. Just this past week, talk show host Bill Maher and several members of the New York Yankees baseball team tested positive despite having been fully vaccinated. But those type of cases are extremely rare. The CDC tracked 9,245 such Covid-19 infections through the end of April, or less than 1% of the 95 million Americans fully vaccinated at the time. (The CDC notes, however, that this figure isn't complete because it isn't able to count every breakthrough case.) In May, the agency changed how it's tracking breakthroughs, shifting to monitor only those that result in hospitalization or death. Those kinds of breakthrough infections are to be expected, and they have been happening for decades with other diseases, from the the flu to mumps. Part of what's at play here is that each person's immune response to a vaccine will be different, in the same way that each person's response to contracting Covid-19 has been different, says Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia. Also important: a virus's incubation period, or the time between getting infected and having symptoms. Those can range widely, from 10 to 14 days for measles and six days for Covid-19, to just a couple of days or less for the flu, says Offit. The shorter the incubation period, the more the virus is able to spread before the body's immune response gets going. Scientists believe that those who have been vaccinated, even if they contract an infection, will have a milder case than they would have otherwise, though it's still being studied. Another question that's also the subject of research is what role variants are playing in these infections. "Right now, it doesn't look like variants are driving breakthrough cases, but it's pretty early," says the CDC's Clark. Thanks to all of you for writing in this week! Next Sunday, we'll be answering the best question we receive again. So if you have any, we want to hear from you. Write to us at CovidQs@bloomberg.net — Emma Court | 
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