A Second Covid-19 Vaccine Is Slated for Approval. How Does It Differ From the First One?

It doesn’t really, and this is excellent news

Dana G Smith
Medium Coronavirus Blog
3 min readDec 17, 2020


Credit: SOPA Images / Contributor / Getty Images

An independent committee advising the U.S. Food and Drug Administration (FDA) endorsed a second vaccine for Covid-19, this one developed by the drug company Moderna. The panel, made up of infectious disease experts, voted 20 to zero (with one abstention) that the benefits of the vaccine outweigh its risks. An emergency use authorization from the FDA is expected to follow in the coming days.

The vote, and the preceding eight-hour meeting, felt a bit like déjà vu. Exactly one week ago, the same committee voted 17 to four (with one abstention) in favor of Pfizer’s Covid-19 vaccine. The two drugs are virtually identical in terms of their mechanism of action, safety, and efficacy.

  • Both use a new method involving mRNA, carried inside lipid nanoparticles, to educate the body’s immune system about the novel coronavirus.
  • Both result in the production of virus-specific antibodies and T cells that defend against infection.
  • Both are roughly 95% effective at preventing Covid-19 (Moderna’s has an efficacy of 94.1%; Pfizer’s is 95% effective. Eleven out of 13,934 people who received the Moderna vaccine developed Covid-19 while eight out of 18,198 people who received the Pfizer vaccine fell ill.)
  • Both were equally good at protecting men and women, older and younger adults, those with and without preexisting health conditions, and all races and ethnicities.
  • Both were moderately effective after the first dose but require two doses for maximum benefit, although the Moderna vaccine is given four weeks apart while the Pfizer vaccine has an interim of three weeks.
  • Both had similar safety profiles, with the most common adverse reactions being pain at the site of the injection, headache, fatigue, and muscle and joint pain. With both vaccines, these side effects were more common after the second dose and in people under the age of 65.

One of the few substantive differences between the two is that Pfizer’s vaccine is currently authorized in 16- and 17-year-olds based on preliminary data, while Moderna only requested authorization for adults over the age of 18. This broader age range was why the Pfizer vaccine received fewer “yes” votes — several of the committee members did not think there was enough data yet to make that call. Both companies are conducting additional studies in children and adolescents.

The other key difference is that the Pfizer vaccine also requires storage at -70 degrees Celsius, necessitating special ultracold freezers, whereas the Moderna vaccine can be stored in regular freezers at -20 degrees Celsius. The difference in temperature has to do with the specific lipid nanoparticles used in the vaccine to package the mRNA.

Notably, neither company provided definitive data about whether the vaccine could prevent asymptomatic infections with SARS-CoV-2, although both said this information is coming based on follow-up antibody tests they’re performing. Moderna did present one slide showing that between the first and second doses, there was a difference in asymptomatic infections between the vaccine and placebo groups, although the numbers were small.

While perhaps the lack of differences sounds a bit anticlimactic, the news is actually very, very good. Two equally effective, equally safe vaccines essentially doubles the number of people who can be vaccinated, getting us out of the pandemic that much faster.



Dana G Smith
Medium Coronavirus Blog

Health and science writer • PhD in 🧠 • Words in Scientific American, STAT, The Atlantic, The Guardian • Award-winning Covid-19 coverage for Elemental