Rapid Response to California’s First Local Dengue: What Serosurveillance Is Already Revealing

Atlanta, Georgia, USA

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After California reported its first locally acquired dengue case in October 2023, followed quickly by additional reports, researchers at Kaiser Permanente Southern California stood up a rapid, system-wide serosurveillance program to understand the true scope of transmission and background immunity. Early results presented at IDWeek 2025 suggest dengue infections in Southern California are underrecognized and that cross-reactive flavivirus antibodies complicate interpretation—underscoring the need for sustained, targeted surveillance.

IDWeek 2025 is being held in Atlanta, Georgia, from October 19 to 22.

How the team moved fast. Beginning in November 2023, the group collected remnant sera from KPSC members with ≤7 days of febrile illness and/or residence in the same or adjacent ZIP codes as locally acquired cases. Specimens were screened by ELISA for antidengue IgM (recent infection) and IgG (past exposure). IgM-positive samples underwent confirmatory testing with dengue NS1 antigen and serotype-specific RT-PCR. Because West Nile virus (WNV) circulates widely in California and can drive serologic cross-reactivity, dengue-positive specimens were also tested for WNV IgM/IgG. Plaque reduction neutralization testing (PRNT) is in progress.

What they found. Of 15,521 remnant specimens collected, 3,990 were tested by ELISA. Among those, 14.3% (571) were DENV IgG-positive, 3.1% (123) were IgM-positive, and 1.2% (48) were positive for both IgM and IgG. Pooling all 171 IgM-positive specimens (IgM-only and IgM/IgG), 9 were confirmed by virologic assays: 4 NS1-positive, 3 RT-PCR positive for DENV-3, and 2 positive by both NS1 and DENV-3 PCR. No DENV-1, -2, or -4 were detected among IgM-positives.

Cross-reactive or coexposure patterns were striking: WNV antibodies were present in 64% of DENV IgG-positives, 37% of IgM-positives, and 85% of IgM/IgG-positives. Those findings highlight the importance of PRNT and molecular testing to distinguish true dengue infection from flavivirus cross-reactivity in regions where WNV is endemic.

“These data represent the first estimates of population immunity to DENV in Southern California and suggest underreporting of DENV infections during recent years,” the authors wrote. “These results will inform estimation of attack rates and the ratio of reported to true infections. These data also serve as a baseline to monitor changes in population seroprevalence to DENV over coming years, and the size of the population at risk for severe dengue disease due to prior infection.”

Why it matters. The team concludes these are the first population-level estimates of dengue immunity in Southern California, and they likely indicate substantial underreporting of infections. The data establish a baseline to track changes in dengue seroprevalence over time, estimate attack rates, and quantify the gap between reported and true infections—key inputs for risk communication, vector-control prioritization, and clinical testing guidance.

Session: Public Health in Transition: Navigating Emerging Infections & Data-Driven Solutions
Poster: Rapid Response to the First Locally Acquired Dengue Infections in California: Dengue Serosurveillance in Southern California
Presenting author: Sara Y. Tartof, PhD, MPH, Epidemiologist/Research Scientist III, Kaiser Permanente Southern California (KPSC)
Contributors: Gregg S. Davis, PhD; Christopher N. Mores, ScD; Jessica Skela, MS; Vennis Hong, MPH; Rudy A. Patrick, PhD; Magdalena E. Pomichowski, MA; Iris Anne C. Reyes, MPH; Madeline Kernahan, MPH; Jonathan P. Bashor, MPH; Adrienne Mackenzie, BS; Michael Aragones, MD; Alec D. Gilfillan, MS; Banshri Kapadia, MS; Adrienne Epstein, PhD (UCSF); Hui Zhou, PhD; Isabel Rodriguez-Barraquer, MD, MPH (UCSF); and Joseph Lewnard, PhD (UC Berkeley).