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Message in a Bottle: What Is in Each of the New Vaccines From Pfizer and Moderna?

December 28, 2020

By Douglas L. Jennings, PharmD, FACC, FAHA, FCCP, FHFSA, BCPS

Douglas L. Jennings, PharmD, FACC, FAHA, FCCP, FHFSA, BCPSIn less than 12 months since the pandemic began, we have had two vaccines with Emergency Use Authorization from the FDA made available to the American public. This is truly a watershed moment for the scientific community as a whole, and for vaccine development technology in particular. In this blog we’ll be taking a look at exactly what’s in these new vaccines and how they protect patients from COVID-19.

Both vaccines have a novel mechanism for imparting immunity to the SARS-CoV-2 virus. The Pfizer vaccine contains a lipid nanoparticle–formulated, nucleoside-modified RNA (modRNA) encoding the SARS-CoV-2 full-length spike, modified by two proline mutations to lock it in the prefusion conformation. The vaccine available from Moderna is chemically similar.

The use of messenger RNA, or mRNA, is the really unique feature of these vaccines. Unlike prior vaccines which introduce either live or inactivated viral particulates to induce an immune response, these vaccines include only the mRNA. Once injected, the mRNA—which is like a blueprint—is picked up by human ribosomes in the host and transcribed into the notorious spike protein that SARS-CoV-2 needs to enter our cells. Once our immune system comes into contact with these newly transcribed spike proteins, T-cells begin to direct B-cells to differentiate and make antibodies, and immunity is achieved.

The effectiveness of these vaccines is nothing short of extraordinary. Both achieved roughly 95% effectiveness in their respective clinical trials, with very good safety profiles. Short-term side effects were limited to mild-to-moderate pain at the injection site, fatigue, and headache. 

With the Pfizer trial now published in the New England Journal of Medicine on December 10th, we can see just how effective these vaccines are. Over 43,000 patients were randomized into this trial; there were 8 cases of COVID-19 with onset at least 7 days after the second dose among participants assigned to receive the vaccine and 162 cases among those assigned to placebo. Perhaps more impressively, the curves for infection in the two groups diverge clearly around day 12 after the first dose, suggesting that protection against infection begins quite promptly.

A few other points of difference. Both vaccines require a 2nd booster dose: Pfizer’s is 21 days later, while Moderna’s is 28 days later. Pfizer requires ultra-cold storage for their vaccine, while Moderna’s does not, which should facilitate more widespread distribution to rural areas for the latter. Lastly, while both vaccines have demonstrated excellent short-term efficacy, neither has long-term data yet, so the duration of protection against COVID-19 remains to be elucidated.  

Dr Jennings is currently an Associate Professor of Pharmacy at Long Island University and the clinical pharmacist for the Heart Transplant and LVAD teams at NewYork- Presbyterian Hospital Columbia University Irving Medical Center.  He is an active researcher in his field, and he has published over 120 peer-reviewed abstracts and manuscripts, primarily focusing on the pharmacotherapy of patients under mechanical circulatory support. As a recognized expert in this area, he has been invited to speak at numerous national and international venues, including meetings in France, Saudia Arabia, India. Finally, Dr. Jennings has been active in professional organizations throughout his career. He is a fellow of the American College of Clinical Pharmacy, the American College of Cardiology, the Heart Failure Society of America, and the American Heart Association.  

Disclaimer: The views and opinions expressed are those of the authors and do not necessarily reflect the official policy or position of Population Health Learning Network. Any content provided by our bloggers or authors are of their opinion and are not intended to malign any religion, ethnic group, club, organization, company, individual or anyone or anything.


Polack FP, Thomas SJ, Kitchin N, et al. Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine [published online ahead of print, 2020 Dec 10]. N Engl J Med. 2020;NEJMoa2034577. doi:10.1056/NEJMoa2034577

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