Your questions, answered "If I got a Moderna vaccine, can I get a Pfizer booster, or vice versa?" — Nina in New York "Is the dosage of the booster shot for Pfizer or Moderna the same as for the first two shots?" — Fredrika in New York "Will the third Pfizer and Moderna shots be formulated to deal with the delta variant, or will they be the same as the first shots?" — Sharon in Pennsylvania Since the news broke that the Biden administration plans to offer coronavirus vaccine booster shots beginning next month for those fully vaccinated with the Pfizer-BioNTech and Moderna vaccines, we have been getting many questions. For starters, there is a distinction between additional doses and booster shots. The Centers for Disease Control and Prevention now recommends an additional dose of the two-dose messenger RNA vaccines for individuals who are moderately to severely immunocompromised since they often do not build enough, or any, protection against the virus. A booster shot is an extra dose given to those who did build enough protection but that protection has started to wane. Pending reviews by federal health agencies, the Department of Health and Human Services (HHS) has put together a plan to start administering booster shots in the fall to eligible individuals — those who completed an mRNA vaccine series at least eight months before. Because boosters have not yet been authorized by the Food and Drug Administration or recommended by the CDC, there are no guidelines. But here are some educated guesses: It is not known whether you will be able to mix vaccines when it comes time for a booster, but the CDC has said in the past that the coronavirus vaccines are not interchangeable, meaning the first and second doses of the mRNA vaccine series should be completed with the same product except in special situations, such as when it cannot be determined which vaccine a person started with or the vaccine is no longer available. "And I think they'll initially follow that same guidance for the boosters," said William Moss, executive director of the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health. Studies are ongoing to assess mixed vaccine schedules, he said. The first and second shots in the mRNA vaccine series are the same, and the boosters are expected to simply be a third shot. So, yes, the dosages for the boosters will likely be the same as they were for the initial shots. And as for whether the boosters will be formulated to target the delta variant — no, at least not initially. Moss said the White House proposal involves using the same formulation based on the sequencing of the original coronavirus strain. But, he added, scientists are working to reformulate the vaccines to target the delta variant in the future. Until then, remember that the vaccines available in the United States have been shown to be highly effective against severe covid-19 infections, hospitalizations and death from delta. All of that said, there are a number of steps to go through before boosters become a reality and, even then, there will likely be some debate among CDC advisers about whether they are needed right now, Moss said. "I'm not sure this is a done deal yet," he said. | 
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