FLUNITY-HD1 Trial: Rationale for Individual-Level Pooled Analysis

A pooled analysis of two pragmatic trials has found that high-dose inactivated influenza vaccines provide superior protection compared to standard-dose vaccines in preventing hospitalizations for severe respiratory infections among older adults.

The findings, detailed in the FLUNITY-HD study and published in The Lancet, indicate that the higher dose is more effective at reducing severe clinical outcomes in the elderly population.

Study Composition and Population

FLUNITY-HD was designed as an individual-level pooled analysis that combined data from two methodologically harmonized, individually randomized trials conducted in Denmark and Spain.

The two component trials targeted specific older adult demographics:

• DANFLU-2 included adults aged 65 years or older.
• GALFLU included community-dwelling adults aged 65-79 years.

By integrating these data sets, researchers were able to compare the effectiveness of the high-dose inactivated influenza vaccine (HD-IIV) against the standard-dose inactivated influenza vaccine (SD-IIV).

Methodological Framework

In a correspondence published on April 11, 2026, the authors of the study clarified the statistical approach used to reach their conclusions. They noted that the DANFLU-2 and GALFLU trials were prospectively harmonized, meaning they shared a consistent structural design from the outset.

This harmonization included the use of the same protocol framework, the same endpoint hierarchy and variable definitions based on the 10th edition of the International Classification of Diseases (ICD-10).

Because of these shared standards, the researchers determined that an individual-level pooled analysis was the most statistically efficient method for analyzing the collective data.

It can reasonably be argued that the FLUNITY-HD trial more closely resembles a single pragmatic trial conducted across two sites than a conventional individual participant data meta-analysis of independent studies.

The Lancet

Clinical Implications for Older Adults

The primary objective of the FLUNITY-HD analysis was to evaluate the vaccine’s ability to prevent hospitalizations. The results demonstrate that the high-dose version provides a higher level of protection against the need for hospital care resulting from severe respiratory infections.

Beyond the primary focus on respiratory hospitalizations, the FLUNITY-HD data set has also been used for secondary analyses. One such investigation focused on the relative impact of high-dose versus standard-dose vaccines on cardiovascular outcomes among the same study population.

The use of pragmatic trials—which observe how interventions perform in real-world clinical practice—suggests that the benefits of the high-dose vaccine are applicable to broader community-dwelling populations of older adults.