Seasonal influenza A(H1N1): key issues

January 2016

1. In some countries in the European Region, the flu virus A(H1N1) is causing severe disease and a number of deaths. Has the pandemic virus come back?

The virus A(H1N1), which causes what was commonly referred to in the past as "swine flu", caused a flu pandemic in 2009. Since then, this virus has continued to circulate in people in Europe and elsewhere, so that it is now a seasonal human flu virus. 

In 2009, the global population had little immunity to A(H1N1) because it was new, and it caused a global epidemic, with 100 000 to 400 000 estimated deaths that year. Now, A(H1N1) circulates as an annual flu virus. So far this season, as in the 2014–2015 season, influenza A(H1N1) is circulating in the European Region, with A(H3N2) and a few influenza B viruses. The seasonal influenza vaccine provides protection against A(H1N1).

Flu News Europe provides a weekly overview and assessment of influenza activity in the Region, based on surveillance data from the 53 countries in the WHO European Region.

2. Is this season's influenza vaccine effective?

Each season, a flu vaccine is developed to protect against the viruses that research suggests will be the most common ones circulating. The 2015–2016 seasonal vaccine includes seasonal influenza A(H1N1) virus, with A(H3N2) and influenza B virus strains. 

There are indications that there is a "good match" between this season's vaccine and most of the seasonal flu viruses – i.e. the viruses in the vaccine and the viruses circulating among people are closely related – and therefore the current vaccine is expected to provide good protection against this year's influenza.

3. Who is at higher risk of health complications from influenza?

When influenza starts to spread every winter, WHO strongly recommends that certain groups be vaccinated against the disease. These include people aged over 65, residents of institutions for older persons and the disabled, pregnant women, anyone with a chronic condition, such as heart or lung disease or chronic neurological conditions, and children under 5 years old. People should contact their medical practitioner if they are unsure whether they at risk.

4. Is the seasonal A(H1N1) different from the A(H1N1) virus that emerged during the 2009 pandemic? 

On the basis of the viruses analysed so far, the seasonal A(H1N1) has not changed significantly from the 2009 virus, and there is no evidence that it has become more harmful. Since its emergence in 2009, A(H1N1) has caused rare cases of severe disease in otherwise healthy, young adults, including pregnant women. In contrast, the A(H3N2) virus is more likely to cause severe disease and death in the elderly.

Clinicians must be aware of this, so that they can start early treatment with an influenza antiviral drug (oseltamivir) in patients who show severe respiratory symptoms. WHO has prepared advice for European clinicians in managing severe complications of influenza.

The same virus strain as was used in the pandemic vaccine is being used in the 2015–2016 seasonal vaccine, namely the A/California/7/2009 (H1N1)pdm09-like strain.

5. When should people consult a doctor if they have flu? 

Irrespective of the flu virus type, influenza is always unpleasant, but it is usually mild, and most people recover quickly. People with symptoms such as coughing, sneezing, sore throat, headache and a slight temperature should rest at home; they can visit their local pharmacy for advice or use painkillers and decongestants. 

People with severe or unusual symptoms and those at increased risk of severe disease (children under 5, people over 65, pregnant women and people with a pre-existing medical conditions) should contact their physician to determine whether antiviral or other treatment is needed.