Influenza viruses belonging to the family Orthomyxoviridae.
The influenza viruses are classified into types A, B and C on the basis of their nucleoproteins. Only types A and B cause human disease of any concern. The subtypes of influenza A viruses are determined by envelope glycoproteins possessing either haemagglutinin (HA) or neuraminidase (NA) activity. High mutation rates and frequent genetic reassortments of these viruses contribute to great variability of the HA and NA antigens. The majority of the currently identified 17 HA and 10 NA subtypes of influenza A viruses are maintained in wild, aquatic bird populations. Humans are generally infected by viruses of the subtypes H1, H2 or H3, and N1 or N2. Minor point mutations causing small changes (“antigenic drift”) occur relatively often. Antigenic drift enables the virus to evade immune recognition, resulting in repeated influenza outbreaks during interpandemic years. Major changes in the HA antigen (“antigenic shift”) are caused by reassortment of genetic material from different A subtypes. Antigenic shifts resulting in new pandemic strains are rare events, occurring through reassortment between animal and human subtypes, for example in co-infected pigs. In 2009, global outbreaks caused by the A(H1N1) strain attained pandemic proportions, gradually evolving into a seasonal epidemiological pattern in 2010.
Respiratory transmission occurs mainly by droplets disseminated by unprotected coughs and sneezes. Airborne transmission of influenza viruses occurs particularly in crowded spaces. Hand contamination followed by direct mucosal inoculation of virus is another possible source of transmission.
Nature of the disease
Influenza is an acute respiratory infection of varying severity, ranging from asymptomatic infection to fatal disease. Typical influenza symptoms include fever with abrupt onset, chills, sore throat and non-productive cough, often accompanied by headache, coryza, myalgia and prostration. Complications of influenza include primary viral pneumonitis, bacterial pneumonia, otitis media and exacerbation of underlying chronic conditions. Illness tends to be most severe in elderly people, in infants and young children, and in immunocompromised individuals. Death resulting from seasonal influenza occurs mainly in elderly people and in individuals with pre-existing chronic diseases.
Influenza occurs all over the world, with an annual global attack rate estimated at 5–10% in adults and 20–30% in children. 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.
Risk for travellers
Travellers, like local residents, are at risk during the influenza season. In addition, groups of travellers (e.g. on cruise ships) that include individuals from areas affected by seasonal influenza may experience out-of-season outbreaks. Travellers visiting countries in the opposite hemisphere during the influenza season are at special risk, particularly if they do not have some degree of immunity through recent infection or regular vaccinations.
Precautions against influenza
During influenza outbreaks, crowded enclosed spaces and close contact with people suffering from acute respiratory infections should be avoided if possible. Frequent hand-washing, especially after direct contact with ill persons or their environment, may reduce the risk of acquiring illness. Ill persons should be encouraged to maintain distance, cover coughs and sneezes with disposable tissues or clothing, and to wash hands. In some situations, physicians may recommend antiviral prophylaxis or early treatment using neuraminidase inhibitors, particularly for individuals at special risk.