Immunization, Vaccines and Biologicals


Influenza is a contagious, acute respiratory illness caused by influenza viruses, usually influenza A or B subtypes. Influenza can cause mild to severe illness, and it may predispose to exacerbations of underlying disease or development of secondary bacterial infections. Some people are at risk for serious influenza complications, such as pregnant women, older people, young children, and people with certain chronic health conditions. Immunization is the best intervention to prevent influenza virus infection.

Influenza viruses can infect humans and other animals. Viruses that infect humans circulate in seasonal epidemics, although some tropical regions experience endemic influenza circulation. Influenza viruses are continuously changing, necessitating annual updates of influenza vaccine formulations. Occasionally, animal influenza viruses may also infect humans. These infections can manifest in a broad range of clinical symptoms from mild disease to death. If new or adapted influenza viruses cause disease in humans, and if they can be efficiently transmitted from person to person, then an influenza pandemic may occur. Pandemics are characterized by the rapid dissemination of a new, virulent influenza A viruses to which there is little or no existing immunity within the population. There have been four influenza pandemics since 1900, with the most recent pandemic occurring in 2009 caused by a new influenza A (H1N1) virus. Animal influenza viruses, including influenza A (H5N1) and influenza A (H7N9) have occasionally caused illness in humans. While efficient human-to-human transmission of these viruses has not been identified, the high case fatality rates of human infection by these viruses underscore the importance of these pathogens to public health.

There are numerous licensed seasonal influenza vaccines available. Several of these vaccines have been prequalified by the WHO for purchase by UN agencies. This process of vaccine prequalification provides independent opinion and advice on the quality, safety, and efficacy of vaccines. There are also several vaccine candidates under development against animal influenza viruses.

WHO has identified several conditions which are associated with elevated risk of complications from influenza virus infection. These groups include pregnant women, children aged 6–59 months, the elderly, individuals with specific chronic medical conditions, and health-care workers. For countries considering the initiation or expansion of programmes for seasonal influenza vaccination, WHO recommends that pregnant women should have the highest priority.

For WHO, the development of vaccines against animal influenza viruses, as well as seasonal influenza vaccines that induce broadly protective and long-lasting immune responses, are high priorities. WHO supports these efforts through provision of technical guidance and advice.

WHO Position papers

Disease burden and surveillance

Vaccine topics

Other WHO resources on influenza

Related partner links

Last updated: 10 October 2015