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Background: Adult studies have demonstrated within-season declines in influenza vaccine effectiveness (VE); data in children are limited., Methods: We conducted a prospective, test-negative study of children 6 months through 17 years hospitalized with acute respiratory illness at 7 pediatric medical centers during the 2015-2016 through 2019-2020 influenza seasons. Case-patients were children with an influenza-positive molecular test matched by illness onset to influenza-negative control-patients. We estimated VE [100% × (1 - odds ratio)] by comparing the odds of receipt of ≥1 dose of influenza vaccine ≥14 days before illness onset among influenza-positive children to influenza-negative children. Changes in VE over time between vaccination date and illness onset date were estimated using multivariable logistic regression., Results: Of 8430 children, 4653 (55%) received ≥1 dose of influenza vaccine. On average, 48% were vaccinated through October and 85% through December each season. Influenza vaccine receipt was lower in case-patients than control-patients (39% vs 57%, P < .001); overall VE against hospitalization was 53% (95% confidence interval [CI]: 46, 60%). Pooling data across 5 seasons, the odds of influenza-associated hospitalization increased 4.2% (-3.2%, 12.2%) per month since vaccination, with an average VE decrease of 1.9% per month (n = 4000, P = .275). Odds of hospitalization increased 2.9% (95% CI: -5.4%, 11.8%) and 9.6% (95% CI: -7.0%, 29.1%) per month in children ≤8 years (n = 3084) and 9-17 years (n = 916), respectively. These findings were not statistically significant., Conclusions: We observed minimal, not statistically significant within-season declines in VE. Vaccination following current Advisory Committee on Immunization Practices (ACIP) guidelines for timing of vaccine receipt remains the best strategy for preventing influenza-associated hospitalizations in children., Competing Interests: Potential conflicts of interest. J. A. E. serves as a consultant to Sanofi Pasteur (payment to self for RSV vaccines), Astra Zeneca (payment to self for coronavirus disease [COVID] vaccines), and Meissa Vaccines (payment to self for RSV vaccines), and has received research support from AstraZeneca (RSV monoclonal antibody prophylaxis), GlaxoSmithKline (GSK antivirals), Merck (CMV antivirals), Novavax and Pfizer (COVID vaccines, RSV vaccines). N. B. H. received grant funding, vaccine donations, and HAI testing from Sanofi Pasteur and grant funding from Quidel. C. J. H.’s institution receives grant support from GSK, Merck, Pfizer, and Moderna for vaccine research studies on which he is an investigator and reports honorarium for educational articles from Pediatric News and Up-to Date. F. M. M. receives research support from the Centers for Disease Control and Prevention (CDC) (Epidemiology of respiratory viruses including COVID), Pfizer (pediatric COVID-19 vaccines), Gilead (pediatric Remdesivir study) and National Institutes of Health (NIH) (Epidemiology of respiratory viruses including COVID), serves on DSMBs for Pfizer (RSV vaccines), Moderna (various vaccines), Meissa (RSV vaccines), and Virometix, and receives royalties from UpToDate for authorship. E. P. S. reports support for the present manuscript from the CDC (grant awarded to Cincinnati Children’s Hospital Medical Center (CCHMC) by the CDC for the conduct of respiratory virus surveillance through the New Vaccine Surveillance Network (NVSN); serves as a consultant to Sanofi Pasteur and receives research support from Pfizer for vaccine research studies (grant awarded to CCHMC by Pfizer for the conduct of MATISSE Maternal RSV vaccine trial) on which she is an investigator and reports a grant from the CDC (grant awarded to CCHMC by the CDC for the conduct of vaccine safety monitoring through the Clinical Immunization Safety Assessment Project [CISA]); reports Participation on a Data Safety Monitoring Board for DMID-sponsored Flu Vaccine in Pediatric Stem Cell Transplant Recipients study from National Institutes of Health, Division of Microbiology and Infectious Diseases; and reports being Chair of Pediatric Infectious Diseases Society International Affairs Committee (unpaid). J. E. S.’s institution received grant support from Merck. G. A. W. received honoraria from Merck for submission of chapters to the Merck Manual textbook and consulting fees from ReViral. L. C. S. and J. A. B. report support for the present manuscript from the CDC (cooperative agreement between Texas Children’s Hospital, Baylor College of Medicine, and CDC to conduct surveillance for respiratory illness through participation in the New Vaccine Surveillance Network [NVSN]) and grants or contracts outside of the submitted work from the CDC (participation in Overcoming COVID-19 network to determine COVID-19 vaccine effectiveness; payment to institution via subcontract through Boston Children’s Hospital). J. V. W. reports compensation paid to self for participation on a Scientific Advisory Board for Quidel and a Data Safety Monitoring Board for GlaxoSmithKline. M. A. S. reports grants or contracts paid to the institution outside of the submitted work from the NIH and payment or honoraria to self for International Adoption articles from Up-to-Date. M. M. M. reports grant or contracts outside of the submitted work to the institution from Sanofi and PATH. E. J. K. reports support for the present manuscript from CDC–New Vaccine Surveillance Network (NVSN) to Seattle Children’s Hospital. P. G. S. reports grants or contracts outside of the submitted work from the CDC (grant number 1 U01 IP001158-01). G. A. W. reports research support to the institution for the current work from the US PHS/CDC and reports payments to self for writing textbook chapters in the Merck Manual – no overlap with content of the current work from Merck & Companies. R. S. reports grant or contracts from the CDC outside of the submitted work. V. A. reports support for the present manuscript from CDC–NVSN. P. A. P. reports grants or contracts from Sanofi-Pasteur outside of the submitted work. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed., (© The Author(s) 2022. Published by Oxford University Press on behalf of Infectious Diseases Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Background: Parent-reported influenza vaccination history may be valuable clinically and in influenza vaccine effectiveness (VE) studies. Few studies have assessed the validity of parental report among hospitalized children., Methods: Parents of 2597 hospitalized children 6 months-17 years old were interviewed from November 1, 2015 to June 30, 2016, regarding their child's sociodemographic and influenza vaccination history. Parent-reported 2015-2016 influenza vaccination history was compared with documented vaccination records (considered the gold standard for analysis) obtained from medical records, immunization information systems, and providers. Multivariable logistic regression analyses were conducted to determine potential factors associated with discordance between the 2 sources of vaccination history. Using a test-negative design, we estimated VE using vaccination history obtained through parental report and documented records., Results: According to parental report, 1718 (66%) children received the 2015-2016 influenza vaccine, and of those, 1432 (83%) had documentation of vaccine receipt. Percent agreement was 87%, with a sensitivity of 96% (95% confidence interval [CI], 95%-97%) and a specificity of 74% (95% CI, 72%-77%). In the multivariable logistic regression, study site and child's age 5-8 years were significant predictors of discordance. Adjusted VE among children who received ≥1 dose of the 2015-2016 influenza vaccine per parental report was 61% (95% CI, 43%-74%), whereas VE using documented records was 55% (95% CI, 33%-69%)., Conclusions: Parental report of influenza vaccination was sensitive but not as specific compared with documented records. However, VE against influenza-associated hospitalizations using either source of vaccination history did not differ substantially. Parental report is valuable for timely influenza VE studies., (Published by Oxford University Press on behalf of The Journal of the Pediatric Infectious Diseases Society 2020.)

Background: Annual United States (US) estimates of influenza vaccine effectiveness (VE) in children typically measure protection against outpatient medically attended influenza illness, with limited data evaluating VE against influenza hospitalizations. We estimated VE for preventing laboratory-confirmed influenza hospitalization among US children., Methods: We included children aged 6 months-17 years with acute respiratory illness enrolled in the New Vaccine Surveillance Network during the 2015-2016 influenza season. Documented influenza vaccination status was obtained from state immunization information systems, the electronic medical record, and/or provider records. Midturbinate nasal and throat swabs were tested for influenza using molecular assays. We estimated VE as 100% × (1 - odds ratio), comparing the odds of vaccination among subjects testing influenza positive with subjects testing negative, using multivariable logistic regression., Results: Of 1653 participants, 36 of 707 (5%) of those fully vaccinated, 18 of 226 (8%) of those partially vaccinated, and 85 of 720 (12%) of unvaccinated children tested positive for influenza. Of those vaccinated, almost 90% were documented to have received inactivated vaccine. The majority (81%) of influenza cases were in children ≤ 8 years of age. Of the 139 influenza-positive cases, 42% were A(H1N1)pdm09, 42% were B viruses, and 14% were A(H3N2). Overall, adjusted VE for fully vaccinated children was 56% (95% confidence interval [CI], 34%-71%) against any influenza-associated hospitalization, 68% (95% CI, 36%-84%) for A(H1N1)pdm09, and 44% (95% CI, -1% to 69%) for B viruses., Conclusions: These findings demonstrate the importance of annual influenza vaccination in prevention of severe influenza disease and of reducing the number of children who remain unvaccinated or partially vaccinated against influenza., (Published by Oxford University Press on behalf of The Journal of the Pediatric Infectious Diseases Society 2020.)