Epidemiology Unit

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What is influenza?

Influenza, commonly referred to as the flu, is a respiratory illness caused by influenza viruses. There are two main types of influenza virus, types A and B. While influenza A viruses are found in many different animals, including birds and swine, humans are considered the primary reservoir for the influenza viruses responsible for seasonal influenza epidemics: influenza A(H1N1), influenza A(H3N2), and influenza B. In the Northern Hemisphere, seasonal influenza epidemics (the flu season) occur annually between October and May, usually peaking in January-February. The severity of each influenza season is related to many factors, including the viral characteristics of circulating strains (such as antigenic type, virulence, and transmissibility), natural or vaccine-related immunity in the population, and the health of the population. Influenza viruses mutate constantly and can combine with other strains in an infected person or animal to create slightly or even drastically new (novel) strains each season. Pandemics occur when a novel strain of influenza emerges that is efficiently transmitted in a sustained way to a susceptible global population.


How is influenza transmitted? 

The influenza virus is transmitted by airborne, droplet, or direct contact with the respiratory secretions of infected persons. Transmission from animals to humans is unusual, but sporadic cases and outbreaks in humans exposed to avian or swine influenza viruses have been documented. People usually get sick two to three days after being exposed to the virus. Outbreaks in residential facilities are common. People sick with influenza should avoid contact with others.


What are the symptoms of influenza?

Influenza is an acute viral respiratory illness characterized by symptoms of fever (100°F or higher), cough, sore throat, and myalgias. Illness can be mild to severe. Most people recover within one to two weeks, but severe illness can result in hospitalization or death. People 65 years and older may not present with the typical influenza-like-illness, but instead with congestive heart failure and no fever. Older people are also at risk for serious influenza-related complications. Other groups at risk for complications include people with a compromised immune system or chronic respiratory condition (such as COPD), pregnant women, young children, and children with neurologic conditions. Antiviral medications may be a treatment option, particularly for people at high risk for complications.


How can you protect yourself?

The best way to prevent influenza is by getting an influenza vaccination each year, washing hands frequently, and avoiding close contact with ill persons. Seasonal influenza vaccines include the strains of influenza A and influenza B considered most likely to circulate that season. Everyone six months of age and older should receive an influenza vaccine every season.


Respiratory Virus Surveillance Report

The County of San Diego performs influenza surveillance year-round. 

Summary reports of past influenza seasons can be found on the Epidemiology Data and Reports page.

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Information for the Public

Everyone six months of age and older should receive a flu vaccination each season, especially pregnant women and those with underlying conditions such as COPD or heart disease. Click here to find influenza vaccination clinics near you.


Information for Health Professionals

Physicians and laboratories are encouraged to report all influenza detections (e.g., rapid test, PCR) to the San Diego County Epidemiology Unit. Hospital laboratories and clinical offices not yet connected to electronic laboratory reporting (ELR) are requested to report via CMR form, Influenza Case Report form, or by simply sending a copy of the laboratory test result to the Epidemiology Unit by fax to (858) 715-6458.

Pediatric (ages 0-17 years) deaths due to influenza are legally reportable to the San Diego County Epidemiology Unit and should be reported via CMR form or Influenza Case Report form.

Influenza outbreaks are legally reportable to the San Diego County Epidemiology Unit and should be promptly reported by telephone at (619) 692-8499 Monday-Friday 8 AM-5 PM, or (858) 565-5255 on evenings, weekends, and County-observed holidays in the following circumstances:

  • Outbreaks in institutions and other residential facilities (e.g., long term care facilities, homes for the developmentally disabled or chronically ill, sleepover camps, prisons) with at least one case of laboratory confirmed influenza in the setting of a cluster (≥2 cases) of influenza-like illness (ILI) within a 72-hour period. Influenza-like illness (ILI) is defined as fever (≥100°F or 37.8°C) and a cough and/or a sore throat, in the absence of a known cause other than influenza.

The following guidance documents and resources are available to assist with preventing, managing, and controlling influenza outbreaks in long-term care facilities and similar settings.

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For more information, contact the Epidemiology Unit at (619) 692-8499 or send us an e-mail.