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Confidential -Low
Interim Influenza Vaccine
Effectiveness Against Laboratory -
Confirmed Influenza, California,
October 2024 –January 2025
Sophie Zhu, PhD and Joshua Quint, PhD
California Department of Public Health
Advisory Committee on Immunization Practices Meeting
April 15, 2025
Confidential -Low
Influenza VEData Type :
Electronic records
Patient interviewsData Sources :
Hospital networks
Public health surveillance
Population :
Pediatric
AdultSetting :
Inpatient
Outpatient
Considerations for calculating vaccine effectiveness
Confidential - Low
Influenza VEData Type :
Electronic records
Patient interviewsData Sources :
Hospital networks
Public health surveillance
Population :
Pediatric
AdultSetting :
Inpatient
Outpatient
Considerations for calculating vaccine effectiveness
Confidential - Low
•January 1, 2023
•All vaccination records must be
reported to the California Immunization
Registry (CAIR)
•Previously, only certain types of providers
were required to report
•June 15, 2023:
•All influenza test results must be
reported to California Reportable Disease
Information Exchange (CalREDIE)
•Previously, only positive results were
reported
New California Data Reporting Requirements
Confidential - Low
Confidential - Low
Pediatric : 6 mo–18 yrs
2023–2024 CA estimates similar to other VE platforms
Confidential - Low
Methods
Confidential - Low
•Dates: October 1, 2024 - January 31, 2025
•Eligibility/inclusion criteria: California residents aged ≥6 months with a test result reported
through the state electronic laboratory reporting (ELR) system
•Outcome (test result): molecular (NAAT) or culture test results for influenza (A/B)
•Exposure (vaccination status): documented dose of seasonal influenza vaccine at least 14
days prior to test date
•Exclusions: duplicate test results for persons with multiple records, laboratories with greater
than 50% weekly positive results
•Analysis: VE = (1 – adjusted OR) x 100%
•Mixed -effects logistic regression model
•Adjusted for continuous age (natural cubic spline), categorical race and ethnicity; testing
week and county as random effectsCase control study (unmatched)
Confidential - Low
Number of positive influenza detections by laboratories
2024 -2025 season to date
Flu Type
Confidential - Low
•About 5% of influenza positive tests
are subtyped, mostly from public
health laboratories
•Influenza A
•53% A(H3)
•43% A(H1)
•4% A not subtyped
•Only ~300 influenza B samples
lineage typed this season
•Persons with severe disease may be
more likely to have results subtyped
Influenza subtyping
Confidential - Low
Total
N = 591,321
Influenza Positive
22.8%
Vaccinated
17.6%Not Vaccinated
82.4%Influenza Negative
77.2%
Vaccinated
25.9%Not Vaccinated
74.1%Study Population
Test Positivity and Vaccination Status
Confidential - Low
Test Negative Test Positive
Total No. 456,437 134,884
Median Age
25th percentile
75th percentile43 yrs
20 yrs
67 yrs30 yrs
11 yrs
55 yrs
Ethnicity - Hispanic or Latino 24% 29%
Race
White
Unknown
Other
Asian
Black
NHPI, AI/AN, or Multiple46%
20%
19%
8%
6%
<1%*39%
22%
23%
8%
6%
<1%*
NHPI = Native Hawaiian or Pacific Islander; AI/AN = American Indian or Alaska Native; * <1% each Study Population
Demographics
Confidential - Low
•Age group by Type (A/B)
•Influenza A subtype (H1N1/H3N2)
•Mode of administration (LAIV/non -LAIV)
•Weekly cumulative
•Month -specific by Type (A/B)Vaccine effectiveness estimates
Confidential - Low
43.7 45.6Vaccine effectiveness by age group
California 2024 –2025
VE (%)
Confidential - Low
VE (%)
Vaccine effectiveness by type and age group
California 2024 –2025
Confidential - Low
Vaccine effectiveness by influenza A subtype
California 2024 –2025
Confidential - Low
Year Vaccine Type VE (95% CI) Count Median Age (IQR)
2024 LAIV 61 (51, 69) 455 7.3 (4.9 –10.4)
2024 Not LAIV 48 (46, 50) 21,980 7.8 (4.4 –12.4)
2023 LAIV 45 (35, 54) 1,387 7.9 (5.3 –10.9)
2023 Not LAIV 52 (51, 54) 66,088 7.3 (4.2 –11.9)Live Attenuated Influenza Vaccine (LAIV)
estimates among children ages 2 –17
Confidential - Low
Positive
Test
Results
VE (%)Cumulative Influenza vaccine effectiveness
California 2024 –2025
Confidential - Low
October November December JanuaryVE (%)Vaccine effectiveness by month of influenza test
California 2024 –2025
Confidential - Low
October November December JanuaryVaccine effectiveness by month and type
California 2024 –2025VE (%)
Confidential - Low
Limitations
1.Incomplete documentation and reporting of vaccination and testing
2.Inability to assess partial/full vaccination status for children aged <9 years
3.Lack of symptom information, test setting, and outcome status (illness,
hospitalization, or death)
4.Incomplete and potentially biased reporting for influenza subtypes
5.Lack of control for other confounders (health seeking behavior, pre -existing
conditions)
Confidential - Low
Summary
1.Current seasonal influenza vaccines provide protection against
laboratory -confirmed influenza for persons aged ≥6 months
•Higher VE for influenza B & younger age groups (<18 years, 18 -49 years)
2.Expanded and improved public health data can be leveraged to
calculate timely in -season influenza effectiveness
•Useful to promote additional prevention measures prior to peak
•Prepare for increased hospital capacity
Confidential - Low
Acknowledgements
California Local Health Departments
CDPH Division of Communicable
Disease Control
•Tomás León
•Monica Sun
•Nancy J. Li
•Cynthia Yen
•Cora Hoover
•Robert Schechter
•Seema Jain
•Erin L. MurrayNCIRD Influenza Division
•Brendan Flannery
•Mark Tenforde
•Sascha Ellington
•Jessie Chung