| In this week's edition of the Covid Q&A, we look at whether vaccination can prevent transmission of Covid-19. 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 comes to us from Michelle in Dunnellon, Florida. Michelle said that while her husband and many neighbors have begun receiving their Covid-19 vaccines, she will have to wait because she's younger than the eligible age. She asks: Once my neighbors have had their second shot, is it safe to socialize with them without a mask or social distancing? Can they still get Covid (without becoming ill themselves) and pass it to me? Great question, Michelle. This has been a source of confusion for a lot of Prognosis readers. We think of vaccines as working by preventing the transmission of a disease. But that isn't necessarily the case.
"The short answer is we don't quite know yet," says Buddy Creech, director of Vanderbilt University's Vaccine Research Program.
Like many questions surrounding Covid-19, there just isn't enough data yet to say whether vaccines can stop the virus from spreading. "The clinical trials of Covid vaccines thus far have focused primarily on preventing the symptoms and severity of Covid rather than evaluating acquisition of the virus and transmission to others," Creech says.
Patients who have been vaccinated can certainly still contract SARS-CoV-2, the virus that causes Covid-19, and pass it on to others. There are two reasons for that, says Matthew Woodruff, an immunologist at Emory University.
First, he says, being vaccinated doesn't guarantee protection. Both the Pfizer-BioNTech and Moderna vaccines have been shown to be extremely effective — about 95% — at preventing Covid symptoms, but that still means they won't work in some people.  A worker prepares a dose of the Pfizer BioNTech in the Bronx borough of New York on Feb. 5. Photographer: Angus Mordant/Bloomberg
"This means that in your neighborhood of 40 vaccinated individuals, even if everyone has followed CDC recommendations, there will still likely be two people at risk of contracting and spreading the disease," Woodruff says. "We do know that those two people will likely have less-severe disease than they would have otherwise, but I don't think anyone would seriously argue that they are incapable of transmission."
Secondly, says Woodruff, being protected from symptoms of a disease isn't the same as being free from infection.
"It is certainly possible that patients that are otherwise protected from symptoms will still have low-level infections that they may not even notice," he says.
Those caveats in mind, the coronavirus spreads most effectively through things like coughing and sneezing, so even just reducing symptomatic cases of the disease seems likely to slow the spread. Recent data have suggested that even one dose of the vaccine can drastically reduce viral infections. There is no reason to believe vaccines won't reduce transmission rates; it's just a question of by how much.
So while disease rates remain high, as they are now in the U.S., it's important to stick to public health recommendations such as social distancing and mask-wearing.
"As more and more individuals become vaccinated, it will become easier and easier to expand our bubble, eventually leading us to the point where masks and distancing will no longer be needed," says Creech. "For us to get there as quickly as possible, our focus still has to be masks, hands, space and vaccines." 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 — Kristen V. Brown | 
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