Explaining the Difference

The Moderna and Pfizer-BioNTech vaccines are similar and “both very good vaccines,” according to Tania Watts, PhD, a professor of immunology at the University of Toronto in Ontario, Canada. 

She noted that there may be some subtle differences in their formulations, “but broadly, they are mRNA and a lipid nanoparticle, so are similar vaccines.”

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As evidence continues to emerge supporting the notion that Moderna may be more effective for patients with cancer and other immunocompromised patients, one underlying reason could simply be that this vaccine contains more mRNA.

The Moderna and Pfizer-BioNTech vaccines both underwent dose-escalation studies in the initial clinical trials when they were tested on humans. Pfizer settled on a dose containing 30 μg of mRNA (for vaccination in adults), and Moderna settled on a dose containing 100 μg of mRNA (for the primary doses in adults).7,8 

Dr Watts said the idea that more mRNA leads to more antigen production “is the most likely explanation” for the differences in responses between the 2 vaccines. She added, however, that “you’d really have to do a dose-escalation with the 2 side by side to really confirm that.” 

The concept of a larger amount of antigen stimulating a greater immune response is not new, Dr Watts noted. Flu vaccines for older people frequently contain higher doses of antigen than those used for younger, less clinically vulnerable populations.

“For a number of years now, there’s been a high-dose flu vaccine offered to people over 65, because it’s well known that our immune system gets weaker with age. So just giving more of the vaccine in this case is known to give a better response,” Dr Watts explained.

Implications for Practice

On the basis of all the current and accumulating evidence, should cancer patients and other clinically vulnerable people be receiving the Moderna vaccine instead of the Pfizer-BioNTech vaccine?

“I think that is a reasonable suggestion,” Dr Watts said. “And in fact, I believe various clinicians are already recommending this informally for their patients. If someone’s already had a course of Pfizer vaccine, and they’re ready for another booster, going for Moderna is not a bad idea. Whereas, for someone who is a healthy person under 65, I think there’s no reason not to just choose any of the available vaccines for the booster.”

“I can tell you, in my clinical practice, most of my patients have finished their primary vaccine series and we are more talking about boosters,” Dr Ma said. “After this study, I’m starting to recommend that they consider using Moderna for their booster.”

Disclosures: Dr Ma reported having no relevant conflicts of interest. Dr Watts reported being an advisor to Crescendo Biologics.


  1. Doukas PG, St Pierre F, Boyer J, Nieves M, Ma S. CLO22-043: Humoral immune response following COVID-19 vaccination in patients with chronic lymphocytic leukemia and other indolent lymphomas: A large, single-center observational study. J Natl Compr Canc Netw. 2022;20(3.5). doi:10.6004/jnccn.2021.7248 
  2. Schieszer J. Moderna COVID-19 vaccine may be more effective in CLL patients. Cancer Therapy Advisor. April 2, 2022.
  3. Nooka AK, Shanmugasundaram U, Cheedarla N, et al. Determinants of neutralizing antibody response after SARS CoV-2 vaccination in patients with myeloma. J Clin Oncol. 2022;JCO2102257. doi:10.1200/JCO.21.02257
  4. Mistry S. Moderna COVID-19 vaccine ‘preferred’ for patients with multiple myeloma. Cancer Therapy Advisor. March 22, 2022.
  5. Obeid M, Suffiotti M, Pellaton C, et al. Humoral responses against variants of concern by COVID-19 mRNA vaccines in immunocompromised patients. JAMA Oncol. 2022;e220446. doi:10.1001/jamaoncol.2022.0446
  6. Blevins Primeau AB. Half of cancer patients lost neutralizing antibodies 6 months after COVID-19 vaccination. Cancer Therapy Advisor. March 25, 2022.
  7. The Pfizer-BioNTech COVID-19 Vaccine. Pfizer and BioNTech. Accessed April 28, 2022. https://www.cvdvaccine-us.com/
  8. Moderna COVID-19 vaccine questions. US Centers for Disease Control and Prevention. Updated March 23, 2022. Accessed April 28, 2022. https://www.cdc.gov/vaccines/covid-19/info-by-product/moderna/moderna-faqs.html#vaccination-schedule-use