That lack of cross comparison matters, as does the lack of information we currently have about things like how different vaccines interact with one another. “We see now that, for example, AstraZeneca is thinking about combining their vaccine with Sputnik V, the Russian vaccine, to try and gain even greater potency and breadth of coverage,” said Dr. Warner Greene, senior investigator at Gladstone Institutes, an independent, nonprofit biomedical research center in San Francisco. But currently, it wouldn’t be a good idea to get one vaccine and then decide you want to try another, because we don’t know how the two vaccines would interact (even though it’s completely possible some of those interactions could be beneficial).

Right now, though, you would not want to get one dose of Pfizer and your second dose of Moderna, said Dr. Purvi Parikh, a professor at New York University’s Grossman School of Medicine and a co-investigator on the Pfizer vaccine trials. The two are both mRNA-based vaccines, but they’re different enough that you can’t assume they’re interchangeable without research to prove it. In fact, she told me, people who get vaccines will be getting cards to document both when they are due for their second shot — and which vaccine they should get when they go back for it.