| In this week's edition of the Covid Q&A, we look at how long it takes for vaccines to provide protection. 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 Paul in the U.K. Paul recently received the Pfizer-BioNTech vaccine. He asks: How long will it take for antibodies to develop in my bloodstream that will give me some protection from Covid-19? Millions of people around the world are now receiving Covid-19 vaccines, so it's likely this question is on a lot of minds. Vaccines work by preparing the body's immune system to respond to a virus in the wild. But that doesn't happen overnight.
For insight, we turned to Shane Crotty, a professor at the La Jolla Institute for Immunology. Two vaccines authorized in the U.S., from Pfizer and Moderna, both use a technology called messenger RNA. Recipients receive an initial dose that primes the immune system, followed weeks later by a second shot that kicks the body's defense into gear. In clinical trials, says Crotty, the Pfizer vaccine appeared to protect against Covid-19 more than seven days after a second shot. The Moderna vaccine provided protection after 14 days.
"Those results are the incredible 95% reduction in Covid-19 cases that everyone has heard about," says Crotty. "For both RNA vaccines!" But you don't have to wait even that long to get some protection from the vaccines.
"Partial protection appeared to occur as early as two weeks" after the first dose, says Crotty. "Since that occurred for both vaccines, it appears to be real protection."  Health-care workers administer a Covid vaccine to residents at St. Johns Missionary Baptist Church in Tampa, Florida. Photographer: Octavio Jones/Getty Images North America That detail is part of why some state and local authorities, including New York Mayor Bill de Blasio, have pushed the U.S. government to allow vaccines earmarked for second doses to be used as first shots. The idea is to provide partial protection for the most amount of people, even it it means delaying second doses —and full protection—for others. Crotty pointed to reports out of Israel, the first country to mass-vaccinate people over 65 with the Pfizer vaccine, that show cases of Covid began to drop two weeks after the first shot.
"All together, those three big data sets suggest partial immunity starts at two weeks after the first immunization, and maximal protection starts at two weeks after the second immunization," says Crotty. 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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