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Influenza A(H1N1)pdm09

In order to avoid confusion, the World Health Organization (WHO) has modified the name of the A(H1N1) virus responsible for the 2009 pandemic to A(H1N1)pdm09 to distinguish it from the A(H1N1) virus that was already in circulation before the pandemic. The latter virus retains the name A(H1N1).

In March 2009, a new flu virus began circulating in Mexico, resulting in many deaths. This virus was responsible for a pandemic that quickly spread throughout the world, significantly affecting humans. The virus strain in question contained human, pig, and bird genes. This strain, being different from that usually in circulation, was previously unknown. Influenza A(H1N1)pdm09—is therefore different from the seasonal flu in terms of virulence.

Influenza A(H1N1)pdm09 specifically affected groups of persons who, under other circumstances, would be mildly affected by the flu, such as adolescents or adults under 50 in good health.

Mode of Transmission

The influenza A(H1N1)pdm09 virus spreads from person to person, like the seasonal-flu virus. Transmission is respiratory, by inhaling droplets spread by an infected person who coughs or sneezes, or through contact, by touching your mouth or nose after having touched an object contaminated by the virus, such as a door handle or counter surface.

Transmission schema

Click on picture to zoom


The symptoms of influenza A(H1N1)pdm09 are similar to those of seasonal flu: fever, cough, muscle aches, fatigue, and diminished appetite. In some cases, these symptoms can be accompanied by nasal discharge, sore throat, nausea, or diarrhea. Complications, when they occur, are generally difficulty breathing, which requires medical consultation.

The influenza A(H1N1)pdm09 virus has been in circulation since 2009. The strain is included in the vaccine offered at no charge to individuals at risk during the flu season.

Pandemic Influenza A (H1N1)pdm09

In September 2010, the World Health Organization (WHO) declared the end of the A(H1N1)pdm09 flu epidemic. Since then, we have been in the post-pandemic phase.

Québec’s population currently has good immunity against the A(H1N1)pdm09 virus, which was acquired either by contracting the virus or by vaccination. Despite the good level of protection in the population, health officials continue to monitor the evolution of the A(H1N1)pdm09 virus in Québec and elsewhere in the world, as is the case with other flu viruses.


At the end of summer 2009, public-health authorities estimated that 18% of Québec’s population had been infected by the virus. The average age of those hospitalized was 30.

More than 40% of hospitalized individuals were from 5 to 49 years old, although the seasonal flu usually results in few hospitalizations in this age group. During the first wave, more than 360,000 Quebeckers had to be hospitalized, including pregnant women and young children, of which more than 400 had to be admitted to intensive care. Having a chronic condition, such as lung or heart disease or diabetes, increased the individual's risk of developing complications.

By the end of the second wave, which occurred in winter 2010, 57% of Québec’s population had been vaccinated against influenza A(H1N1)pdm09.

Updated on December 19, 2011

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