Pfizer has released the full data from its phase 3 trial of an mRNA flu vaccine candidate, with the potential new shot demonstrating 34.5% greater efficacy compared to a control vaccine.
Of 9,225 people received given Pfizer’s mRNA vaccine, 57 developed the flu, compared to 87 of the 9,251 people given Sanofi’s Fluzone shot. Pfizer published the results in the New England Journal of Medicine on Nov. 19.
Pfizer’s mRNA candidate “met the criteria for both noninferiority and superiority” against A strains of influenza, but not B strains, the authors wrote. Most of the flu cases seen in the trial were caused by A strains, which are generally more common and more severe than influenza B.
Pfizer originally announced its success against influenza A in the phase 3 study in October 2023. Moderna, too, had previously missed against influenza B with its mRNA vaccine candidate, and a combo COVID-flu shot from Pfizer and BioNTech also struggled against the B strain in phase 3.
Moderna then altered its vaccine’s formula and, in June, finally announced it had conquered influenza B.
Pfizer’s modRNA shot led to similar side effects as the Sanofi control vaccine, the authors wrote in the paper, but at a higher rate. The mRNA candidate caused 70.1% of people to have an immune reaction at the injection site, compared to 43.1% for the control, as well as whole-body reactions like fatigue and headache in 65.8% of participants compared to 48.7% in the control arm. Fever developed in 5.6% of those given the modRNA shot, compared to 1.7% of those given the control.
One patient given modRNA developed a grade 3 reaction at the injection site and grade 4 anaphylaxis, which was determined to be related to the vaccine. There were no cases of myocarditis or pericarditis, the heart-swelling side effect that has been used by opponents of vaccines to disparage mRNA vaccines.
“Frequencies of severe and life-threatening adverse events, serious adverse events, and adverse events leading to withdrawal from the trial were low and similar in the two trial groups,” the authors wrote.
Rather than using a weakened form of the flu virus or proteins from the virus, mRNA vaccines instead provide the human body with instructions to make viral proteins itself. Once the body uses the mRNA to make flu proteins, the immune system can recognize those proteins and build protection against the virus.
Since first being developed to combat the COVID-19 pandemic, mRNA vaccines have been proven to be safe, effective and cost-effective alternatives to traditional vaccines. There are currently no approved mRNA shots for flu. Still, Moderna plans to submit its candidate, mRNA-1010, for approval in the U.S., Europe, Canada and Australia by January 2026, according to a Nov. 20 announcement.