Pfizer Inc. (PFE) might be onto something with its mRNA flu vaccine. According to results from a Phase 3 study the company funded, this next-generation shot outperformed traditional flu vaccines against the strains that actually mattered during the 2022-2023 flu season. The catch? It also caused more immediate side effects.
The trial put an experimental quadrivalent modRNA vaccine head-to-head with a standard inactivated flu shot. Researchers wanted to see if the same mRNA technology that powered COVID vaccines could deliver better protection against seasonal influenza. Spoiler alert: it did, though not without some bumps along the way.
How the Study Worked
This was no small experiment. The trial enrolled 18,476 healthy adults between ages 18 and 64, splitting them roughly evenly between the mRNA vaccine and a licensed quadrivalent flu shot that served as the control. The timing was deliberate, running during the 2022-2023 flu season when researchers could measure real-world effectiveness.
The primary objective was straightforward: count how many people developed laboratory-confirmed influenza accompanied by flu-like symptoms at least two weeks after vaccination. Researchers also tracked immune responses, side effects, and any serious adverse events over several months.
The Results Tell an Interesting Story
The mRNA vaccine achieved relative efficacy of 34.5% compared to the traditional shot. That number came from 57 confirmed influenza cases among mRNA recipients versus 87 cases in the control group. The vaccine met the statistical criteria for both noninferiority and superiority, which is research-speak for "it's at least as good, and actually better."
Most of the flu cases that season were driven by A/H3N2 and A/H1N1 strains. B strains barely showed up, which turned out to be important for interpreting the results. When researchers measured antibody responses using hemagglutination inhibition tests, the mRNA vaccine hit noninferiority thresholds against influenza A strains but fell short for B strains. That makes sense given how little B virus was circulating.
The Side Effect Trade-Off
Here's where things get real. While both vaccines produced mostly mild to moderate reactions, the mRNA candidate showed notably higher reactogenicity. Local reactions like sore arms occurred in 70.1% of mRNA recipients compared to 43.1% in the control group. Systemic symptoms hit 65.8% of mRNA recipients versus 48.7% of those who got the traditional shot.
Fever was more common too, affecting 5.6% of mRNA vaccine recipients compared to just 1.7% in the control group. Nothing dangerous, but definitely more noticeable. The researchers concluded that the mRNA vaccine delivered statistically superior efficacy and stronger immune responses to the predominant A strains, though at the cost of higher short-term reactogenicity.
What This Means for Flu Prevention
This study represents a potential shift in seasonal flu prevention. Traditional flu vaccines have been the standard for decades, but mRNA technology might offer a path to better protection, especially against the strains that cause most cases each season. The question is whether people will accept more noticeable side effects in exchange for better protection.
Meanwhile, in M&A News
The flu vaccine space is heating up. Last week, Merck & Co. Inc. (MRK) agreed to acquire Cidara Therapeutics, Inc. (CDTX) for $221.50 per share in cash, valuing the transaction at approximately $9.2 billion. Cidara's lead candidate, CD388, takes a different approach: a small molecule neuraminidase inhibitor conjugated to a proprietary Fc fragment of a human antibody, designed to prevent both influenza A and B.
PFE Price Action: Pfizer stock is up 3.13% at $25.18 at publication on Friday.
