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Notice to Health Care Professionals – Influenza A(H1N1)pdm09 predominant seasonal influenza strain

Surveillance Summary

Influenza activity in Canada has continued to increase since late-November 2013, consistent with the usual timing of the seasonal influenza epidemic. Influenza A(H1N1)pdm09, which first emerged in 2009, causing the 2009 influenza pandemic, has continued to circulate as a seasonal influenza virus strain and has emerged as the predominant virus this season to date, in both laboratory detections and paediatric and adult hospitalizations.
Of the laboratory confirmed influenza-associated hospitalizations reported for the month of December 2013, the majority (99%) were associated with influenza A. Of the cases that were subtyped (n=254), 96% were associated with A(H1N1)pdm09. By age group, influenza-associated hospitalizations were highest among adults 20-64 years of age (approximately 53%), followed by children under 5 years of age (22%). Of the influenza-associated ICU admissions and deaths reported in December 2013, the majority were due to A(H1N1)pdm09 in the 45-64 year old age group. Please refer to FluWatch for up-to-date information on influenza activity in Canada.

The United States is experiencing a similar influenza season with A(H1N1)pdm09 circulating and causing severe respiratory illness among young and middle-aged adults, many of whom were infected with A (H1N1) pdm09.

Of the influenza viruses antigenically characterized to date at the National Microbiology Laboratory (NML), all influenza A viruses and the majority of influenza B viruses were similar to the strains recommended by the WHO for the 2013-14 seasonal influenza vaccine, indicating that the vaccine is a good match to the circulating influenza viruses.



It is not too late for Health Care Professionals and the general public to receive vaccine if they have not already been vaccinated this season. Vaccination is the most effective way to prevent influenza. Health Care Professionals should use every opportunity to give influenza vaccine to individuals at risk who have not been immunized during the current season, even after influenza activity has been documented in the community.

The National Advisory Committee on Immunization (NACI) recommends that to reduce the morbidity and mortality associated with influenza, immunization programs should focus on those at high risk of influenza-related complications, those capable of transmitting influenza to individuals at high risk of complications and those who provide essential community services.  As significant illness and societal costs also occur with seasonal influenza in people who may not be considered at high risk of complications (i.e. healthy people aged 5 to 64 years), NACI also encourages influenza vaccine for all Canadians aged 6 months and older.

Please refer to the Statement on Seasonal Influenza Vaccine for 2013-2014 for details.


Although vaccination remains the first-line of defense against influenza, antiviral drugs can be used to reduce morbidity and mortality from influenza. It is important that antiviral drugs be started as early as possible after the onset of symptoms. Clinicians are encouraged to review guidance on the use of antiviral drugs for treatment and prophylaxis of influenza, available from the Association of Medical Microbiology and Infectious Disease CanadaExternal site.