Influenza vaccine effectiveness against outpatient acute respiratory illness with laboratory-confirmed influenza, United States, 2024-25 season

During the 2024-25 U.S. influenza season, the vaccine demonstrated moderate effectiveness of approximately 33% against outpatient medically attended influenza, with specific protection ranging from 27% to 40% across different virus subtypes.

The Gist

🛡️ The Big Picture: A Shield Against the Flu

Imagine the flu virus as a sneaky burglar trying to break into your house (your body). The flu vaccine is like a high-tech security system installed before the burglar arrives.

This study looked at how well that security system worked during the 2024–2025 flu season in the United States. The researchers wanted to know: If you got the security system (the vaccine), were you less likely to get caught by the burglar (the flu) and end up at the doctor's office?

🔍 The Detective Work: How They Studied It

The researchers acted like detectives using a clever trick called the "Test-Negative Design."

Imagine a waiting room full of people who all have a cough and a fever (a "respiratory illness").

1. Group A (The Burglars): These people tested positive for the flu. They were the "cases."
2. Group B (The False Alarms): These people had the same symptoms but tested negative for the flu. They were the "controls."

The detectives then asked everyone in the room: "Did you install the security system (get the flu shot) this year?"

By comparing how many people in Group A had the shot versus how many in Group B had the shot, they could calculate how much the vaccine reduced the risk of getting sick.

📊 The Results: How Good Was the Shield?

The study found that the 2024–25 flu vaccine was moderately effective. Here is the breakdown:

• Overall Protection: The vaccine reduced the risk of getting sick enough to visit a doctor by about one-third (33%).
  • Analogy: Think of it like wearing a raincoat in a heavy storm. It won't keep you 100% dry, but it will keep you significantly less wet than if you stood in the rain with nothing on.
• Against Different "Burglars" (Virus Types):
  • Flu A (H1N1): The vaccine was quite good here (37% effective).
  • Flu A (H3N2): This was the trickiest burglar. The vaccine offered less protection (27% effective), but still some.
  • Flu B: The vaccine worked well here too (40% effective).

👶 Who Got the Best Protection?

• Kids (Ages 8 months to 4 years): They had the best protection! The vaccine was like a super-shield for them, reducing their risk by about 50%.
• Older Adults (65+): This group had the hardest time. The vaccine didn't show a statistically significant drop in risk for them in this specific study.
  • Why? As we age, our immune systems get a bit slower to react, like an old alarm system that sometimes takes a moment to go off. Also, the H3N2 virus (the tricky burglar) is particularly good at evading older immune systems.

⏳ The "Time Bomb" Factor

The study also looked at how long the protection lasted.

• Fresh Protection: If you got the shot recently (within 2 months), the protection was strongest.
• Waning Protection: As time passed (more than 4 months), the "shield" got a little weaker.
  • Analogy: It's like a battery. A fresh battery powers your flashlight brightly. After a few months, the light gets dimmer. This is why getting the shot before the flu season starts is crucial, but getting it too early might mean the battery is running low by the time the storm hits.

🔄 The "Repeat Customer" Question

A big question was: If you got the flu shot last year, does getting it again this year help, or does your body get "tired" of it?

• The Finding: It helps! Getting the shot this year was beneficial whether you got it last year or not.
• The Analogy: Think of the flu virus as a thief who changes their disguise every year. Even if you recognized the thief last year (had the shot), they are wearing a new mask this year. Getting a new shot updates your "Wanted" poster so your immune system knows who to look for. The study showed that getting the new update didn't hurt your protection; it actually helped.

🧬 The "Genetic Match" Issue

The researchers also looked at the genetic makeup of the viruses.

• The vaccine was designed to fight specific versions of the flu.
• The viruses circulating in the wild were slightly different (they had "mutated").
• Analogy: It's like the vaccine was designed to lock a specific type of door. The burglars (viruses) slightly changed the shape of their lock picks. The vaccine still worked, but not as perfectly as if the locks hadn't changed. This "mismatch" is likely why the protection wasn't 100%.

💡 The Bottom Line

Even though the vaccine wasn't a perfect shield (it didn't stop everyone), it was still a very important tool.

• The "One-Third" Rule: By reducing the risk of getting sick by one-third, the vaccine prevented thousands of people from ending up in the hospital or dying.
• The Takeaway: Getting the flu shot is like putting on a seatbelt. It might not stop a crash from happening, but it drastically increases your chances of walking away from it without serious injury.

In short: The 2024–25 flu season was tough, but the vaccine provided a solid layer of defense, especially for children, and helped keep the overall burden of the flu lower than it would have been otherwise.

Technical Summary

1. Problem Statement

The 2024–25 influenza season in the United States was classified as high severity, with co-circulation of Influenza A(H1N1)pdm09 and A(H3N2) viruses. While annual vaccination is the primary prevention strategy, VE fluctuates based on viral antigenic drift, host factors, and time since vaccination. There was a need to quantify the protection offered by the 2024–25 vaccine against medically attended outpatient acute respiratory illness (ARI), specifically stratified by virus type/subtype, age group, time since vaccination, and prior season vaccination history. Additionally, understanding the impact of genetic mismatches between vaccine components and circulating strains (particularly A(H3N2) and B/Victoria) was critical.

2. Methodology

• Study Design: A prospective, test-negative case-control study conducted by the US Flu VE Network.
• Study Population: Outpatients aged ≥8 months presenting with mild-to-moderate ARI (symptoms including cough) within 7 days of onset.
• Sites: Seven states (Arizona, Michigan, Missouri, Ohio, Pennsylvania, Texas, Washington) across outpatient clinics, urgent care, and emergency departments.
• Data Collection Period: October 6, 2024, to May 10, 2025.
• Inclusion/Exclusion:
  • Included: 6,793 patients meeting criteria (symptoms, timing, no prior clinical flu test).
  • Excluded: Patients with prior clinical test results, those outside flu circulation periods, and those testing positive for SARS-CoV-2 (to avoid confounding due to correlated vaccination behaviors).
• Laboratory Methods:
  • Testing: Nasal/oropharyngeal swabs tested via RT-PCR for Influenza (type/subtype) and SARS-CoV-2.
  • Genetic Characterization: Whole-genome sequencing (WGS) of influenza-positive specimens (n=1,380) to determine HA clades/subclades.
  • Serology: Microneutralization (MN) assays performed on a subset of adults (n=232) to quantify antibody titers against the predominant circulating A(H3N2) virus (A/Wisconsin/154/2024).
• Vaccination Status: Determined via electronic health records (EHR), state immunization registries, or plausible self-report.
• Statistical Analysis:
  • VE estimated as $(1 - \text{adjusted Odds Ratio}) \times 100$.
  • Models: Multivariable logistic regression adjusted for study site, age, underlying health conditions, and month of illness onset.
  • Stratification: Analyzed by influenza type/subtype, age group, time since vaccination (14–60, 60–120, >120 days), and prior season (2023–24) vaccination status.

3. Key Contributions

• Comprehensive Seasonal Data: Provides the first end-of-season VE estimates for the 2024–25 US season, covering a high-severity period.
• Subtype and Age Granularity: Offers detailed VE breakdowns for A(H3N2), A(H1N1)pdm09, and B/Victoria across specific age cohorts, highlighting significant disparities in protection (e.g., high protection in young children vs. low/non-significant protection in adults ≥65).
• Prior Season Vaccination Analysis: Rigorously evaluates the "repeat vaccination" hypothesis, finding no statistically significant difference in VE between those vaccinated only in the current season versus those vaccinated in both the current and prior seasons.
• Immunological Correlates: Links serological data (antibody titers) with VE estimates, confirming that current season vaccination boosts antibody levels regardless of prior season status.
• Viral Characterization: Documents the specific genetic subclades circulating (e.g., A(H3N2) subclade J.2 and B/Victoria subclade C.3.1) and their prevalence in the study population compared to national surveillance.

4. Key Results

• Overall VE: The vaccine reduced the risk of outpatient medically attended influenza by 33% (95% CI: 24–41).
• VE by Subtype:
  • A(H3N2): 27% (95% CI: 14–39).
  • A(H1N1)pdm09: 37% (95% CI: 24–48).
  • B/Victoria: 40% (95% CI: 12–59).
• VE by Age Group:
  • Children (8 months–4 years): Highest protection. VE was 50% against any influenza, 41% against A(H3N2), and 73% against A(H1N1)pdm09.
  • Adults (≥65 years): VE was 4% (95% CI: -42 to 35), which was not statistically significant. This aligns with historical trends of lower VE against A(H3N2) in older adults.
• Time Since Vaccination: VE was highest (56%) for those vaccinated 14–59 days prior to illness onset. Protection waned over time, dropping to 16% (>120 days), though confidence intervals overlapped.
• Prior Season Vaccination:
  • Among adults, VE for current season vaccination was 33% for those unvaccinated the prior season and 17% for those vaccinated in both seasons.
  • The interaction p-value was 0.26, indicating no statistically significant difference in VE based on prior season vaccination status.
• Serology:
  • Antibody titers (GMT) were significantly higher in vaccinated controls (GMT: 44) compared to unvaccinated controls (GMT: 14).
  • Titers did not differ significantly based on prior season vaccination status, supporting the VE findings.
• Genetic Findings:
  • A(H3N2) viruses were predominantly subclade J.2 (69%).
  • B/Victoria viruses were predominantly subclade C.3.1 (43% in study vs. 12.9% nationally), suggesting a potential antigenic mismatch contributing to lower VE against B/Victoria in children.

5. Significance

• Public Health Impact: Despite being a "modestly effective" vaccine (33%), the study emphasizes that in a high-burden season (estimated 43–76 million illnesses), vaccination prevents a substantial number of medical visits, hospitalizations, and deaths.
• Clinical Guidance: The findings reinforce the recommendation for annual vaccination for all eligible individuals, particularly highlighting the strong protection in young children.
• Vaccine Policy: The lack of significant "blunting" from prior season vaccination supports the current CDC recommendation for annual vaccination regardless of previous history.
• Future Considerations: The low VE in adults ≥65 against A(H3N2) and the potential antigenic mismatch in B/Victoria subclades underscore the need for continued monitoring of viral evolution and the potential benefit of enhanced vaccines (high-dose/adjuvanted) for older adults, although VE for these specific vaccines in this season was comparable to standard doses in the analysis.
• Limitations: The study notes potential misclassification of vaccination status (though minimized by EHR data) and the lack of systematic RSV testing, though RSV co-infection was deemed unlikely to bias influenza VE estimates significantly.

Conclusion: The 2024–25 influenza vaccine provided moderate protection against outpatient illness, with efficacy varying significantly by age and virus subtype. The data supports continued annual vaccination efforts, particularly for children, while highlighting the ongoing challenge of protecting older adults against A(H3N2).