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From Morbidity and Mortality Weekly Report (MMWR), Vol. 74, No. 6 — Aaron M. Frutos and colleagues, CDC Influenza Division, with the CDC Influenza Vaccine Effectiveness Collaborators.
Summary
- Known before: CDC tracks flu vaccine effectiveness (VE) every season; annual vaccination is recommended for everyone eligible aged ≥ 6 months.
- New here: interim 2024–25 estimates from four networks, October 2024 to February 2025.
- Implication: the vaccine reduced flu-related outpatient visits and hospitalizations. Anyone eligible who hasn't been vaccinated should be, as long as flu is circulating.
Background
Flu viruses keep evolving, so CDC checks VE every season. Vaccination prevents hundreds of thousands of outpatient visits, tens of thousands of hospitalizations and thousands of deaths each year.
- A new vaccine formula: with no influenza B/Yamagata detected since 2020, the FDA recommended in March 2024 switching from quadrivalent (four antigens) to trivalent (three) vaccines.
- This season's viruses: 97% of positive specimens were influenza A; of subtyped A viruses, 52% were A(H3N2) and 47% A(H1N1)pdm09.
Methods
All four networks use a test-negative, case-control design: people seeking care for acute respiratory illness (ARI) who tested positive for flu are cases; those who tested negative are controls.
| Network | Patients |
|---|---|
| IVY (Investigating Respiratory Viruses in the Acutely Ill) | hospitalized adults ≥ 18 |
| NVSN (New Vaccine Surveillance Network) | children 6 months–17 years, outpatient and hospitalized |
| U.S. Flu VE | children 8 months–17 years and adults, outpatient |
| VISION | children and adults, outpatient and hospitalized |
- VE = (1 − adjusted odds ratio) × 100%, adjusted for region, age, timing and other factors.
- Vaccinated = at least one dose of the 2024–2025 vaccine ≥ 14 days before illness; people vaccinated < 14 days before, or positive for SARS-CoV-2, were excluded.
- Size: IVY 3,175 hospitalized adults; NVSN 4,611 children (2,969 outpatient, 1,642 hospitalized); U.S. Flu VE 3,344 outpatients (1,134 children, 2,210 adults); VISION 139,558 outpatient and 32,671 hospital encounters.
Results
Vaccination among controls: children 22% (VISION) to 34% (NVSN) outpatient and 27%–40% inpatient; adults 34% outpatient and 35%–39% inpatient; adults ≥ 65, 54%–59% outpatient and 45%–46% inpatient.
Effectiveness against any flu:
| Group | Outpatient visits | Hospitalization |
|---|---|---|
| Children and adolescents (< 18) | 32% (U.S. Flu VE), 59% (NVSN), 60% (VISION) | 63% (NVSN), 78% (VISION) |
| All adults (≥ 18) | 36% (U.S. Flu VE), 54% (VISION) | 41% (IVY), 55% (VISION) |
| Adults 18–64 | 37% (U.S. Flu VE), 56% (VISION) | 48% (IVY), 51% (VISION) |
| Adults ≥ 65 | 51% (VISION); U.S. Flu VE not significant (18%) | 38% (IVY), 57% (VISION) |
By subtype:
- Children: against A(H1N1)pdm09, 72% (NVSN) and 53% (U.S. Flu VE) outpatient and 63% (NVSN) against hospitalization; against A(H3N2), 42% outpatient and 55% against hospitalization (NVSN). U.S. Flu VE's outpatient H3N2 estimate was not significant (16%).
- Adults: against H1N1, 42% outpatient (U.S. Flu VE), not significant against hospitalization (IVY, 39%); against H3N2, 51% against hospitalization (IVY), not significant outpatient (U.S. Flu VE, 25%).
Virus genetics (to February 3, 2025): all 286 A(H3N2) viruses sequenced belonged to clade 2a.3a.1, which includes the vaccine strain (A/Massachusetts/18/2022). Of 158 A(H1N1)pdm09 viruses, most were in clade 5a.2a and the rest in 5a.2a.1, which includes the vaccine strain (A/Wisconsin/67/2022).
Discussion
- The vaccine worked for children, adolescents and adults. Despite more H3N2 — usually linked to lower VE — estimates matched 2023–24 and higher-VE seasons of the past 15 years, and resembled interim results from Canada (54% overall) and South America's 2024 southern-hemisphere vaccine (34% against influenza A).
- With high flu activity and severity this season, more vaccination could prevent more illness, visits, hospitalizations and deaths.
- Regional differences: H3N2 made up 67% of subtyped A specimens in U.S. Flu VE versus 48% in NVSN; U.S. Flu VE found no significant outpatient VE against H3N2, similar to 2018–19 and to Europe this season. Vaccines are reformulated every year, and VE can drop when circulating viruses drift from vaccine strains.
Limitations
- The estimates are interim; end-of-season results may differ.
- Vaccination status may be misclassified — shots at workplaces and other non-traditional sites may not reach immunization registries.
- Anyone with one dose counted as vaccinated, though children 6 months–8 years not previously vaccinated twice need 2 doses — possibly lowering VE estimates.
- Unmeasured confounding: prior vaccination, prior infection and underlying conditions weren't controlled.
Implications
Vaccination is the best way to prevent flu and flu hospitalization. Everyone eligible aged ≥ 6 months who hasn't had the 2024–2025 vaccine should get it while flu circulates locally.
Sources
Based on "Interim Estimates of 2024–2025 Seasonal Influenza Vaccine Effectiveness — Four Vaccine Effectiveness Networks, United States, October 2024–February 2025," MMWR 74(6), Centers for Disease Control and Prevention (PDF). MMWR material is in the public domain. Confidence intervals, the full author and collaborator lists, and disclosures are in the original.
Лицензия: CC0 1.0 (общественное достояние) · По материалам www.cdc.gov
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