Seasonal influenza differs from pandemic influenza in that it occurs each year, typically during a specific time of the year. Seasonal flu generally causes less illness because the population has some immunity leftover from previous, similar influenza strains. In the Northern Hemisphere, winter is the time for seasonal influenza, but the exact timing and duration of influenza seasons vary. While influenza outbreaks can happen as early as October, activity usually peaks in January or later.
The United States 2017–2018 influenza season was a high severity season with high levels of outpatient clinic and emergency department visits for influenza-like illness, high influenza-related hospitalization rates, and elevated and geographically widespread influenza activity across the country for an extended period (Garten et al., 2018).
During the 2017–2018 influenza season, influenza-like illness activity began increasing in November, reaching an extended period of high activity during January–February, and remaining elevated through March. Influenza A(H3N2) viruses predominated through February and were predominant overall for the season; influenza B viruses predominated from March onward (Garten et al., 2018).
The figure below shows peak seasonal flu activity for the United States by month for the 1982–1983 through 2017-2018 flu seasons. The “peak month of flu activity” is the month with the highest percentage of respiratory specimens testing positive for influenza virus infection during that influenza season. During this 36-year period, flu activity most often peaked in February (15 seasons), followed by December (7 seasons), March (6 seasons), and January (6 seasons) (CDC, 2018d).
Source: CDC, 2018d.
Influenza is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness resulting in hospitalization or even death. “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 (bedridden) (CDC, 2015PB).
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, 2015PB).
The incubation period for influenza is usually 2 days, but can vary from 1 to 4 days. The severity of illness depends on whether the immune system has been exposed to related virus variants. Somewhat surprisingly, only about 50% of infected people will develop the classic clinical symptoms of influenza (CDC, 2015PB).
Systemic symptoms and fever usually last from 2 to 3 days, rarely more than 5 days. They may be decreased 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, 2015PB).
*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:
* Not everyone with flu will have a fever.
The most frequent complication associated with influenza is pneumonia, especially secondary bacterial pneumonia (eg, Streptococcus pneumoniae, Haemophilus influenzae, or 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, 2015PB).
Other complications can include myocarditis 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 less than 1 per 1,000 cases. The majority of deaths occur among individuals 65 years of age and older (CDC, 2015PB).
If vaccine supply is limited, vaccination efforts should focus on:
Influenza is primarily a community-based infection that is transmitted in households and community settings. 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).
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 can also occur through direct 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.
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 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:
Watch this fascinating 3-minute video showing how influenza is transmitted and replicated.