ATrain Education


Continuing Education for Health Professionals

Influenza 2013-14: Fighting Complacency (retired)

Module 5

Clinical Features of Influenza

“Classic” influenza is characterized by the abrupt onset of fever, myalgia, sore throat, nonproductive cough, and headache. The fever is usually 101°F to 102°F, and accompanied by prostration. The onset of fever is often so abrupt that the exact hour is recalled by the patient. Myalgias mainly affect the back muscles. Cough is believed to be a result of tracheal epithelial destruction. Additional symptoms may include runny nose, headache, substernal chest burning, and ocular symptoms such as eye pain and sensitivity to light (CDC, 2012PB).

The incubation period for influenza is usually 2 days, but can vary from 1 to 4 days. The severity of influenza illness depends on the prior immunologic experience with antigenically related virus variants. In general, only about 50% of infected persons will develop the classic clinical symptoms of influenza (CDC, 2012PB).

Systemic symptoms and fever usually last from 2 to 3 days, rarely more than 5 days. They may be ameliorated by such medications as aspirin* or acetaminophen. Recovery is usually rapid, but some patients may have lingering depression and lack of strength or energy for several weeks (CDC, 2012PB).

*Aspirin should not be used for infants, children, or teenagers because they may be at risk for contracting Reye syndrome following an influenza infection.


Symptoms of the Flu

People who have the flu often feel some or all of these symptoms:

  • Fever* or feeling feverish, chills
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Muscle or body aches
  • Headaches
  • Fatigue
  • Vomiting and diarrhea—more common in children than adults

* Not everyone with flu will have a fever.


Transmission in Humans

In humans, influenza is primarily transmitted from person to person via large virus-laden droplets (particles more than 5 microns in diameter) that are generated when infected individuals cough or sneeze. These large droplets can then settle on the mucosal surfaces of the upper respiratory tracts of susceptible people who are nearby (within 3 feet).

Cone-Shaped Dispersion of Sneeze Particles

image: photo of man sneezing and showing sneeze particles

This photograph captures a sneeze in progress, revealing the plume of salivary droplets as they are expelled in a large cone-shaped array from this man’s open mouth, thereby dramatically illustrating the reason for covering your mouth when coughing or sneezing, in order to protect others from germ exposure. Source: James Gathany, CDC PHIL, 2009.

Transmission may also occur through direct contact or indirect contact with respiratory secretions, such as when touching surfaces contaminated with influenza virus and then touching the eyes, nose, or mouth. Adults can transmit influenza from the day before symptom onset to approximately 5 days after symptoms begin. Children can transmit influenza to others for 10 or more days (CDC, 2012PB). Infected individuals may be able to pass the flu to someone else before knowing they are sick and also after symptoms develop.

Most healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick. Some people, especially young children and people with weakened immune systems, might be able to infect others for an even longer time.

A recent German study yielded information about how and when influenza spreads within a group of people. In the study involving 180 participants, evidence of pre-symptomatic shedding of the influenza virus was observed in 30% of samples 1 day prior to the onset of symptoms. Shedding of virus was greatest on days 1 to 3 of illness (Suess et al., 2012).

Other findings included:

  • Quantity of virus shed appeared to be the same for asymptomatic and symptomatic infected subjects.
  • Those infected with influenza B exhibited the highest viral shedding load.
  • Antiviral therapy induced a milder clinical course and faster illness resolution but had no effect on viral shedding.
  • Vaccinated and unvaccinated patients did not differ in terms of clinical course and shedding.
  • Viable viral shedding, as measured by viral culture, did not persist as long as PCR positivity. (Suess et al., 2012)


Flu is unpredictable, and its severity can vary widely from one season to the next, depending on:

  • Which flu viruses are circulating
  • How much flu vaccine is available
  • When vaccine is available
  • How many people get vaccinated
  • How well the flu vaccine is matched to flu viruses that are causing illness


The most frequent complication is pneumonia, especially secondary bacterial pneumonia (eg, Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus). Primary influenza viral pneumonia is an uncommon complication but has a high fatality rate. Reye syndrome is a complication that occurs almost exclusively in children taking aspirin, primarily in association with influenza B (or varicella zoster), and presents with severe vomiting and confusion, which may progress to coma due to swelling of the brain (CDC, 2012PB).

Other complications can include myocarditis, ear infections, sinus infections, and worsening of chronic bronchitis and other chronic pulmonary diseases. People with congestive heart failure may have a worsening of the condition triggered by the flu. Death is reported in 0.5 to 1.0 per 1,000 cases. The majority of deaths occur among individuals 65 years of age and older (CDC, 2012PB).


Who Is at Higher Risk for Flu-Related Complications?

  • Children younger than 5—especially those <2 years old
  • Adults 65 years of age and older
  • Pregnant women
  • American Indians and Alaskan Natives
  • People with certain medical conditions (asthma, neurologic and neurodevelopmental conditions, chronic lung or heart disease, blood or endocrine disorders, kidney or liver disorders, metabolic disorders)
  • Additionally, people who have a weakened immune system, are <19 years old, and are receiving long-term aspirin therapy, have chronic obstructive pulmonary disease, or are morbidly obese


Laboratory Diagnosis

When laboratory diagnosis is needed, the influenza virus can be isolated from throat and nasopharyngeal swabs obtained within 3 days of onset of illness. A minimum of 48 hours is required to grow the sample in a culture medium, and 1 to 2 additional days to identify the virus type. Obviously culture is helpful in defining the etiology of local epidemics but not in individual case management(CDC, 2012PB).

Serologic confirmation of influenza requires demonstration of a significant rise in influenza immunoglobulin G (IgG), antibodies that are fighting the influenza infection. The acute-phase specimen should be taken less than 5 days from onset, and a convalescent specimen taken 10 to 21 days (preferably 21 days) following onset (CDC, 2012PB).

  • Online
  • Study the course, take the test, register and pay, print your certificate. Done!

  • Print Option
  • Click "Print Option" to print a hard copy with mail-in instructions.

  • Take the Test