Interim Estimates of 2025-26 Seasonal Influenza Vaccine Effectiveness

Key Takeaways

• In interim estimates, vaccine effectiveness against influenza-associated outpatient visits was 38%–41% in children and adolescents and 22%–34% in adults; effectiveness against influenza-associated hospitalization was 41% and 30%, respectively.
• Subtype-specific outpatient estimates varied, with lower reported protection against A(H3N2) than against influenza B in children and adolescents in networks with statistically significant estimates, and a separate adult influenza B estimate was reported from the VISION Network.
• The authors described ongoing circulation of antigenically drifted viruses, characterized the estimates as preliminary, and said surveillance will continue through the season.

Interim findings from a CDC MMWR report linked 2025–26 influenza vaccination with fewer influenza-associated outpatient visits and hospitalizations while viruses were still circulating nationwide.

The analysis included children, adolescents, and adults across outpatient care and inpatient settings in three U.S. surveillance networks. Outcomes centered on laboratory-confirmed influenza among patients seeking care for acute respiratory illness during the current season, capturing medically attended illness across multiple clinical settings in a single interim assessment. The report also noted that interim vaccine effectiveness was lower than in recent influenza seasons.

Investigators drew data from the New Vaccine Surveillance Network, the U.S. Flu Vaccine Effectiveness Network, and the VISION Network from September 2025 through February 2026. NVSN enrolled pediatric outpatients and inpatients, U.S. Flu VE enrolled outpatients, and VISION evaluated outpatient and inpatient encounters using medical records. The analysis used a test-negative case-control approach, comparing patients with acute respiratory illness who tested positive for influenza with those who tested negative. Multivariable logistic regression was used to estimate vaccine effectiveness for laboratory-confirmed influenza-associated outpatient visits and hospitalizations after adjustment for study site, patient age, date of illness, and other potential confounders. Together, the three systems produced interim estimates across multiple surveillance settings and patient populations.

Virologic surveillance showed that most subtyped influenza A–positive specimens were A(H3N2), and investigators described the predominant A(H3N2) and circulating B/Victoria viruses as antigenically different from vaccine strains. CDC characterization also identified subclade K as the dominant group among sequenced A(H3N2) viruses during the reporting period. Among children and adolescents in NVSN, outpatient effectiveness against A(H3N2) was 35%. Across networks, outpatient effectiveness against influenza B ranged from 45% to 71% in children and adolescents, and VISION reported 63% in adults. These subtype-specific findings showed variation by virus type, subtype, network, and age group.

The authors characterized the estimates as preliminary because influenza transmission was still underway and end-of-season values could shift as the season progressed. They also noted that unmeasured confounding might remain because factors such as underlying conditions and prior-season vaccination were not modeled. Vaccination status could have been misclassified through self-report, incomplete documentation outside medical systems, or pediatric dosing complexities. Small sample sizes and unstable model estimates also limited reporting in some strata and prevented estimation for influenza A(H1N1)pdm09. The interim findings were presented with these stated analytic constraints.

In the discussion, the report concluded that influenza vaccination was associated with a lower likelihood of influenza-associated outpatient visits and hospitalizations, even during circulation of antigenically drifted A(H3N2) viruses.

They placed that interpretation alongside the reported antigenic differences between circulating viruses and the season’s vaccine components. The report also said CDC will continue monitoring vaccine effectiveness as influenza circulation continues through the season. The closing perspective keeps the interim estimates tied to ongoing seasonal surveillance rather than a final end-of-season measure.