Moderna 2nd dose timing7/7/2023 The agency updated its initial guidance after it received feedback that some flexibility might be helpful to people, especially if there are challenges around returning on a specific date, says CDC spokesperson Kristen Nordlund. ![]() Why does the CDC now allow up to 42 days between doses of the Pfizer and Moderna vaccines? Two doses of either vaccine lowers the risk by about 95 percent. One dose of the Pfizer vaccine can reduce the average person’s risk of getting a symptomatic infection by about 50 percent, and one dose of the Moderna shot can do so by about 80 percent. “You can think of it like a gradient,” he adds. ![]() Two-shot vaccinations aim for maximum benefit: the first dose primes immunological memory, and the second dose solidifies it, says Thomas Denny, chief operating officer of the Duke Human Vaccine Institute. Most COVID vaccines elicit this response by presenting the immune system with copies of the novel coronavirus’s spike proteins, which adorn its surface like a crown. Vaccines are designed to create immunological memory, which gives our immune system the ability to recognize and fend off invading foes even if we have not encountered them before. So what gives? How long can you go on a single shot and still stay safe? And what happens if your second shot isn’t available on time? Scientific American explores the potential risks and benefits of delaying vaccine doses. Their data is in a new preprint paper, released before peer review. ![]() And developers of the University of Oxford–AstraZeneca vaccine-which is authorized for use in the U.K.-suggest even longer stretches are possible, saying their shot performs better when its doses are spaced 12 weeks apart. Centers for Disease Control and Prevention has updated its guidance to say that people can wait up to 42 days between doses, though the agency still advises individuals to stick to the initial schedule. The original recommended interval was 21 days between doses for the Pfizer vaccine and 28 days for the Moderna shots, the two currently authorized in the U.S. So some scientists have suggested postponing the second shots of two-dose vaccines to make more available for people to get their first doses. Vaccine shortages and distribution delays are hampering efforts to curb the SARS-CoV-2 pandemic.
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