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About Seasonal Influenza (flu)

Information Supplied By Public Health Canada

Human influenza, or the flu, is a respiratory infection caused by the influenza virus. Strains circulate every year, making people sick. Influenza typically starts with a headache, chills and cough, followed rapidly by fever, loss of appetite, muscle aches and fatigue, running nose, sneezing, watery eyes and throat irritation. Nausea, vomiting and diarrhea may also occur, especially in children.
Most people will recover from influenza within a week or ten days, but some - including those over 65 and adults and children with chronic conditions, such as diabetes and cancer - are at greater risk of more severe complications, such as pneumonia. Between 4 000 and 8 000 Canadians can die of influenza and its complications annually, depending on the severity of the season.

Influenza is caused by influenza A and B viruses and occurs in Canada every year, generally during late fall and the winter months. Influenza A viruses are the most common cause of annual influenza epidemics. Outbreaks of influenza B are generally more localized and in any one year may be restricted to one region of the country. An association between influenza outbreaks, especially those caused by type B virus, and cases of the rare, but serious, Reye syndrome has been noted.

The annual incidence of influenza varies widely, depending on the virulence of circulating strains and the susceptibility of the population, which is affected by antigenic changes in the virus, vaccine match and vaccine coverage. People at greatest risk of serious infections, complications, hospitalization and/or death are children aged 6-23 months, those with chronic medical conditions (especially cardiopulmonary diseases) and the elderly. Although many other respiratory viruses can cause influenza-like illness during the year, influenza virus is usually the predominant cause of serious respiratory infections in a community.

Influenza A viruses are classified into subtypes based on their hemagglutinin (H) and neuraminidase (N) antigens. Recently circulating strains have possessed one of three H and one of two N antigens, and the subtypes are designated accordingly (e.g., H3N2, H1N1). Antibodies to these antigens, particularly to H antigen, can protect an individual against a virus carrying the same antigen. During inter-pandemic periods, minor H antigen changes (referred to as drifts) are common, and the greater the change the less the cross-immunity will be to the previously circulating virus. It is this antigenic variation from one influenza virus subtype to another that is responsible for continued outbreaks of influenza, necessitating annual reformulation and administration of the influenza vaccine.

Since 1997, two influenza A subtypes, H3N2 and H1N1, have been circulating in the human population. Influenza B viruses have evolved into two antigenically distinct lineages since the mid-1980s, represented by B/ Yamagata/16/88-like and B/Victoria/2/87-like viruses. The B/Victoria lineage first re-appeared in 2001 after an absence of more than 10 years in North America, and since that time viruses belonging to the two influenza B lineages have caused outbreaks in different influenza seasons. The antigens of influenza B viruses are much more stable than those of influenza A viruses and, although antigenic variation does occur, it is less frequent.

Between 1996 and 2005, six of the nine seasons (1997-98, 1998-99, 1999-2000, 2001-02, 2003-04, 2004-05) were predominantly influenza A seasons (84%-99% of laboratory detections being influenza A). Two seasons (1996-97 and 2002-03) were considered mixed seasons (61% and 58% of laboratory detections being influenza A and 39% and 42% being influenza B, respectively), and one season (2000-01) was a predominantly influenza B season (68% of laboratory detections being influenza B). Influenza A is typically associated with greater morbidity and mortality than influenza B and typically affects the elderly, whereas influenza B is more often seen in young children. As well, influenza A/H3N2-like viruses tend to be associated with more severe illness than influenza A/H1N1-like or influenza A/H1N2-like viruses.

In four of the six predominantly influenza A seasons, 41%-46% of laboratory-confirmed influenza cases were in persons 65 years of age and older. In those same seasons, children less than 5 years of age accounted for less than 20% of laboratory-confirmed cases. In the mixed seasons and influenza B season, children less than 5 years of age accounted for 24%-32% of laboratory-confirmed cases, whereas persons 65 years of age and older accounted for 7% to 19% of laboratory-confirmed cases.

Pandemic influenza is usually associated with a major antigenic change (referred to as a shift) and the rapid global spread of influenza A virus with a different H and possibly a different N antigen from strains circulating previously. Canada, like other countries, was affected by the major influenza pandemics that occurred in 1889-90, 1918-19, 1957-58 and 1968-69.

The National Advisory Committee on Immunization (NACI) produces a Statement on Influenza Vaccination each year that contains specific information and recommendations regarding the vaccine to be used in the forth-coming season. It is published in the Canada Communicable Disease Report (CCDR) and is available at www.naci.gc.ca.

Source: Canadian Immunization Guide, 7^th edition, 2006

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