Questions & Answers

What causes influenza?
Viruses cause influenza. There are two basic types, A and B, which can cause clinical illness in humans. Their genetic material differentiates them. Influenza A can cause moderate to severe illness in all age groups and infects humans and other animals. Influenza B causes milder disease and affects only humans, primarily children.

Subtypes of the type A influenza virus are identified by two antigens (proteins involved in the immune reaction) on the surface of the virus. These antigens can change, or mutate, over time. When a "shift" (major change) occurs, a new influenza virus is born and an epidemic is likely among the unprotected population. This happened when the novel H1N1 influenza virus appeared in March of 2009 and led to a major pandemic, lasting until the summer of 2010.

How does influenza spread?
Influenza is transmitted through the air from the respiratory tract of an infected person. It can also be transmitted by
direct contact with respiratory droplets.

How long does it take to develop symptoms of influenza after being exposed?
The incubation period of influenza is usually two days but can range from one to four days.

What are the symptoms of influenza?
Typical influenza disease is characterized by abrupt onset of fever, aching muscles, sore throat, and non-productive cough. Additional symptoms may include runny nose, headache, a burning sensation in the chest, and eye pain and sensitivity to light. Typical influenza disease does not occur in every infected person. Someone who has been previously exposed to similar virus strains (through natural infection or vaccination) is less likely to develop serious clinical illness.

How serious is influenza?
Although many people think of influenza as the "flu" or just a common cold, it is really a specific and serious respiratory disease that can result in hospitalization and death. In the United States, the number of influenza-associated deaths has increased since 1990. This increase is due in part to the substantial increase in the number of people age 65 years or older, who are at increased risk for death from influenza complications. The Centers for Disease Control and Prevention (CDC) estimates that from the 1976–77 influenza season to the 2006–07 season, influenza-associated deaths ranged from a low of about 3,000 to a high of about 49,000. It is estimated that approximately 43–89 million people became ill with 2009 pandemic H1N1 in the U.S. from April 2009 to April 2010.

Influenza disease can occur among people of all ages; however, the risks for complications, hospitalizations, and deaths are higher among people age 65 years or older, young children, and people of any age who have certain medical conditions. Case reports and epidemiologic studies also indicate that pregnancy can increase the risk for serious medical complications from influenza.

In nursing homes, up to 60% of residents may become infected, with up to a 30% fatality rate in the infected. Risk for influenza-associated death is highest among the oldest of the elderly: people age 85 years and older are 16 times more likely to die from an influenza-associated illness than people age 65–69 years.

Hospitalization from influenza-related complications is also high among children age 24 months and younger—comparable to rates for people age 65 and older. There were 153 laboratory-confirmed influenza-related pediatric deaths reported during the 2003–04 influenza season. In the following four influenza seasons, the annual number of pediatric deaths reported to CDC ranged from 47 during the 2004–05 season to 88 during the 2007–08 season. During April 2009–March 2010, over 300 deaths in children due to 2009 H1N1 influenza were reported, and over 1,000 deaths were estimated to have occurred in children.

How is a pandemic different from an epidemic of influenza?
Occasionally, major influenza epidemics expand to a pandemic. The first recording of such an event was in 1580, and at least seven international pandemics have occurred in the nineteenth and twentieth centuries. The "Spanish flu" epidemic of 1918–1919 caused an estimated 21 million deaths worldwide, including more than 500,000 Americans. On June 11, 2009, the World Health Organization (WHO) officially declared that the spread of the novel influenza A (H1N1) virus had become a pandemic. The novel virus, which first appeared in Mexico during the spring of 2009, had demonstrated person-to-person transmission on multiple continents, meeting the definition of a pandemic. The pandemic was officially declared over by the WHO Director-General on August 10, 2010. It was projected, however, that the 2009 H1N1 viruses would likely continue to circulate for some years to come, taking on the behavior of a seasonal influenza virus.

How many people in the United States are hospitalized with seasonal influenza in a typical year?
A study conducted by CDC and published in the Journal of American Medical Association (JAMA) on September 15, 2004, provided new information on the number of people in the United States who are hospitalized from seasonal influenza-related complications each year. The study was based on records from 1979 to 2001 from about 500 hospitals across the United States. The study concluded that, on average, more than 200,000 people in the United States are hospitalized each year for respiratory and cardiac-related illnesses associated with seasonal influenza virus infections.

What are possible complications from influenza?
The most frequent complication of influenza is bacterial pneumonia. Viral pneumonia is a less common complication but has a high fatality rate. Other complications include inflammation of the heart and worsening of pulmonary diseases (e.g., bronchitis).

Reye's syndrome is a complication that occurs almost exclusively in children—patients suffer from severe vomiting and confusion, which may progress to coma because of swelling of the brain. To decrease the chance of developing Reye's syndrome, infants, children, and teenagers should not be given aspirin for fever reduction or pain relief.

Questions and answers about influenza vaccine

Technically reviewed by the Centers for Disease Control and Prevention, December 2011