What is the new influenza A(H1N1)?

This is a new influenza A(H1N1) virus that has never before circulated among humans. This virus is not related to previous or current human seasonal influenza viruses.

How do people become infected with the virus?

The virus is spread from person-to-person. 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.

There are no known instances of people getting infected by exposure to pigs or other animals.

The place of origin of the virus is unknown.

What are the signs and symptoms of infection?

Signs of influenza A(H1N1) are flu-like, including fever, cough, headache, muscle and joint pain, sore throat and runny nose, and sometimes vomiting and diarrhoea.

Why are we 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 which helps limit infections. Some countries also use seasonal influenza vaccines to reduce illness and deaths.

But influenza A(H1N1) is a new virus and one to which most people have no or little immunity and, therefore, this virus could cause more infections than are seen with seasonal flu. WHO is working closely with manufacturers to expedite the development of a safe and effective vaccine but it will be some months before it is available.

The new influenza A(H1N1) appears to be as contagious as seasonal influenza, and is spreading fast particularly among young people (from ages 10 to 45). The severity of the disease ranges from very mild symptoms to severe illnesses that can result in death. The majority of people who contract the virus experience the milder disease and recover without antiviral treatment or medical care. Of the more serious cases, more than half of hospitalized people had underlying health conditions or weak immune systems.

Most people experience mild illness and recover at home. When should someone seek medical care?

A person should seek medical care if they experience shortness of breath or difficulty breathing, or if a 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 - is adequate for recovery in most cases. (A non-aspirin pain reliever should be used by children and young adults because of the risk of Reye's syndrome.)

Use of pandemic influenza A (H1N1) vaccines

Up-to-date as of 23 July 2009 (Originally posted on 2 May 2009 and revised on 27 May and 12 July 2009)

Will it be possible to deliver influenza A (H1N1) vaccine simultaneously with other vaccines?

Inactivated influenza vaccine can be given at the same time as other injectable non-influenza vaccines, but the vaccines should be administered at different injection sites. Advice is being developed at present by regulatory authorities on co-administration of pandemic and seasonal influenza vaccines and will be provided when available.

How can a person who wishes to be vaccinated against influenza A (H1N1) receive the vaccine?

Once the first doses of pandemic influenza A(H1N1) vaccine become available, national health authorities will decide how to implement national vaccination campaigns.