CDPH Influenza Surveillance Program
The California Department of Public Health (CDPH) Influenza Surveillance Program is a collaborative effort between CDPH, the Centers for Disease Control and Prevention (CDC), Kaiser Permanente, California local health jurisdictions and participating sentinel providers and laboratories. Annual influenza epidemics follow a winter seasonal pattern in the United States with typical activity peaking during late December to early February. CDPH obtains and analyzes clinical, pharmacy and laboratory data year-round in an effort to determine the timing and impact of influenza activity and to determine how well circulating strains of the virus match those used in the current influenza vaccines. Each surveillance method is described more fully below.
CDC Influenza Sentinel Providers
Sentinel providers (physicians, nurse practitioners, and physician assistants) situated throughout California report on a weekly basis the number of patients seen with influenza-like illness (ILI) and the total number of patients seen for any reason. ILI is defined as any illness with fever (≥100°F or 37.8°C) AND cough and/or sore throat (in the absence of a known cause other than influenza). At present, over 150 sentinel providers report ILI data, allowing CDPH to attain the CDC goal of 1 sentinel provider per 250,000 population. Sentinel providers also submit specimens from patients with ILI to the CDPH Viral and Rickettsial Disease Laboratory or local public health laboratories for influenza testing. Additional information about the California Sentinel Provider program can be found at: http://www.cdph.ca.gov/programs/dcdc/Pages/CaliforniaSentinelProviderProgram.aspx.
Kaiser Inpatient Data
The CDPH Influenza Surveillance Program collaborates with Kaiser Permanente to monitor trends in influenza-related hospitalizations. There are two regions of Kaiser Permanente in California: Kaiser Permanente Northern California and Kaiser Permanente Southern California. Together, they insure more than 6 million Californians, or roughly one-sixth of the state’s population. Thus, influenza activity among Kaiser Permanente patients is thought to be generally representative of influenza activity for the entire state.
Kaiser Permanente inpatients with an admission diagnosis including the keywords “flu,” “influenza,” “pneumonia,” or variants of the keywords are defined as pneumonia and influenza (P&I)-related admissions. The number of P&I admissions is divided by the total number of hospital admissions occurring in the same time period to estimate the percentage of P&I admissions. Admissions for pregnancy, labor and delivery, birth, and outpatient procedures are excluded from the denominator. Influenza admissions that have subsequent laboratory confirmation for influenza are also tracked.
Kaiser Permanente Pharmacy Data
The number of prescriptions for medications active against influenza, such as amantadine, rimantadine, zanamivir and oseltamivir, are used to serve as indicators of influenza activity. This component of the project assesses the number of influenza antiviral prescriptions filled weekly by all Kaiser outpatient pharmacies in California.
Severe and Fatal Influenza Case-Based Surveillance
Public health surveillance for persons severely ill with influenza is important for detecting the emergence of novel influenza viruses and to characterize populations at risk for complications due to influenza. Information gathered from surveillance can lead to new prevention measures, education campaigns, and strategies for vaccine and antiviral use that will help protect the public. CDPH is working with local health jurisdictions to enhance surveillance and to amend current reporting regulations for influenza according to the California reportable disease list for medical providers (Section 2500).
Currently, the following cases are reportable:
- Influenza deaths in laboratory-confirmed cases for ages 0- 64 years
In addition, CDPH requests that the following cases are reported on a voluntary basis:
- Laboratory-confirmed influenza cases ages 0-64 years requiring intensive care
Laboratory confirmation can include any positive test performed by any clinical, commercial or local public health laboratory (LPHL), including rapid antigen test (as rapid antigen tests may yield a relatively high proportion of false positive results when influenza prevalence is low, it is recommended that a positive rapid antigen test result be followed up with confirmatory testing using one of the other indicated methods), direct fluorescence assay, culture or polymerase chain reaction (PCR).
The laboratory component of the project involves use of data from hospital, academic, private and public health laboratories located throughout California. These laboratories report the number of laboratory-confirmed influenza and other respiratory virus detections and isolations on a weekly basis. Reporting laboratories are divided into the following networks:
The Respiratory Laboratory Network (RLN) is composed of 28 local public health laboratories that offer PCR testing for influenza A and B. Some RLN laboratories are also able to identify five other common respiratory viruses (respiratory syncytial virus, adenovirus, and parainfluenza virus types 1-3) using the R-mix shell vial culture system. Participating laboratories include: Alameda, Contra Costa, El Dorado, Fresno, Imperial, Kern, Long Beach, Los Angeles, Marin, Monterey, Orange, Placer, Riverside, Sacramento, San Bernardino, San Diego, San Francisco, San Joaquin, San Luis Obispo, San Mateo, Santa Barbara, Santa Clara, Shasta, Solano, Sonoma, Stanislaus, Tulare, and Ventura.
California Sentinel Laboratories are a network of clinical, commercial, academic and hospital laboratories located throughout California that provide weekly data on the number of laboratory-confirmed influenza and other respiratory virus detections and isolations. These labs use various testing methods, including rapid test, direct fluorescent assay, viral culture and polymerase chain reaction (PCR). Participating laboratories include: Kaiser Permanente Northern California Regional Laboratory, Kaiser Permanente Southern California Regional Laboratory, Stanford Medical Center, UCSF, San Francisco General Hospital, Children’s Hospital Oakland, Children’s Hospital Central California, UCLA Medical Center, Long Beach Memorial Hospital, Children’s Hospital Los Angeles, El Centro Regional Medical Center, Pioneers Memorial Hospital, Rady Children’s Hospital San Diego, and San Ysidro Health Center.
The CDPH Viral and Rickettsial Disease Laboratory (VRDL) serves as a reference laboratory for the state of California. A fraction of the influenza viruses isolated at participating laboratories are forwarded to VRDL for further antigenic and genetic characterization. Complete antigenic characterization enables detection of new strain variants and provides a method for monitoring how well circulating influenza strains match those used in the current influenza vaccines. In addition, sentinel providers located throughout California submit specimens from patients with influenza-like illnesses for respiratory virus PCR and straintyping to VRDL. The VRDL performs surveillance for antiviral resistance on a limited basis. The VRDL works closely with the RLN and the CDC to develop and adapt new assays for novel influenza viruses and other emerging pathogens.