Seasonal Flu

The seasonal flu is an infectious disease caused by RNA viruses of the family Orthomyxoviridae (the influenza viruses) that affects birds and mammals. The name influenza comes from the Italian influenza, meaning “influence” (Latin: influentia). Influenza is a viral infectious disease that is considered one of the most important causes of respiratory tract infections. Symptoms are similar to those of the common cold, however, are more severe, and their beginning is generally abrupt. Influenza can affect a significant number of persons of all age groups during the occurrence of an epidemic. This disease frequently requires medical attention and hospitalization, contributing significantly to economical losses, excessive numbers of hospitalizations, and deaths. The ability of influenza A and B viruses to suffer gradual antigenic changes in their two surface antigens, the hemagglutinin (H) and the neuraminidase (N), complicates vaccination against this disease. Epidemics of influenza were responsible for an average of 36,000 deaths per year in countries such as the United States during the 1990s, affecting all age groups but mainly children less than 2 years old and adults older than 65 years.

Influenza viruses of types A and B are associated with the epidemics and outbreaks typical of influenza “seasons” (seasonal influenza). Influenza type C is thought to be associated primarily with milder common cold-like illnesses. Type A viruses are further subdivided into subtypes (e.g., H1N1 and H3N2). During an influenza season, one of the two influenza A subtypes or influenza B can be predominant, while in other years, all three viruses may be found (this is considered in the production of vaccines for protection against influenza viruses). Influenza viruses undergo rapid genetic evolution that eventually results in changes to the virus's antigenic characteristics. Influenza vaccine virus strains are selected each year to make sure that the vaccine is matched as closely as possible to the currently circulating influenza strains. Other subtypes of influenza A viruses occur in animals, and all 16 HA and 9 NA subtypes are found in birds (mainly in water fowl); interspecies transmission (1918 pandemic) and viral reassortment (1957, 1968 pandemics) may give rise to new subtypes able to infect and be easily transmissible between humans and cause the next pandemic.

Influenza is mainly transmitted by droplets disseminated by unprotected coughs and sneezes. Short-distance airborne transmission of influenza viruses may occur particularly in crowded, enclosed spaces. Hand contamination and direct inoculation of virus is another possible source of transmission.

From a geographical point of view, influenza is a global disease, occurring according to the season in specific regions. In temperate regions, influenza is a seasonal disease occurring typically in winter months: it affects the northern hemisphere from November to April and the southern hemisphere from April to September. In tropical areas, there is no clear seasonal pattern, and influenza circulation is year-round, typically with several peaks during rainy seasons.

The administration of the influenza vaccine for children (up to 6 years old) is recommended at a minimum age of 6 months for trivalent inactivated influenza vaccine (TIV) and at age 2 years for live, attenuated influenza vaccine (LAIV). Influenza vaccine can be administered annually to children aged 6 months through 18 years. For healthy, nonpregnant individuals (i.e., those who do not have underlying medical conditions that predispose them to influenza complications) aged 2 through 49 years, either LAIV or TIV may be used. Children receiving TIV should receive 0.25 mL if aged 6 through 35 months or 0.5 mL if aged 3 years or older. Two doses can be administered (separated by at least four weeks) to children aged younger than 9 years who are receiving influenza vaccine for the first time or who were vaccinated for the first time during the previous influenza season but received only one dose. The timing of the vaccination is important. This should be done before the start of the influenza season, but this depends on where the individual is geographically located. However, vaccine for visitors to the opposite hemisphere may not be obtainable before arrival at the travel destination. For travelers in the highest risk groups for severe influenza who have not been or cannot be vaccinated, the prophylactic use of antiviral drugs such as zanamivir or oseltamivir is indicated in countries where they are available. Amantadine and rimantadine may also be considered when the circulating strains are known to be susceptible. However, the latter drugs are not active against influenza B, and high frequencies of resistance in H3N2 and less often H1N1 viruses make them unreliable for prevention at the time of this writing.

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