{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,11,12]],"date-time":"2026-11-12T18:06:44Z","timestamp":1794506804632,"version":"4.9.1"},"reference-count":41,"publisher":"MDPI AG","issue":"10","license":[{"start":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T00:00:00Z","timestamp":1760054400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/100020955","name":"GlaxoSmithKline Biologicals SA","doi-asserted-by":"crossref","award":["221526"],"award-info":[{"award-number":["221526"]}],"id":[{"id":"10.13039\/100020955","id-type":"DOI","asserted-by":"crossref"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Vaccines"],"abstract":"<jats:p>Background: Some studies suggest that, thanks to the mechanisms of immune-mediated attenuation, influenza vaccination reduces severity of influenza illness in breakthrough infections. This study aimed to assess whether influenza vaccination attenuates severity of laboratory-confirmed influenza among Italian adults. Methods: This secondary analysis included all influenza cases detected during respiratory surveillance studies conducted in outpatient and inpatient settings in Genoa (Italy), throughout the 2023\/2024 and 2024\/2025 seasons. Here, we compared viral load and the count of influenza-related symptoms in outpatients, alongside all-cause in-hospital mortality and radiologically confirmed pneumonia in inpatients, between vaccinated and unvaccinated adults. Results: The study included 188 influenza cases diagnosed in primary care and 281 influenza cases identified among inpatients. Of these, 37.2% and 31.7%, respectively, were vaccinated, constituting breakthrough infections. Compared to unvaccinated adults, vaccinated outpatients had a slightly lower viral load (difference in cycle threshold values of 1.36 corresponding to about 0.51 log10 reduction in the number of copies\/mL; p = 0.077), primarily driven by influenza A(H1N1)pdm09. Vaccinated outpatients also reported 9% fewer influenza-related symptoms than unvaccinated counterparts [prevalence ratio 0.91; 95% confidence interval (CI): 0.84, 0.99]. Among hospitalized older adults, influenza vaccination was associated with 64% reduced odds of in-hospital death (odds ratio 0.36; 95% CI: 0.12, 0.94). Conversely, no association between vaccination and development of pneumonia was found. Conclusions: This study corroborates the idea that influenza vaccination attenuates disease severity in breakthrough infections. These effects are, however, dependent on the measure of severity used.<\/jats:p>","DOI":"10.3390\/vaccines13101046","type":"journal-article","created":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T12:32:47Z","timestamp":1760099567000},"page":"1046","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Effect of Influenza Vaccination on the Disease Severity and Viral Load Among Adult Outpatients and Inpatients"],"prefix":"10.3390","volume":"13","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-9608-8892","authenticated-orcid":false,"given":"Alexander","family":"Domnich","sequence":"first","affiliation":[{"name":"Hygiene Unit, San Martino Polyclinic Hospital-IRCCS for Oncology and Neurosciences, 16132 Genoa, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Vincenzo","family":"Paolozzi","sequence":"additional","affiliation":[{"name":"Department of Health Sciences (DISSAL), University of Genoa, 16132 Genoa, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Giada","family":"Garzillo","sequence":"additional","affiliation":[{"name":"Department of Health Sciences (DISSAL), University of Genoa, 16132 Genoa, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2433-9610","authenticated-orcid":false,"given":"Andrea","family":"Orsi","sequence":"additional","affiliation":[{"name":"Hygiene Unit, San Martino Polyclinic Hospital-IRCCS for Oncology and Neurosciences, 16132 Genoa, Italy"},{"name":"Department of Health Sciences (DISSAL), University of Genoa, 16132 Genoa, Italy"},{"name":"Interuniversity Research Centre on Influenza and Other Transmissible Infections (CIRI-IT), 16132 Genoa, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8463-8487","authenticated-orcid":false,"given":"Giancarlo","family":"Icardi","sequence":"additional","affiliation":[{"name":"Hygiene Unit, San Martino Polyclinic Hospital-IRCCS for Oncology and Neurosciences, 16132 Genoa, Italy"},{"name":"Department of Health Sciences (DISSAL), University of Genoa, 16132 Genoa, Italy"},{"name":"Interuniversity Research Centre on Influenza and Other Transmissible Infections (CIRI-IT), 16132 Genoa, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2025,10,10]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"3199","DOI":"10.1016\/j.vaccine.2018.04.063","article-title":"Estimating the annual attack rate of seasonal influenza among unvaccinated individuals: A systematic review and meta-analysis","volume":"36","author":"Somes","year":"2018","journal-title":"Vaccine"},{"key":"ref_2","unstructured":"World Health Organization (WHO) (2022). 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