Influenza virus types A and B are both common causes of acute respiratory illnesses, although influenza A viruses are the principal cause of large epidemics, as well as pandemics. Children are efficient transmitters of influenza viruses and those 5-9 years of age typically manifest the highest rates of infection and illness. However, severe mobidity and mortality are more common among elderly people and in specific high-risk groups. Although morbidity, mortality and affected risk groups appear to be similar all over the world, in many developing countries the disease burden and socioeconomic impact of influenza are largely unknown.
Influenza viruses undergo frrequent changes in their surface antigens. Immunity resulting from infection by one influenza virus does not protect fully against antigenic or genetic variants of the same subtype (influenza A viruses) or type (influenza B viruses). As a consequence, influenza outbreaks occur every year. New influenza vaccines must be designed annually to match the circulating viruses which are expected to cause the next epidemic.
Efficacious and safe inactivated vaccines remain the cornerstone of influenza prophylaxis in most countries. In industrialized countries, influenza vaccines offer approximately 70-90% protection against clinical disease in healthy adults, provided there is a good match between the vaccine antigens and circulating virus(es). Among elderly people not living in institutions, vaccination may reduce the number of hospitalizations by 25-39% and has also been shown to reduce overall mortality by 39-75% during influenza seasons.