Current Weekly Influenza Update



2024 - 2025 Weekly Influenza Update: 09/29/2024 - 05/17/2025 (Week 40 - 20)
Fresno County Department of Public Health

The 2024 - 2025 influenza season began on 09/29/2024 (week 40). This surveillance report monitors influenza activity and is updated weekly through the end of the 2024 - 2025 season. Providers must report laboratory-confirmed influenza-associated fatal pediatric cases in those 0-17 years old.1 Fresno County Public Health obtains the number of influenza-associated fatal cases between 0 and 17 years of age from investigations. The number of influenza-associated deaths in those 18 years and older are obtained from vital statistics records. National death and viral surveillance data is from the CDC’s FluView2 applications. California death data is from the California Respiratory Virus Weekly Report.3

INFLUENZA ASSOCIATED DEATHS LOCAL, STATE AND NATIONAL

Table 1: Cumulative Influenza Associated Deaths for the 2024 - 2025 Influenza Season by Location and Age Range

Location 0-17 Years Old 18-64 Years Old 65 Years or Older All Ages
Fresno County4 09/29/2024-05/17/2025 1 6 14 21
California3 06/30/2024-05/17/2025 30 401 1176 1607
Nationwide2 09/29/2024-05/10/2025 254 4043 13205 17502

INFLUENZA TYPING AT NATIONAL, REGIONAL, STATE AND LOCAL LEVELS

Figure 1 shows the cumulative distribution of influenza types from national, CDC Region 9 and California from 09/29/2024 to 03/15/2025 (week 11). CDC Region 9 includes California, Arizona, Hawaii and Nevada.2

NationalRegion 9California01020304050
A (H3)A (H1N1)pmd09A (subtyping not performed)H3N2vB VictoriaB YamagataB (lineage unspecified)Figure 1: Cumulative Influenza Types from National, CDC Region 9 and CaliforniaPercent (%)

Figure 2 shows the weekly distribution of influenza types in CDC Region 9 from 09/29/2024 to 03/15/2025 (week 11). CDC Region 9 includes California, Arizona, Hawaii and Nevada.2

Oct 2024Nov 2024Dec 2024Jan 2025Feb 2025Mar 202502004006008001000120014001600
A (H3)A (H1N1)pmd09A (subtyping not performed)H3N2vB VictoriaB YamagataB (lineage unspecified)Figure 2: Influenza Types by Disease Week from CDC Region 9Week StartNumber of Cases

Figure 3 shows the number of influenza positive tests by disease week and influenza type in Fresno County from 09/29/2024 to 05/10/2025 (week 19). The number of weekly tests changes throughout the season (figure 4). The data for figure 3 comes from both public health and clinical laboratories.5

Oct 2024Nov 2024Dec 2024Jan 2025Feb 2025Mar 2025Apr 2025May 2025050100150200250
A (H3)A (H1N1)pmd09A (subtyping not performed)B VictoriaB YamagataB (lineage unspecified)Figure 3: Influenza Types by Disease Week in Fresno CountyWeek StartNumber of Cases

Figure 4 shows the number of weekly influenza tests, and the percentage of those test that are positive for influenza in Fresno County from 09/29/2024 to 05/10/2025 (week 19). The data for figure 4 comes from both public health and clinical laboratories.5

020040060080010001200Oct 2024Nov 2024Dec 2024Jan 2025Feb 2025Mar 2025Apr 2025May 202505101520
Figure 4: Influenza Tests and Percent Positivity by Disease Week in Fresno CountyWeek StartNumber of Influenza TestsPositive Influenza Tests (%)​*​​*​Maybe Unstable Due to a Small Sample Size​*​Maybe Unstable Due to a Small Sample Size

INFLUENZA-LIKE-ILLNESS (ILI) AT EMERGENCY DEPARMENTS

Figure 5 shows the percent of ED visits attributed to influenza-like illness (ILI) in previous seasons and the current season as of 05/17/2025 (week 20). The baseline and alert curves provide reference points to monitor ILI activity in the current season. The baseline curve and alert thresholds are estimated using prior seasons’ data from 2018 to 2023. The baseline curve is the average expected percent of ED visits due to ILI. The alert curve is the upper 95% confidence limit based off percent ED visits due to ILI. Because COVID-19 and influenza have similar symptoms, ILI from the 2019-2020 season onward may be skewed. Data on previous seasons and current seasons is collected through EpiCenter,6 a syndromic surveillance system.

404142434445464748495051525312345678910111213141516171819202122230123456
2019-202020-212021-222022-232023-242024-25AverageAlertFigure 5: Emergency Department Visits Due to ILI in Fresno CountyDisease WeekPercent (%)
REFERENCES/TECHNICAL NOTES

  1. CDPH Reportable Diseases and Conditions.

  2. CDC FluView.

  3. California Respiratory Virus Weekly Report.

  4. Counts include all influenza deaths that occurred within Fresno County among Fresno County residents.

  5. Data from clinical sentinel and public health laboratories in the Respiratory Laboratory Network (RLN) reporting for Fresno County. These laboratories report the number of laboratory-confirmed influenza positive and negative tests on a weekly basis. When possible, influenza subtype information is also reported. For more information on the RLN, see the California Respiratory Virus Weekly Report. (reference 3).

  6. EpiCenter.

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