Cause
Influenza viruses of types A and B are associated with the epidemics and outbreaks typical of influenza “seasons.” Influenza type C is thought to be associated primarily with milder common cold-like illnesses. Type A viruses are further subdivided into subtypes (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. Influenza viruses undergo rapid genetic evolution which eventually results in changes to the virus’ 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); inter-species transmission (1918 pandemic) and viral reassortment (1957, 1968 pandemics) may give rise to new subtypes able to infect and easily transmissible between humans and cause the next pandemic.
Transmission
Respiratory transmission occurs mainly 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.
Nature of the disease
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, non-productive cough and, often accompanied by headache, coryza, myalgia and prostration. Complications of Influenza viral infection include: primary influenza viral pneumonitis, bacterial pneumonia, otitis media and exacerbation of underlying chronic conditions. Illness tends to be most severe in the elderly and in infants and young children, and in immunocompromised hosts. Death resulting from seasonal influenza occurs mainly in the elderly and in individuals with preexisting chronic diseases.
Geographical distribution
Worldwide. 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 around typically with several peaks during rainy seasons.
Risk for travellers
In the tropics, seasonal infl uenza can occur throughout the year. However, in the southern hemisphere, peak activity occurs between April and September and in the northern hemisphere, between November and March. Infl uenza transmission may be enhanced in crowded conditions associated with air travel, cruise ships and tour groups. Elderly people and individuals with certain medical conditions such as respiratory and cardiac disease, diabetes mellitus or any immunosuppressive condition are particularly at risk of more severe disease.
Prophylaxis
Vaccination before the start of the influenza season. However, vaccine for visitors to the opposite hemisphere may not be obtainable before arrival at the travel destination (see Chapter 6). For travellers 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 then unreliable for prevention currently.
Precautions
Whenever possible, avoid crowded enclosed spaces and close contact with people suffering from acute respiratory infections. Hand-washing after direct contact with ill persons or their environment may reduce the risk of illness. Ill persons should be encouraged to practise cough etiquette (maintain distance, cover coughs and sneezes with disposable tissues or clothing, wash hands).
Vaccine
Influenza viruses constantly evolve, with rapid changes in their characteristics. To be effective, infl uenza vaccines need to stimulate immunity that protects against the principal strains of virus circulating at the time. Every year, the composition of infl uenza vaccines is modifi ed separately for the northern and southern hemispheres. Since the antigenic changes in circulating infl uenza viruses can occur abruptly and at different times of the year, there may be signifi cant differences between prevailing infl uenza strains in the northern and southern hemispheres. The internationally available vaccines contain three inactivated viral strains, the composition of which is modifi ed every 6 months to ensure protection against the strains prevailing in each infl uenza season. The composition of vaccines is therefore adjusted for the hemisphere in which the vaccine will be used. Thus, a vaccine obtainable in one hemisphere may offer only partial protection against infl uenza infection in the other hemisphere; although in some years, the viruses in the vaccine may be antigenically identical. Available infl uenza vaccines do not protect against avian infl uenza. Travellers with conditions placing them at high risk for complications of infl uenza should be regularly vaccinated every year. In years in which the northern and southern hemisphere infl uenza vaccine strains differ, those high-risk individuals travelling from one hemisphere to the other shortly before or during the other hemisphere’s infl uenza season should obtain vaccination for the opposite hemisphere in a travel clinic. Where this is not possible, the traveller should arrange vaccination as soon as possible after arriving at the travel destination. Otherwise, receiving a vaccination at least 2 weeks before travel is advisable. One dose of vaccine is given by intramuscular injection for individuals aged over 9 years. Two doses are administered at least 4 weeks apart for immunocompromised people and for children aged 6 months – 9 years; those aged 3–36 months should receive half the dose of the adult vaccines injections. Mild local reactions such as pain or swelling at the injection site are common. Systemic reactions such as fever are less common. Vaccination is relatively contraindicated in case of egg allergy.