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Influenza (Seasonal)

Provider Information

Annual influenza activity follows a winter seasonal pattern in the United States from October through May, with a peak typically occurring in late December to early February.

The best way to prevent seasonal flu is to get vaccinated every year. CDC recommends annual influenza vaccination for everyone 6 months and older with any licensed, age-appropriate flu vaccine (IIV, RIV4, or LAIV4) with no preference expressed for any one vaccine over another.

The following groups of people are more likely to get serious flu-related complications if they get sick with flu:

  • Children younger than 5 years (especially less than 2 years old)
  • Adults 65 years of age and older
  • Pregnant women (and women up to two weeks postpartum)
  • Residents of congregate living nursing homes and other long-term care facilities
  • American Indians and Alaska Natives
  • People who haveunderlying medical conditions, including:
    • Asthma
    • Neurological and neurodevelopmental conditions
    • Chronic lung disease
    • Heart disease
    • Blood disorders
    • Endocrine disorders
    • Kidney disorders
    • Liver disorders
    • Metabolic disorders
    • Weakened immune system due to disease or medication
    • People younger than 19 years of age who are receiving long-term aspirin therapy
    • People with obesity (body mass index [BMI] of 40 or more)


Contra Costa Public Health collects and analyzes both clinical and laboratory data throughout the year in an effort to help determine the timing and impact of influenza virus spread in the community. The information gathered is used for targeted outreach to health care partners and the community around disease prevention and control activities.

Report to Public Health (per Title 17, California Code of Regulations (CCR) §2500)

  1. Laboratory-Confirmed influenza-related pediatric deaths (ages 0-17 years)
  2. Acute Respiratory Outbreaks in both healthcare and congregate living settings, such as residential living facilities (retirement community, assisted living, board & care, skilled nursing facilities, rehabilitation centers, homeless and evacuation shelters, camps, jails, etc.) An outbreak is defined as:
    • One lab-confirmed influenza case in a healthcare or congregate living setting.
    • 2 or more cases of new onset respiratory illness within 72 hours (3 days) in a healthcare or congregate living settings.

How to Report

Please contact Contra Costa Public Health Communicable Disease Programs
Phone: 925-313-6740 | Fax: 925-313-6465

  • Laboratory-Confirmed/influenza-related pediatric deaths (fax report)
  • Acute Respiratory Outbreaks (phone report 24/7)

Clinical Management

Obtain an exposure history (within 10 days from symptom onset) for all patients in which influenza has been suspected. History should include:

  • Residence in a congregate living setting
  • Travel (dates and travel destination)
  • Close contact (same exposure) with similar symptoms or clinical diagnosis
  • Animal exposure (pigs and poultry. Animals that can be infected with variant influenza viruses)

Laboratory Testing

Laboratory testing with real-time reverse-transcription polymerase chain reaction (rRT-PCR) is the preferred testing method when there is strong clinical suspicion of influenza, even if the rapid test is negative.

  • Rapid influenza tests may vary in terms of sensitivity and specificity, when compared with rRT-PCR, with sensitivities ranging from 50-70%. This means that false positives are common when influenza prevalence is low and false negatives can occur when influenza prevalence is high.
  • Influenza testing by rRT-PCR is encouraged in the situations listed below:
    • A patient who is hospitalized or in the intensive care unit (ICU)
    • A patient who lives in a congregate living setting
    • A patient who has recently traveled (document travel dates and destination)
    • A patient who has recently been exposed to animals such as pigs or poultry (that can be infected with variant influenza viruses)
  • Influenza Specimen Collection – Desk Guide (CDC)
  • Influenza Specimen Collection – Visual Guide (CDC)


Infection Control

Congregate Living Setting Outbreaks

Additional Resources

See more information designed specifically for health care providers.