What is 2009 H1N1 influenza?

2009 H1N1 influenza (also called Swine Flu) is caused by a new strain of influenza virus. It has spread to many countries. Like other flu viruses, 2009 H1N1 spreads from person to person through coughing, sneezing, nasal secretions, and sometimes through touching objects contaminated with the virus. Signs of 2009 H1N1 can include: fatigue, fever, sore throat, muscle aches, chills, coughing, and sneezing. Some people also have diarrhea and vomiting. Most people feel better within a week. But some people get pneumonia or other serious illnesses. Some people have to be hospitalized and some die.

How is 2009 H1N1 different from regular (seasonal) flu?

Seasonal flu viruses change from year to year, but they are closely related to each other. People who have had flu infections in the past usually have some immunity to seasonal flu viruses (their bodies have built up some ability to fight off the viruses). The 2009 H1N1 flu virus is a new virus strain. It is very different from seasonal flu viruses. Most people have little or no immunity to 2009 H1N1 flu (their bodies are not prepared to fight off the virus).

2009 H1N1 influenza vaccine

Vaccines are being made to protect against 2009 H1N1 influenza. These vaccines are produced just like seasonal flu vaccines. They are expected to be as safe and effective as seasonal flu vaccines. They will not prevent ''influenza-like'' illnesses caused by other viruses. They will not prevent seasonal flu. You should also get seasonal influenza vaccine, if you want to be protected against seasonal flu. Inactivated (killed) vaccine is injected into the muscle, like the annual flu shot. This information describes the inactivated vaccine. A live, intranasal vaccine is also available. It is described in a separate monograph.

Some inactivated H1N1 vaccine contains a preservative called thimerosal to keep it free from germs. While some people have suggested that thimerosal may be related to developmental problems in children, that theory has not been supported by research. Thimerosal-free vaccine is also available.

Who should get 2009 H1N1 influenza vaccine and when?


Groups recommended to receive 2009 H1N1 vaccine first are:

  • Pregnant women,
  • People who live with or care for infants younger than 6 months of age,
  • Health care and emergency medical personnel,
  • Anyone from 6 months through 24 years of age,
  • Anyone from 25 through 64 years of age with certain chronic medical conditions or a weakened immune system.

These groups should also be vaccinated:

  • Healthy 25-64 year olds,
  • Adults 65 and older.

The Federal government is providing this vaccine for receipt on a voluntary basis. However, state law or employers may require vaccination for certain persons.


Get vaccinated as soon as the vaccine is available. Children through 9 years of age should gettwo dosesof vaccine, about a month apart. Older children and adults need only one dose.

Some people should not get the vaccine or should wait

You should not get 2009 H1N1 flu vaccine if you have asevere (life-threatening) allergy to eggs, or to any other substance in the vaccine. Tell the person giving you the vaccine if you have any severe allergies.

Also tell them if you have ever had a life-threatening allergic reaction after a dose of seasonal flu vaccine, or Guillain Barré Syndrome (a severe paralytic illness also called GBS). These may not be reasons to avoid the vaccine, but the medical staff can help you decide.

If you are moderately or severely ill, you might be advised to wait until you recover before getting the vaccine. If you have a mild cold or other illness, there is usually no need to wait.

Pregnancy or breastfeeding are not reasons to avoid getting 2009 H1N1 flu vaccine.

Inactivated 2009 H1N1 vaccine may be given at the same time as other vaccines, including seasonal influenza vaccine.

What are the risks from 2009 H1N1 influenza vaccine?

A vaccine, like any medicine, could cause a serious problem, such as a severe allergic reaction. But the risk of any vaccine causing serious harm, or death, is extremely small. The virus in inactivated 2009 H1N1 vaccine has been killed, so you cannot get influenza from the vaccine. The risks from inactivated 2009 H1N1 vaccine are expected to be similar to those from seasonal flu vaccine:

Mild problems:

  • soreness, redness, tenderness, or swelling where the shot was given
  • fainting (mainly adolescents)
  • headache, muscle aches
  • fever
  • nausea

If these problems occur, they usually begin soon after the shot and last 1-2 days.

Severe problems:

  • Life-threatening allergic reactions to vaccines are very rare. If they do occur, it is usually within a few minutes to a few hours after the shot.
  • In 1976, an earlier type of swine flu vaccine was associated with cases of Guillain-Barré Syndrome (GBS). Since then, flu vaccines have not been clearly linked to GBS.

What if there is a severe reaction?

What should I look for?

  • Any unusual condition, such as a high fever or behavior changes. Signs of a serious allergic reaction can include difficulty breathing, hoarseness or wheezing, hives, paleness, weakness, a fast heartbeat, or dizziness.

What should I do?

  • Call a doctor, or get the person to a doctor right away.
  • Tell the doctor what happened, the date and time it happened, and when the vaccination was given.
  • Ask your provider to report the reaction by filing a Vaccine Adverse Event Reporting System (VAERS) form. Or you can file this report through the VAERS website at Web Site, or by calling1-800-822-7967. VAERS does not provide medical advice.

Vaccine injury compensation

If you or your child has a reaction to the vaccine, your ability to sue is limited by law. However, a federal program has been created to help pay for the medical care and other specific expenses of certain persons who have a serious reaction to this vaccine. For more information about this program, call1-888-275-4772or visit the program's website at: Web Site.

How can I learn more?

  • Ask your provider. They can give you the vaccine package insert or suggest other sources of information.
  • Call your local or state health department.
  • Contact the Centers for Disease Control and Prevention (CDC): call1-800-232-4636(1-800-CDC-INFO) or visit CDC's website at Web Siteor Web Site.

2009 H1N1 Inactivated Influenza Vaccine Information Statement. U.S. Department of Health and Human Services/Centers for Disease Control and Prevention National Immunization Program. 10/2/2009.

AHFS® Consumer Medication Information. © Copyright, The American Society of Health-System Pharmacists, Inc., 7272 Wisconsin Avenue, Bethesda, Maryland. All Rights Reserved. Duplication for commercial use must be authorized by ASHP.

Selected Revisions: October 1, 2009.