Enrollment in DANFLU-2 trial was significantly more likely for those receiving single-page letter versus two-page letters
By Elana Gotkine HealthDay Reporter
TUESDAY, July 8, 2025 (HealthDay News) — Shorter-form messaging seems to be preferable for recruiting participants to enroll in clinical trials, according to a research letter published online June 25 in the Journal of the American Medical Association.
Niklas Dyrby Johansen, M.D., Ph.D., from Copenhagen University Hospital in Denmark, and colleagues examined whether varying digital recruitment letter layout and color scheme would impact trial enrollment in a randomized experiment conducted within the context of the DANFLU-2 (A Pragmatic Randomized Trial to Evaluate the Effectiveness of High-Dose Quadrivalent Influenza Vaccine vs Standard-Dose Quadrivalent Influenza Vaccine in Older Adults) trial. Layout and color scheme were varied in a 3×3 factorial design. Invitees were randomly assigned to receive one of three layout variants — a single-page letter, two-page letter, or enhanced two-page letter with additional colored text boxes — and to receive one of three color schemes (dark red, blue, or green). Across all letters, the text content was identical.
A total of 934,049 Danish citizens aged 65 years and older were randomly assigned to receive different letter types; letters were successfully delivered to 99.95 percent. Overall, 160,451 participants were enrolled in DANFLU-2 during the 2023 to 2024 season. The researchers found that enrollment in DANFLU-2 was significantly more likely among participants who received the single-page letter versus the two-page letter and enhanced two-page letter (16.28 percent versus 16.02 and 15.29 percent, respectively). Across the color schemes, no differences were observed. There were no interactions noted between letter layout and color scheme.
“These findings do underline the importance of carefully considering design and style of participant-facing material in clinical trials, including recruitment letters, particularly in decentralized pragmatic trials where broadly distributed, low-touch recruitment strategies are often necessary,” the authors write.
Several authors disclosed ties to pharmaceutical companies, including Sanofi, which funded the DANFLU-2 trial.
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