992. 2016–2017 Influenza Burden of Disease and End-of-Season Influenza Vaccine Effectiveness (VE) Estimates for Preventing Influenza-Related Hospitalization Among Canadian Adults: An Analysis From the Canadian Immunization Research Network (CIRN) Serious Outcomes Surveillance (SOS) Network

Abstract Background To inform public health decision making around influenza prevention and treatment, ongoing surveillance of the influenza burden of disease and assessment of influenza vaccine effectiveness (VE) is critical. The Canadian Immunization Research Network (CIRN) Serious Outcomes Surveillance (SOS) Network conducts active surveillance each influenza season to characterize the burden of influenza disease and to provide estimates of influenza VE to prevent influenza-related hospitalization in Canadian adults (≥16 years of age). Methods Active surveillance for influenza was conducted at 13 hospitals in four provinces beginning on November 15, 2016 and ending April 30, 2017. Patients admitted with any respiratory diagnosis or symptom were eligible for enrolment. Eligible patients had a nasopharyngeal swab collected and tested for influenza using polymerase chain reaction (PCR). Patients who tested positive for influenza were considered cases; patients who tested negative for influenza were eligible to become matched controls. Detailed demographic and medical information were obtained from the medical record. Influenza VE was estimated as 1 − odds ratio (OR) of influenza in vaccinated vs. unvaccinated patients × 100% using conditional logistic regression, with corresponding 95% confidence intervals (CIs). Results A total of 1,431 influenza cases were enrolled; the majority were influenza A (n = 1,299) and 100% of patients with known influenza A subtype were A/H3N2. Among all influenza cases, 144 (10.1%) patients were admitted to the intensive care unit (ICU) and 91 (6.4%) patients died within 30 days of discharge. Overall adjusted influenza VE for prevention of influenza-related hospitalization in all ages was 23.3% (95% CI: 2.9–39.4%), with slightly lower VE observed in patients ≥65 years (VE: 19.4%; 95% CI: −7.8–39.8%) and higher VE observed in patients <65 years (VE: 47.9%; 95% CI: 9.9–69.9%). Conclusion Overall, influenza VE was low but effective (VE: 23%) for preventing influenza-related hospitalization during the 2016–2017 season in Canada. Given the low influenza VE observed, continued assessment of influenza VE is crucial to inform immunization policy in Canada and to emphasize the importance of the development and utilization of improved influenza vaccines. Disclosures M. K. Andrew, GSK: Grant Investigator, Research grant. Pfizer: Grant Investigator, Research grant. sanofi pasteur: Grant Investigator, Research grant. T. F. Hatchette, GSK: Grant Investigator, Research grant. Pfizer: Grant Investigator, Research grant. Abbvie: Consultant, Speaker honorarium. J. McElhaney, GSK: Scientific Advisor, Speaker honorarium. sanofi pasteur: Scientific Advisor, Speaker honorarium. A. McGeer, GSK: Grant Investigator, Research grant. Hoffman La Roche: Grant Investigator, Research grant. sanofi pasteur: Grant Investigator, Research grant. A. Poirier, sanofi pasteur: Investigator, Research grant. Actelion: Grant Investigator, Research grant. J. Powis, GSK: Grant Investigator, Research grant. Merck: Grant Investigator, Research grant. Roche: Grant Investigator, Research grant. Synthetic Biologics: Investigator, Grant recipient. M. Semret, GSK: Grant Investigator, Research grant. Pfizer: Grant Investigator, Research grant. S. Trottier, CIHR: Grant Investigator, Research grant. S. A. McNeil, GSK: Grant Investigator, Research grant. Pfizer: Grant Investigator, Research grant. Merck: Collaborator and Consultant, Contract clinical trials and Speaker honorarium. Novartis: Collaborator, Contract clinical trials. sanofi pasteur: Collaborator, Contract clinical trials.

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150. Relative Effectiveness of High-Dose and Standard-Dose Influenza Vaccine Against Influenza-Related Hospitalization Among Older Adults—United States, 2015–2017

Open Forum Infectious Diseases ◽

10.1093/ofid/ofy209.020 ◽

2018 ◽

Vol 5 (suppl_1) ◽

pp. S10-S10

Author(s):

Joshua Doyle ◽

Lauren Beacham ◽

Elif Alyanak ◽

Manjusha Gaglani ◽

Emily T Martin ◽

...

Keyword(s):

United States ◽

Older Adults ◽

Influenza Vaccine ◽

Influenza A ◽

Standard Dose ◽

Relative Effectiveness ◽

The United States ◽

Vaccine Effectiveness ◽

High Dose ◽

Research Grant

Abstract Background Seasonal influenza causes substantial morbidity and mortality, and older adults are disproportionately affected. Newer vaccines have been developed for use in people 65 years and older, including a trivalent inactivated vaccine with a 4-fold higher dose of antigen (IIV-HD). In recent years, the use of IIV-HD has increased sufficiently to evaluate its effectiveness compared with standard-dose inactivated influenza vaccines (IIV-SD). Methods Hospitalized patients with acute respiratory illness were enrolled in an observational vaccine effectiveness study at 8 hospitals in 4 states participating in the United States Hospitalized Adult Influenza Vaccine Effectiveness Network during the 2015–2016 and 2016–2017 influenza seasons. Predominant influenza A virus subtypes were H1N1 and H3N2, respectively, during these seasons. All enrolled patients were tested for influenza virus with polymerase chain reaction. Receipt and type of influenza vaccine was determined from electronic records and chart review. Odds of laboratory-confirmed influenza were compared among vaccinated and unvaccinated patients. Relative odds of laboratory-confirmed influenza were determined for patients who received IIV-HD or IIV-SD, and adjusted for potential confounding variables via logistic regression. Results Among 1,744 enrolled patients aged ≥ 65 years, 1,105 (63%) were vaccinated; among those vaccinated, 621 (56%) received IIV-HD and 484 (44%) received IIV-SD. Overall, 315 (18%) tested positive for influenza, including 97 (6%) who received IIV-HD, 86 (5%) who received IIV-SD, and 132 (8%) who were unvaccinated. Controlling for age, race, sex, enrollment site, date of illness, index of comorbidity, and influenza season, the adjusted odds of influenza among patients vaccinated with IIV-HD vs. IIV-SD were 0.72 (P = 0.06, 95% CI: 0.52 to 1.01). Conclusion Comparison of high-dose vs. standard-dose vaccine effectiveness during 2 recent influenza seasons (1 H1N1 and 1 H3N2-predominant) suggested relative benefit (nonsignificant) of high-dose influenza vaccine in protecting against influenza-associated hospitalization among persons aged 65 years and older; additional years of data are needed to confirm this finding. Disclosures H. K. Talbot, sanofi pasteur: Investigator, Research grant. Gilead: Investigator, Research grant. MedImmune: Investigator, Research grant. Vaxinnate: Safety Board, none. Seqirus: Safety Board, none.

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988. Effectiveness of Seasonal Influenza Vaccines Against Influenza A(H3N2) Illness Among Children Aged <18 Years, US Flu VE Network, 2010–2018

Open Forum Infectious Diseases ◽

10.1093/ofid/ofy210.825 ◽

2018 ◽

Vol 5 (suppl_1) ◽

pp. S292-S292

Author(s):

Brendan L Flannery ◽

Jessie Chung ◽

Michael L Jackson ◽

Lisa A Jackson ◽

Arnold S Monto ◽

...

Keyword(s):

Influenza Vaccine ◽

Influenza A ◽

The United States ◽

Vaccine Effectiveness ◽

Influenza Viruses ◽

Older Children ◽

Research Support ◽

Illness Onset ◽

Negative Controls ◽

Research Grant

Abstract Background Interim estimates of 2017–2018 influenza vaccine effectiveness (VE) against influenza A(H3N2)-related illness in the United States indicated better protection among young children than among older children and adolescents. We examined VE against influenza A(H3N2) illness during five A(H3N2)-predominant seasons from 2010–2011 through 2016–2017 to investigate differences between VE among younger vs. older children. Methods We analyzed data from 11,736 outpatients aged <18 years with medically attended acute respiratory illnesses enrolled at US Flu VE Network study sites during five influenza A(H3N2)-predominant seasons. Respiratory specimens from all enrollees were tested for influenza viruses using reverse transcription PCR. Children with documented receipt of the recommended number of doses of current season inactivated influenza vaccine at least 14 days before illness onset were considered fully vaccinated; partially vaccinated children and those who received live attenuated influenza vaccine were excluded. Vaccine effectiveness was estimated as 100 × (1 – adjusted odds ratio) from multivariable logistic regression adjusting for study site, age, sex, presence of high-risk medical conditions, and days from illness onset to enrollment comparing odds of vaccination among A(H3N2)-positive cases vs. influenza-negative controls. Results A total of 1,854 influenza A(H3N2) cases and 9,882 influenza-negative controls were included; 494 (28%) influenza A(H3N2) cases and 3,637 (41%) controls were fully vaccinated before illness onset. VE ranged from 26% (95% confidence interval [CI], −17% to 53%) to 60% (38%–75%) among children aged 6 months–4 years and from 9% (−16% to 29%) to 66% (37%–82%) among 5–17 year olds (figure). During 2012–2013 and 2014–2015, A(H3N2) VE estimates were significantly higher among younger compared with older children (P < 0.05); in other seasons before 2017–2018, A(H3N2) VE estimates were similar among younger and older children. Conclusion Higher VE against A(H3N2) viruses in younger vs. older children in some seasons suggests immunologic differences in response to vaccine components. Overall, inactivated influenza vaccine provided moderate protection against A(H3N2)-related illness among children. Disclosures M. L. Jackson, sanofi pasteur: Grant Investigator, Research support. L. A. Jackson, Novartis: Grant Investigator, Research support. R. K. Zimmerman, sanofi pasteur: Grant Investigator, Research support. Pfizer: Grant Investigator, Research support. Merck: Grant Investigator, Research support. M. P. Nowalk, Merck: Grant Investigator, Research support. Pfizer: Grant Investigator, Research support. M. R. Griffin, MedImmune: Grant Investigator, Research support. H. K. Talbot, sanofi pasteur: Investigator, Research grant. Gilead: Investigator, Research grant. MedImmune: Investigator, Research grant. Vaxinnate: Safety Board, none. Seqirus: Safety Board, none. J. J. Treanor, Novartis: Board Member and Consultant, Consulting fee.

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Epidemiology of Influenza Viruses in Canada over the 2011–2012 to 2013–2014 Seasons: A Study from the Serious Outcomes Surveillance (SOS) Network of the Canadian Immunization Research Network (CIRN)

Open Forum Infectious Diseases ◽

10.1093/ofid/ofx163.735 ◽

2017 ◽

Vol 4 (suppl_1) ◽

pp. S314-S314

Author(s):

May Elsherif ◽

Todd Hatchette ◽

Jason Leblanc ◽

Lingyun Ye ◽

Melissa K Andrew ◽

...

Keyword(s):

Clinical Trials ◽

Health Research ◽

Influenza A ◽

Respiratory Illness ◽

Influenza Viruses ◽

Research Network ◽

Influenza B ◽

Acute Respiratory Illness ◽

Grant Recipient ◽

Research Grant

Abstract Background Influenza virus activity varies seasonally and within season. Epidemiology of serious influenza outcomes is contingent on the prevalent circulating strain/s and susceptible age group/s. Given the strain variability over the 2011–2012 through 2013–2014 seasons in Canada, this study examined the clinical and epidemiological profiles of different influenza strains causing adult hospitalizations. Methods During these three influenza seasons, the Serious Outcomes Surveillance (SOS) Network of the Canadian Immunization Research Network (CIRN) enrolled adults hospitalized with acute respiratory illness across Canada. Nasopharyngeal swabs (NPs) from influenza cases were tested for strain characterization using real-time reverse transcriptase polymerase chain reaction (rtRT-PCR). A primary assay differentiated A and B influenza viruses. Subsequently, influenza A viruses were subtyped as H1N1 or H3N2, and influenza B lineages were differentiated as Victoria or Yamagata. Laboratory results were compared with patient demographic data and clinical outcomes. Results Over three consecutive influenza seasons, 3394 cases of hospitalized acute respiratory illness were laboratory-confirmed as influenza. At 72.4%, influenza A was predominant across all seasons, while influenza B caused 27.6%. Most of the influenza A cases were due to H3N2 (58.7%), while H1N1 accounted for 41.3%. For influenza B, the Yamagata lineage was predominant at 88.4% whereas the Victoria lineage accounted for 11.6%. Outcome analyses are presented for each influenza A subtype and influenza B lineage, overall and per season. Considering serious outcomes in patients ≥65, higher proportions of patients hospitalized with the H1N1 strain experienced intensive care unit (ICU) admission and need for mechanical ventilation, while higher proportions of patients hospitalized with B/Yamagata and H3N2 died within 30 days of admission. Conclusion Comprehensive collection of surveillance data paired with NP specimens by the CIRN SOS Network was conducive to broader understanding of influenza strain activity and associated outcomes at the subtype and lineage level. This data is important to make informed recommendations for the use of multicomponent influenza vaccines. Disclosures M. Elsherif, Canadian Institutes of Health Research: Investigator, Research grant. Public Health Agency of Canada: Investigator, Research grant. GSK: Investigator, Research grant. T. Hatchette, GSK: Grant Investigator, Grant recipient; Pfizer: Grant Investigator, Grant recipient. Abbvie: Speaker for a talk on biologics and risk of TB reactivation, Speaker honorarium.M. K. Andrew, GSK: Grant Investigator, Research grant. Pfizer: Grant Investigator, Research grant. Sanofi-Pasteur: Grant Investigator, Research grant. J. McElhaney, GSK Vaccines: Scientific Advisor, Speaker honorarium. A. Mcgeer, Hoffman La Roche: Investigator, Research grant. GSK: Investigator, Research grant. sanofi pasteur: Investigator, Research grant. J. Powis, Merck: Grant Investigator, Research grant. GSK: Grant Investigator, Research grant. Roche: Grant Investigator, Research grant. Synthetic Biologicals: Investigator, Research grant. M. Semret, GSK: Investigator, Research grant. Pfizer: Investigator, Research grant. S. Trottier, Canadian Institutes of Health Research: Investigator, Research grant. L. Valiquette, GSK: Investigator, Research grant. S. McNeil, GSK: Contract Clinical Trials and Grant Investigator, Research grant. Merck: Contract Clinical Trials and Speaker’s Bureau, Speaker honorarium. Novartis: Contract Clinical Trials, No personal renumeration. sanofi pasteur: Contract Clinical Trials, No personal renumeration

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Influenza Vaccine Effectiveness and Waning Effect in Hospitalized Older Adults. Valencia Region, Spain, 2018/2019 Season

International Journal of Environmental Research and Public Health ◽

10.3390/ijerph18031129 ◽

2021 ◽

Vol 18 (3) ◽

pp. 1129

Author(s):

Ainara Mira-Iglesias ◽

F. Xavier López-Labrador ◽

Javier García-Rubio ◽

Beatriz Mengual-Chuliá ◽

Miguel Tortajada-Girbés ◽

...

Keyword(s):

Older Adults ◽

Influenza Vaccine ◽

Influenza A ◽

Vaccine Effectiveness ◽

Surveillance Network ◽

Adjuvanted Vaccine ◽

Clinical Registries ◽

Populations At Risk ◽

Influenza A H1n1 ◽

Hospitalized Older Adults

Influenza vaccination is annually recommended for specific populations at risk, such as older adults. We estimated the 2018/2019 influenza vaccine effectiveness (IVE) overall, by influenza subtype, type of vaccine, and by time elapsed since vaccination among subjects 65 years old or over in a multicenter prospective study in the Valencia Hospital Surveillance Network for the Study of Influenza and other Respiratory Viruses (VAHNSI, Spain). Information about potential confounders was obtained from clinical registries and/or by interviewing patients and vaccination details were only ascertained by registries. A test-negative design was performed in order to estimate IVE. As a result, IVE was estimated at 46% (95% confidence interval (CI): (16%, 66%)), 41% (95% CI: (−34%, 74%)), and 45% (95% CI: (7%, 67%)) against overall influenza, A(H1N1)pdm09 and A(H3N2), respectively. An intra-seasonal not relevant waning effect was detected. The IVE for the adjuvanted vaccine in ≥75 years old was 45% (2%, 69%) and for the non-adjuvanted vaccine in 65–74 years old was 59% (−16%, 86%). Thus, our data revealed moderate vaccine effectiveness against influenza A(H3N2) and not significant against A(H1N1)pdm09. Significant protection was conferred by the adjuvanted vaccine to patients ≥75 years old. Moreover, an intra-seasonal not relevant waning effect was detected, and a not significant IVE decreasing trend was observed over time.

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Should Sex Be Considered an Effect Modifier in the Evaluation of Influenza Vaccine Effectiveness?

Open Forum Infectious Diseases ◽

10.1093/ofid/ofy211 ◽

2018 ◽

Vol 5 (9) ◽

Cited By ~ 6

Author(s):

Catharine Chambers ◽

Danuta M Skowronski ◽

Caren Rose ◽

Gaston De Serres ◽

Anne-Luise Winter ◽

...

Keyword(s):

Older Adults ◽

Sex Differences ◽

Confidence Interval ◽

Influenza Vaccine ◽

Influenza A ◽

Vaccine Effectiveness ◽

Absolute Difference ◽

Effect Modifier ◽

Potential Interactions

Abstract We investigated sex as a potential modifier of influenza vaccine effectiveness (VE) between 2010–2011 and 2016–2017 in Canada. Overall VE was 49% (95% confidence interval [CI], 43% to 55%) for females and 38% (95% CI, 28% to 46%) for males (absolute difference [AD], 11%; P = .03). Sex differences were greatest for influenza A(H3N2) (AD, 17%; P = .07) and B(Victoria) (AD, 20%; P = .08) compared with A(H1N1)pdm09 (AD, 10%; P = .19) or B(Yamagata) (AD, –3%; P = .68). They were also more pronounced in older adults ≥50 years (AD, 19%; P = .03) compared with those <20 years (AD, 4%; P = .74) or 20–49 years (AD, –1%; P = .90) but with variation by subtype/lineage. More definitive investigations of VE by sex and age are warranted to elucidate these potential interactions.

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Effects of Prior Influenza Exposure on Immunogenicity of Influenza Vaccine

Open Forum Infectious Diseases ◽

10.1093/ofid/ofaa181 ◽

2020 ◽

Author(s):

Jia-Qian Cao ◽

Peng-Fei Jin ◽

Zhao-Zhun Zeng ◽

Li Zhang ◽

Fan-Yue Meng ◽

...

Keyword(s):

Clinical Trials ◽

Influenza Virus ◽

Immune Responses ◽

Influenza Vaccine ◽

Hemagglutination Inhibition ◽

Cross Reactivity ◽

Vaccine Effectiveness ◽

Prior Exposure ◽

Post Vaccination ◽

Blood Samples

Abstract Background To investigate effects of prior influenza exposure on vaccine-elicited humor immune responses to circulating influenza variants. Method We randomly selected 360 participants in previous clinical trials stratified by age. Blood samples before and 28 days after vaccination were collected and tested by hemagglutination-inhibition tests against both vaccine strains and circulating variants during the 2015–2016 influenza seasons in China. The antigenic map was plotted and antigenic distance was calculated. Results Subjects with H1-priming had higher cross-reactive antibodies titers against A/JiangsuTinghu/11019/2015(H3N2) compared with subjects with B-priming did (Padjusted=0.038). Subjects with H1-priming also had higher cross-reactive antibodies titers against A/Jiangsu Qinhuai/11059/2015(H3N2) than subjects with both H1 and B priming did (Padjusted=0.036). Nevertheless, subjects with no H1 and B-priming had higher cross-reactive antibodies titers against A/Jiangsu Qinhuai/11059/2015(H3N2) than subjects with both H1 and B priming did (Padjusted=0.012). Antigenic distance was well-matched with serological results. Besides, age-specific differences in human post-vaccination responses against the identical circulating strain was noted. And children had most cross-reactive response to both H3N2 and B-yamagata subtypes. Conclusion Our results suggest that prior exposure to H1 or B influenza virus may influence cross-reactivity of H3-specific post-vaccination responses and consequently could influence the vaccine effectiveness. Our findings also support that there are age-specific differences in human post-vaccination responses.

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Evaluation of Influenza Vaccine Effectiveness Among Young Children Receiving Consecutive Versus Nonconsecutive Vaccination During Influenza A(H3N2)-Predominant Seasons

Journal of the Pediatric Infectious Diseases Society ◽

10.1093/jpids/piaa080 ◽

2020 ◽

Author(s):

Suchitra Rao ◽

Angela Moss ◽

Molly M Lamb ◽

Edwin J Asturias

Keyword(s):

Influenza Vaccine ◽

Influenza A ◽

Care Setting ◽

Case Control ◽

Vaccine Effectiveness ◽

Urgent Care ◽

Control Analysis ◽

Negative Case ◽

Case Control Analysis ◽

Urgent Care Setting

Abstract A test-negative case-control analysis of 1478 children aged 6 months to 8 years of age seeking care at an emergency/urgent care setting with influenza like illness during the 2016-17 and 2018-19 (H3N2 predominant) influenza seasons demonstrated that influenza vaccine effectiveness did not vary significantly by the prior seasons’ vaccination status. Clinical Trials Registration NCT02979626.

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Influenza Vaccine Effectiveness by A(H3N2) Phylogenetic Subcluster and Prior Vaccination History: 2016–2017 and 2017–2018 Epidemics in Canada

The Journal of Infectious Diseases ◽

10.1093/infdis/jiaa138 ◽

2020 ◽

Author(s):

Danuta M Skowronski ◽

Siobhan Leir ◽

Suzana Sabaiduc ◽

Catharine Chambers ◽

Macy Zou ◽

...

Keyword(s):

Confidence Interval ◽

Influenza Vaccine ◽

Influenza A ◽

Vaccine Effectiveness ◽

Glycosylation Sites ◽

Vaccination History ◽

H3n2 Vaccine

Abstract Background The influenza A(H3N2) vaccine was updated from clade 3C.3a in 2015–2016 to 3C.2a for 2016–2017 and 2017–2018. Circulating 3C.2a viruses showed considerable hemagglutinin glycoprotein diversification and the egg-adapted vaccine also bore mutations. Methods Vaccine effectiveness (VE) in 2016–2017 and 2017–2018 was assessed by test-negative design, explored by A(H3N2) phylogenetic subcluster and prior season’s vaccination history. Results In 2016–2017, A(H3N2) VE was 36% (95% confidence interval [CI], 18%–50%), comparable with (43%; 95% CI, 24%–58%) or without (33%; 95% CI, −21% to 62%) prior season’s vaccination. In 2017–2018, VE was 14% (95% CI, −8% to 31%), lower with (9%; 95% CI, −18% to 30%) versus without (45%; 95% CI, −7% to 71%) prior season’s vaccination. In 2016–2017, VE against predominant clade 3C.2a1 viruses was 33% (95% CI, 11%–50%): 18% (95% CI, −40% to 52%) for 3C.2a1a defined by a pivotal T135K loss of glycosylation; 60% (95% CI, 19%–81%) for 3C.2a1b (without T135K); and 31% (95% CI, 2%–51%) for other 3C.2a1 variants (with/without T135K). VE against 3C.2a2 viruses was 45% (95% CI, 2%–70%) in 2016–2017 but 15% (95% CI, −7% to 33%) in 2017–2018 when 3C.2a2 predominated. VE against 3C.2a1b in 2017–2018 was 37% (95% CI, −57% to 75%), lower at 12% (95% CI, −129% to 67%) for a new 3C.2a1b subcluster (n = 28) also bearing T135K. Conclusions Exploring VE by phylogenetic subcluster and prior vaccination history reveals informative heterogeneity. Pivotal mutations affecting glycosylation sites, and repeat vaccination using unchanged antigen, may reduce VE.

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Influenza Vaccine Effectiveness Among Children for the 2017–2018 Season

Journal of the Pediatric Infectious Diseases Society ◽

10.1093/jpids/piz077 ◽

2019 ◽

Vol 9 (4) ◽

pp. 468-473 ◽

Cited By ~ 2

Author(s):

Lauren N Powell ◽

Rodolfo E Bégué

Keyword(s):

Influenza Vaccine ◽

Influenza A ◽

Influenza Season ◽

Vaccine Effectiveness ◽

Influenza B ◽

Immunization Registry ◽

Previous Season ◽

Waning Immunity ◽

Small Benefit ◽

Susceptible Populations

Abstract Background The 2017–2018 influenza season was of high severity. Circulating influenza strains change periodically, making it important to determine vaccine effectiveness on an annual basis, especially for susceptible populations. The primary aim of our study was to estimate the effectiveness of the influenza vaccine among children. Secondary aims were to assess the effect of previous season vaccination and intraseasonal waning of immunity. Methods Children 6 months to 17 years of age tested for influenza during the 2017–2018 season were included. Clinical charts were reviewed, and immunization status was confirmed via the Louisiana Immunization Registry. Influenza vaccine effectiveness (IVE) was estimated in a test-negative design by comparing vaccination status of influenza-positive vs influenza-negative cases. Results A total of 3595 children were included, 26% of whom tested positive for influenza, mostly type A (79%); 15% had received an influenza vaccine prior to illness: 8% among the influenza-positive and 17% among influenza-negative cases (P <.0001). IVE for the 2017–2018 influenza season was 52% overall (95% confidence interval, 38%–62%), 49% for influenza A, and 60% for influenza B. While receiving current year (2017–2018) vaccine had the most effect, receiving the previous year (2016–2017) vaccine had a small benefit and no interference. We found no evidence of waning immunity of the vaccine for the 2017–2018 season. Conclusions IVE was moderate for children. Previous year vaccination had a small but significant benefit and there was no evidence of waning immunity in our cohort. Ongoing national and local surveillance is important to understand the benefit of influenza vaccination.

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