close up of a COVID-19 spike

Which COVID-19 vaccine should you get?

In front of the press, President-elect Joe Biden rolled up his sleeve for the Pfizer vaccine and Dr. Anthony Fauci for the Moderna vaccine. While some tried to read something into their choices, these vaccination events were likely arranged to instill public trust in both vaccines. Regardless, the public wants to know which vaccine is best.

The comparisons start based on their physical properties. Both are spike protein messenger RNA (mRNA) vaccines that require two doses. The Pfizer vaccine doses are spaced 21 days apart while Moderna doses are 28 days apart.

A frequently mentioned difference is that the Pfizer vaccine is stored at ultra-low temperatures while Moderna's is maintained at regular icebox temperatures. Neither dosing interval nor storage temperature matters much.

A week is a small amount of time in a pandemic with the potential to stretch for years, and both vaccines are thawed and given at room temperature.

The attempt is made to judge the vaccines on clinical trials results. For example, the efficacy preventing COVID-19 illness was 95 percent for the Pfizer vaccine and 94 percent for Moderna. They also compare the vaccines based on the frequency of non-serious side effects such as fatigue, fever and body aches.

Comparisons based on data from separate trials are inexact, to say the least. The trials were conducted differently. Case in point, COVID-19 tests were performed using different methods that may not be equally sensitive. Likewise, the study populations varied, which can affect the outcomes.

Pfizer enrolled volunteers 16 years and older while Moderna enrolled those 18 and older. The breakdown by age matters, as Pfizer and Moderna found that younger people tend to suffer minor side effects more often.

The study populations varied in other ways as well.

The Pfizer trial was conducted in the United States, Argentina, Brazil and South Africa, and the efficacy varied by country. On the other hand, the Moderna trial was performed only in the United States.

Since the start of the vaccine roll out, some are favoring one vaccine based on the occurrence of severe allergic reactions known as anaphylaxis. Pfizer, the first COVID-19 vaccine given Emergency Use Authorization in the United States and United Kingdom, has been given to far more people than the Moderna vaccine.

So, it comes as no surprise that the majority of anaphylaxis cases have been associated with the Pfizer vaccine. Current U.S. Centers for Disease Control and Prevention estimates place the risk of anaphylaxis at 11 cases for every million first doses of the Pfizer vaccine given.

Moderna hasn't been given long enough for similar estimates to be made. People with a history of anaphylaxis should let the health care professional know before vaccination. Of note, all those who suffered anaphylaxis related to the two vaccines have recovered.

In summary, current information doesn't favor one vaccine over the other. Demand for the vaccines has far outstripped our current supply.

So, which COVID-19 vaccine is right for you? The first one that you can get.

Vaccine Smarts is written by Sealy Institute for Vaccine Sciences faculty members Drs. Megan Berman, an associate professor of internal medicine, and Richard Rupp, a professor of pediatrics at the University of Texas Medical Branch. For questions about vaccines, email