Influenza Vaccination and Symptom Burden in Ambulatory Patients with Confirmed Influenza: Evidence from Case-ascertained Household Transmission Studies

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BACKGROUND: The extent to which influenza vaccination may temper disease remains uncertain. We compared symptoms between influenza vaccinated and unvaccinated ambulatory persons with influenza. METHODS: We conducted household transmission studies across multiple US sites during consecutive influenza seasons from 2017-2018 through 2022-2023 (excluding 2020-2021 season with minimal influenza activity). We collected daily symptom diaries and respiratory specimens. Three scores were calculated, varying weights on specific symptoms: 1) influenza-like illness (ILI); 2) lower respiratory; and 3) all symptoms. For each score, we examined the daily mean score between those vaccinated and unvaccinated against influenza using weighted multivariable linear mixed effects and frailty models. RESULTS: Study included 869 children (41% vaccinated, 47% female, 58% white, non-Hispanic) and 653 adults (42% vaccinated, 65% female, 72% white, non-Hispanic) with laboratory-confirmed influenza. In multivariable analyses, the mean difference in the daily ILI score was -0.16 (95% CI: -0.32, -0.01) between vaccinated and unvaccinated children and -0.18 (95% CI: -0.34, -0.01) for the lower respiratory score. There was no significant difference among adults. Influenza vaccination was not associated with time to symptom alleviation among children. However, ILI symptom alleviation occurred earlier among vaccinated (HR: 1.30, 95% CI: 1.04, 1.62) adults. CONCLUSION: Ambulatory children with influenza who were vaccinated experienced less ILI symptom burden compared to those who were not vaccinated against influenza, but vaccination was not associated with symptom duration. Conversely, among ambulatory adults with influenza, vaccination and symptom burden were not associated, but vaccination was associated with faster alleviation of ILI symptoms.

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