Influenza report

Last updated: 3/29/23

The County of Santa Clara Public Health Department provides regular summaries of current data during each influenza season. Influenza season typically occurs between October through the following May, but can fluctuate. Influenza can also be referred to as “flu.”

The County of Santa Clara Public Health Department uses a range of methods to monitor trends in influenza activity during influenza season. We monitor reported emergency department influenza-like illness (ILI) data, influenza laboratory data provided by clinical laboratories, and severe and fatal influenza cases.

Influenza activity

Influenza activity in this current season has decreased since the peak in December.  Influenza A wastewater concentrations have dropped throughout Santa Clara County, while Influenza B concentrations remain low.

Influenza wastewater data are updated on every Monday, Wednesday and Friday during influenza season, except holidays.

All other metrics in this dashboard are updated every Wednesday during influenza season, except holidays. Weekly data are preliminary and subject to change.

Refer to the California Department of Public Health Influenza website for California influenza data and the Centers for Disease Control and Prevention Influenza Surveillance website for United States influenza data. 

Please expand each section below for more information.

    This graph demonstrates the weekly percentage of total emergency department visits that are associated with influenza-like illness (such as fever, congestion, sneezing, sore throat, runny nose and cough) for each influenza season, by week. Please note: Symptoms of ILI may be caused by multiple respiratory pathogens, including influenza, COVID, and RSV. This graph does not distinguish whether ED visits for these symptoms ultimately resulted in a diagnosis of influenza and/or any other type of respiratory illness.

    The regional baseline is calculated by the CDC as the mean percentage of ILI patient visits during non-influenza season weeks for the most recent three seasons (excluding the COVID pandemic) plus two standard deviations. Refer to U.S. Influenza Surveillance: Purpose and Methods | CDC.

    Source: ESSENCE (Electronic Surveillance System for the Early Notification of Community-Based Epidemics). Facilities included in this dataset are Good Samaritan Hospital, Kaiser Permanente Santa Clara Medical Center, Kaiser Permanente San Jose Medical Center, O'Connor Hospital, Regional Medical Center of San Jose, St. Louise Regional Hospital, Stanford Medical Center, and Valley Medical Center.

    This graph demonstrates the number of specimens collected during this influenza season that tested positive for influenza. It further indicates the number of specimens that tested positive for Influenza Strain A or Influenza Strain B.

    PCR, rapid, and point-of-care influenza results for Santa Clara County residents and cases with unknown residency data are counted in this graph. Limitations for this graph include potential missed influenza cases tested by alternate laboratories in the county or missed Santa Clara County cases of residents who are tested outside of the county.

    Sources: Stanford Health Care, Sutter Health/Palo Alto Medical Foundation, O'Connor Hospital, Santa Clara Valley Medical Center, and St. Louise Regional Hospital. Specimens collected by other laboratories within Santa Clara County are not included in this graph.

    This graph demonstrates the total number of laboratory-confirmed ICU admissions and deaths due to influenza among individuals ages 0 to 64 years old, by episode week, during this current influenza season. Episode week is defined as the earliest of the following weeks, if available: Week of onset, week of diagnosis, week of death, week of laboratory specimen collection date, or week of report receipt. ICU admissions and deaths due to influenza among individuals 65 and older are not reportable in the state of California and therefore are not reflected in this graph. 

    ICU cases and deaths by age group information will be provided once there are at least 10 cases or 10 deaths to report. Counts by age group are not shown if the counts are too small to display. Cases who die after being hospitalized in the ICU may be removed from the count of ICU hospitalizations to avoid counting them twice.

    This graph demonstrates the weekly total number of laboratory-confirmed outbreaks occurring in community settings and healthcare settings, by week, during this influenza season.

    An outbreak is defined as at least one case of laboratory-confirmed influenza in the setting of a cluster (equal or greater than 2 cases) of influenza-like illness (ILI) within a 72-hour period. All outbreaks are investigated by SCCPHD.

    Examples of community settings include, but are not limited to, schools, childcare centers, assisted living facilities, and congregate settings. Examples of healthcare settings include, but are not limited to, skilled nursing facilities, acute care hospitals, urgent care centers and clinics, and surgical centers.

    Source: CalREDIE (California Reportable Diseases Information Exchange)

    The County of Santa Clara Public Health Department provides an overview of Influenza A and B virus concentration in wastewater samples collected at wastewater treatment facilities that serve people in Santa Clara County.  Four wastewater treatment facilities receive wastewater from designated geographic areas, known as “sewersheds,” within Santa Clara County.

    Monitoring concentrations of Influenza A and B virus in wastewater can detect levels of infection caused by Influenza A and B virus within a community.

    Source: Sewer Coronavirus Alert Network (SCAN) project by Stanford University

    Select a sewershed of interest. Information about areas in Santa Clara County within each sewershed are described below. Select "All sewersheds in Santa Clara County" to view all 4 sewersheds in one graph.

    San Jose, Gilroy, Sunnyvale and Palo Alto sewershed views will display concentrations of Influenza A and Influenza B virus. Please note that testing for Influenza B virus concentration began in November 2022.

    Only concentrations of Influenza A virus will be shown when viewing all 4 sewersheds in Santa Clara County. Influenza A is the predominant type of influenza strain circulating in the community.

    When reviewing these graphs, it is important to look at the overall, sustained trends, instead of measurements on any given day. Wastewater samples tend to have some variability simply due to the nature of environmental samples. 

    Influenza A and B are shed in feces by individuals who are infected and can be measured in wastewater. This can include individuals experiencing symptoms of influenza illness, or those who are infected but asymptomatic.

    As is already the case for COVID, wastewater monitoring for influenza holds the potential to be a tool to identify surges across the county. This is important because most influenza infections are not tested or reportable, and so wastewater data can provide information on trends in influenza activity in the community. A higher concentration of Influenza A and B in wastewater may mean more influenza is being transmitted locally.

    Wastewater data isn't connected to a single household or even small neighborhoods. Samples are taken from water that combines the output of thousands of households.

    Public Health is monitoring levels of Influenza A and B virus in wastewater in partnership with Stanford University’s Wastewater SCAN Project. This effort includes all four wastewater treatment plant partners in Santa Clara County and builds on techniques developed during the COVID pandemic to track community virus transmission through wastewater analysis.

    Wastewater samples are collected 7 days a week from all 4 wastewater treatment facilities within Santa Clara County. Results from the wastewater samples are available within 24 to 48 hours.

    The 4 wastewater treatment facilities collectively serve approximately 1,901,352 people, representing 98% of Santa Clara County’s total population.

    Wastewater samples are collected at the following wastewater treatment plants:

    Sewershed Wastewater treatment facility Approximate number of people in Santa Clara County served
    San Jose San Jose-Santa Clara Regional Wastewater Facility 1,419,393
    Palo Alto Palo Alto Regional Water Quality Control Plant 215,544
    Sunnyvale Donald M. Somers Water Pollution Control Plant 161,021
    Gilroy South County Regional Wastewater Authority 105,394

    Source: U.S. Census Bureau, 2020 American Community Survey block data

    San Jose-Santa Clara Regional Wastewater Facility serves people in the cities of San Jose, Santa Clara and Milpitas; Cupertino Sanitary District (City of Cupertino and nearby unincorporated areas); West Valley Sanitation District (Cities of Campbell, Los Gatos, Monte Sereno, and Saratoga); County Sanitation District No. 2-3 (unincorporated area); and Burbank Sanitary District (unincorporated area).

    Palo Alto Regional Water Quality Control Plant serves people in Los Altos, Los Altos Hills, Mountain View, Palo Alto, Stanford University, and the East Palo Alto Sanitary District. Note: A portion of wastewater received at the Palo Alto Regional Water Quality Control Plant comes from San Mateo County.

    Donald M. Somers Water Pollution Control Plant serves people in the city of Sunnyvale; Rancho Rinconada portion of Cupertino; and Moffett Federal Airfield.

    South County Regional Wastewater Authority serves people in the cities of Gilroy and Morgan Hill.

    Protocol of Sample Collection and Testing: Samples for testing consist of “settled solids” from wastewater entering each treatment plant. Samples are collected 7 days a week from the wastewater treatment plants and are transported daily to a commercial lab for analysis. Laboratory staff process the samples to quantify Influenza A and B in the wastewater. These procedures use methods to concentrate solids and extract RNA and detect viral RNA. Results are typically ready within 24 hours of sample drop-off and are uploaded to this page. The quick turnaround and posting are some of the major advantages of utilizing this data.

    Interpretation of Plots: Data reflected in the table below shows the results for Influenza A and B, and the concentrations are “normalized” by the concentration of a plant virus that is harmless to humans but is shed in stool (pepper mild mottle virus, or PMMoV). Normalizing by PMMoV adjusts for changes in the amount of feces in the sample and the efficiency of the procedures from day to day.

    Wastewater typically contains waste from a variety of sources, such as from your shower or a commercial process. These types of waste do not have stool (which is the source of Influenza A and B) so they dilute the sample. Therefore, we adjust this sample to the known amount of feces in the wastewater to get an accurate result. 

    The curves displayed are the “5-day trimmed average” of the sample results. This is done by using samples from each day with the previous 4 days, eliminating the maximum and minimum among the 5 samples, and then taking the mean.

    The County of Santa Clara Public Health Department also provides wastewater monitoring data on COVID.

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