Influenza

Description

Influenza is caused by infection with either influenza A or B viruses. Influenza A viruses are further classified into subtypes on the basis of two surface proteins: hemagglutinin (H) and neuraminidase (N). Both influenza A and B viruses undergo continual minor antigenic change (i.e., drift), but influenza B viruses evolve more slowly and are not divided into subtypes. Influenza A (H1N1), A (H1N2), A (H3N2), and influenza B viruses currently circulate globally.

Occurrence

In the Northern Hemisphere, seasonal epidemics of influenza generally occur during the winter months on an annual or near annual basis and are responsible for approximately 36,000 deaths in the United States each year. Influenza virus infections cause disease in all age groups. Rates of infection are highest among infants, children, and adolescents, but rates of serious morbidity and mortality are highest among persons 65 years of age and persons of any age who have medical conditions that place them at high risk for complications from influenza (e.g., chronic cardiopulmonary disease). Children aged <2 years have rates of influenza-related hospitalization that are as high as those in the elderly. The emergence of a novel human influenza A virus could lead to a global pandemic, during which rates of morbidity and mortality from influenza-related complications could increase dramatically.

Risk for Travelers

The risk for exposure to influenza during international travel depends on the time of year and destination. In the tropics, influenza can occur throughout the year, while in the temperate regions of the Southern Hemisphere most activity occurs from April through September. In temperate climates, travelers can also be exposed to influenza during the summer, especially when traveling as part of large tourist groups with travelers from areas of the world where influenza viruses are circulating. Influenza vaccine should be recommended before travel for persons at high risk for complications of influenza if 1) influenza vaccine was not received during the preceding fall or winter, 2) travel is planned to the tropics, 3) travel is planned with large groups of tourists at any time of year, or 4) travel is planned to the Southern Hemisphere from April through September. In North America, travel-related influenza vaccination should take place by spring when possible, because influenza vaccine may not be available during the summer. Travelers at high risk for influenza-related complications who plan summer travel should consult with their physicians to discuss the symptoms and risks of influenza before embarking.

Clinical Presentation

Uncomplicated influenza illness is characterized by the abrupt onset of constitutional and respiratory signs and symptoms (e.g., fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis). Among children, otitis media, nausea, and vomiting are also commonly reported with influenza illness. Respiratory illness caused by influenza is difficult to distinguish from illness caused by other respiratory pathogens on the basis of symptoms alone, and laboratory testing can aid in diagnosis. Influenza illness typically resolves relatively quickly for most persons, although cough and malaise can persist for >2 weeks. Influenza can exacerbate chronic conditions (e.g., pulmonary or cardiac disease), leading to secondary infections and severe complications. Influenza-related deaths can result from pneumonia as well as from exacerbations of cardiopulmonary conditions and other chronic diseases.

Prevention

Vaccines

Annual vaccination of persons at high risk for complications before the influenza season is the most effective measure for preventing influenza and associated complications. Two types of influenza vaccine are currently available for use in the United States: inactivated vaccine, administered by intramuscular injection, and live, attenuated influenza vaccine (LAIV), administered by nasal spray. LAIV is approved for use only in healthy persons 5-49 years of age. Annual influenza vaccination is recommended for the following groups who are at high risk for complications from influenza: