What is the pandemic (H1N1) 2009 virus?

24 February 2010

This is an influenza virus that had never been identified as a cause of infections in people before the current H1N1 pandemic. Genetic analyses of this virus have shown that it originated from animal influenza viruses and is unrelated to the human seasonal H1N1 viruses that have been in general circulation among people since 1977.

Antigenic analysis has shown that antibodies to the seasonal H1N1 virus do not protect against the pandemic H1N1 virus. However, other studies have shown that a significant percentage of people age 65 and older do have some immunity against the pandemic virus. This suggests that some people in the older age group may have some cross protection from exposure to viruses that have circulated in the more distant past.

After early outbreaks in North America in April 2009 the new influenza virus spread rapidly around the world. By the time WHO declared a pandemic in June 2009, a total of 74 countries and territories had reported laboratory confirmed infections. To date, most countries in the world have confirmed infections from the new virus.

Unlike typical seasonal flu patterns, the new virus caused high levels of summer infections in the northern hemisphere, and then even higher levels of activity during cooler months in this part of the world.

The new virus has also led to patterns of death and illness not normally seen in influenza infections. Most of the deaths caused by the pandemic influenza have occurred among younger people, including those who were otherwise healthy. Pregnant women, younger children and people of any age with certain chronic lung or other medical conditions appear to be at higher risk of more complicated or severe illness. Many of the severe cases have been due to viral pneumonia, which is harder to treat than bacterial pneumonias usually associated with seasonal influenza. Many of these patients have required intensive care.

How do people become infected with the virus?

The pandemic H1N1 virus is spread from person to person, similar to seasonal influenza viruses. It is transmitted as easily as the normal seasonal flu and can be passed to other people by exposure to infected droplets expelled by coughing or sneezing that can be inhaled, or that can contaminate hands or surfaces.

To prevent spread, people who are ill should cover their mouth and nose when coughing or sneezing, stay home when they are unwell, clean their hands regularly, and keep some distance from healthy people, as much as possible.

What are the signs and symptoms of typical infection?

Signs of the pandemic influenza are flu-like, including malaise, fever, cough, headache, muscle and joint pain, sore throat and runny nose, and sometimes vomiting and diarrhoea.

The majority of people with pandemic influenza experience mild illness and recover fully without treatment.

When should someone seek medical care?

People should seek medical care if they experience shortness of breath or difficulty breathing, or if a fever, and especially high fever, continues more than three days. For parents with a young child who is ill, seek medical care if a child has fast or labored breathing, continuing fever or convulsions (seizures).

Supportive care at home - resting, drinking plenty of fluids and using a pain reliever for aches and pains - is adequate for recovery in most cases. A non-aspirin pain reliever should be used for children or adolescents under age 18.

Why was WHO so worried about this flu when hundreds of thousands die every year from seasonal epidemics?

Seasonal influenza occurs every year and the viruses change each year - but many people have some immunity to the circulating virus that helps limit infections. Some countries also use seasonal influenza vaccines to reduce illness and deaths.

By contrast, the pandemic H1N1 was a new virus when it emerged and most people had no or little immunity to it. In addition, one of the lessons from history is that influenza pandemics can kill millions. Finally, there was no pandemic influenza vaccine at the outset.

The global impact of the current pandemic has not yet been estimated. Typically, the numbers of deaths from seasonal influenza or past pandemics are estimated using statistical models.

By contrast, the currently reported counts of over 16,000 deaths from pandemic H1N1 represent individually tested and confirmed deaths, primarily reported from countries with adequate resources for widespread laboratory testing. This approach has never been used to count seasonal or previous pandemic deaths and results in a significant underestimate.

A more accurate assessment of mortality from the pandemic, using statistical models, will likely be possible in about one to two years.

(updated from 11 June 2009 version)