Why the Flu Vaccine Is Less Effective This Season

This season, the flu vaccine has shown lower effectiveness than expected, raising questions among health experts. Various factors have contributed to this reduced protection, prompting a closer look at how influenza viruses and vaccines interact each year.

Lower Than Expected Effectiveness for the 2026 Flu Vaccine

As France’s seasonal influenza vaccination campaign stretches toward its January 31, 2026 deadline, public debate has centered on one striking figure: the overall effectiveness of this year’s vaccine stands at just 36.5%, according to a report from Santé publique France. This number, while sobering, reflects longstanding challenges unique to the ever-shifting nature of the influenza virus.

The Perennial Gamble of Virus Forecasting

Each flu season sets off a months-long race among vaccine producers and global health experts — notably those coordinated by the World Health Organization (WHO) — to predict which viral strains will dominate when winter arrives. Back in February 2025, specialists made their annual projections. However, as Professor Antoine Flahault from Université Paris-Cité – Inserm points out, these decisions are essentially bets placed nearly a year in advance: “The composition is based on a forecast… carried out in February… regarding which virus will circulate next season.” This time around, the dominant clade K variant of A/H3N2, currently responsible for most cases in France, slipped past those predictions.

After the fact, says epidemiologist Flahault, “the measured effectiveness falls within expectations…on the lower end typical for seasonal flu vaccines,” often below 40%.

The Persistent Value of Vaccination

Still, even with modest protection rates, flu vaccination remains an essential public health measure. Several factors explain this decision:

• Reduces severe forms and complications, especially in those most exposed.
• Protects high-risk groups: people over 65, immunocompromised individuals or those with chronic illnesses, and pregnant women.
• Uptake is improving; more than half of French residents over age 65 have already received their shot this season—a notable rise from last year’s participation.

An Uncertain Path for the Epidemic’s Evolution

France recently saw a first peak in flu cases during the Christmas holidays. Yet experts caution that another surge could hit in late January—a scenario not unfamiliar after previous winters. “That is very possible,” notes Flahault. If no fresh spike materializes by month’s end, health authorities may soon be able to draw conclusions about the trajectory of this year’s epidemic.

In sum, despite its limited efficacy and inherent unpredictability, this season’s influenza vaccine still represents a crucial barrier against complications—especially among society’s most vulnerable. The coming weeks will determine whether France faces another wave or can finally close the book on this challenging flu season.