Pfizer brings mRNA flu vaccine into Phase 3 testing

Pfizer on Wednesday announced it has started a Phase 3 trial of a messenger RNA vaccine it’s developing for influenza, making the shot the second of its kind to begin late-stage testing this year.  

The study will enroll 25,000 U.S. adults 18 years or older and judge the shot’s effectiveness against influenza. The vaccine is a “quadrivalent” shot encoding for four viral strains recommended by the World Health Organization, Pfizer said.  

Vaccines based on messenger RNA technology came of age during the coronavirus pandemic, proving to be powerfully effective at preventing COVID-19’s worst health outcomes. But the reach of mRNA technology, so far, is limited. Developers aim to see whether the approach can be used to combat other infections. Seasonal influenza is now the first to be evaluated in large-scale clinical tests.   

Despite the widespread availability of multiple flu vaccines, there remains room for improvement. Flu infections cause anywhere from 140,000 to 710,000 hospitalizations and 12,000 to 52,000 deaths each year, according to the Centers for Disease Control and Prevention. Available shots are typically only 40% to 60% effective even when they’re well matched to circulating strains, leaving gaps in protection. 

mRNA vaccine developers believe they can do better. The technology underlying their shots can be mobilized more quickly than other methods, making it easier to design and adapt a shot closer to flu season. However, it’s unclear whether the approach will lead to a superior shot. Early study results from Moderna’s candidate were mixed, for instance.

Both Pfizer and Moderna now have shots in Phase 3 testing, but their studies are designed differently. While Pfizer’s study is larger and geared towards evaluating efficacy, Moderna’s trial is enrolling about 6,000 participants and is designed to test the shot’s safety and ability to spur an immune response. Moderna dosed its first study volunteer in June.  

Pfizer gained rights to its flu vaccine prospect from BioNTech, the co-developer of the companies’ COVID-19 shot Comirnaty, in a collaboration the two struck in 2018.  

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