Your questions, answered
“If you have the opportunity to get more than one vaccine, should you?” — Bob in California
Please don't get more than one vaccine right now. The three available in the United States — Pfizer's, Moderna's and Johnson & Johnson's — were authorized to be given as separate and complete immunizations, not in combination or sequence.
As we noted in January, swapping the Pfizer or Moderna second dose should be done only in “exceptional situations," according to the Centers for Disease Control and Prevention. The agency also cautioned that those mRNA vaccines aren't interchangeable. Two months later, that guidance remains.
Because demand continues to outstrip supply in most parts of the country, it's also worth considering the many people who haven't been vaccinated yet. If you're aware of available doses nearby and would like to further decrease your chances of exposure to the coronavirus, even after full vaccination, consider reaching out to unvaccinated members of your community. The more people around you who have immunity, the harder it is for a virus to spread.
“It’s a bit too early to be getting greedy about taking multiple vaccines just yet,” University of Colorado immunologist Ross Kedl told Bloomberg News earlier this month. “Let's all just be content with one for now until everyone gets a shot.”
Additionally, it is not clear how beneficial that extra vaccine would be. Here's a rough analogy: If you have two bike helmets and you're already wearing one, everyone is safer if you lend the spare to a bareheaded cyclist than if you try to stack a helmet on a helmet.
It's not out of the question that we may need booster shots in the future should protection afforded by this round of vaccinations wane. If so, a recommendation will come after evaluation by medical researchers, statisticians, health regulators and others — not the result of “let's try this at home” personal experiments.
Pfizer and Moderna, for instance, are testing additional boosters and potentially revised vaccines in response to variants. Vaccine combinations are also being studied in clinical trials: In February, the U.K. government announced a trial to give one shot of one vaccine, followed by a second shot of another (such as an AstraZeneca dose with a Pfizer dose 12 weeks later). Another clinical trial in Europe will test the AstraZeneca vaccine and Russia's Sputnik vaccine given 29 days apart. Such studies are in the early stages. The first results from U.K.'s trial might come over the summer.