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Content on this page was developed during the 2009-2010 H1N1 pandemic and has not been updated.

  • The H1N1 virus that caused that pandemic is now a regular human flu virus and continues to circulate seasonally worldwide.
  • The English language content on this website is being archived for historic and reference purposes only.
  • For current, updated information on seasonal flu, including information about H1N1, see the CDC Seasonal Flu website.

Key Facts About 2009 H1N1 Flu Vaccine

March 8, 2010, 11:00 AM ET

A flu vaccine is the single best way to protect against influenza illness. This season, there is a seasonal flu vaccine to protect against seasonal flu viruses and a 2009 H1N1 vaccine to protect against the 2009 H1N1 influenza virus (sometimes called “swine flu”).

This page contains information about the 2009 H1N1 flu vaccine.

There are two kinds of 2009 H1N1 vaccines being produced:

  • A 2009 H1N1 "flu shot" — an inactivated vaccine (containing killed virus) that is given with a needle, usually in the arm. The indications for who can get the 2009 H1N1 flu shot are the same as for seasonal flu shots. The flu shot is approved for use in people 6 months of age and older, including healthy people, individuals with chronic medical conditions and pregnant women. The same manufacturers who produce seasonal flu shots are producing 2009 H1N1 flu shots for use in the United States this season. The 2009 H1N1 flu shot is being made in the same way that the seasonal flu shot is made.
  • The 2009 H1N1 nasal spray flu vaccinea vaccine made with live, weakened viruses that do not cause the flu (sometimes called LAIV for "live attenuated influenza vaccine"). The indications for who can get the 2009 H1N1 nasal spray vaccine are the same as for seasonal nasal spray vaccine. LAIV is approved for use in healthy* people 2 years to 49 years of age who are not pregnant. The nasal spray vaccine for use in the United States is being made by MedImmune, the same company that makes the seasonal nasal spray vaccine called “FluMist®.” The 2009 H1N1 nasal spray vaccine is being made in the same way as the seasonal nasal spray vaccine.

About 2 weeks after vaccination, antibodies that provide protection against 2009 H1N1 influenza virus infection will develop in the body.

The 2009 H1N1 vaccine will not protect against seasonal influenza viruses.

Got your flu vaccine? CDC and Johns Hopkins want to know how you're feeling.
The vaccine for 2009 H1N1 flu will be the same for the entire 2009-2010 influenza season, which extends into the spring of 2010. The "2009" in the name only relates to the year the virus was first identified; it does not have to do with how long the vaccine will work or the year in which it should be administered. The 2009 H1N1 strain is not included in the 2009-2010 seasonal flu vaccine because it was identified after manufacturers had started making the seasonal flu vaccine.

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When to Get Vaccinated

Vaccination against 2009 H1N1 should begin now and continue throughout the influenza season, which extends into the spring of 2010. This is because the timing and duration of flu activity can vary. Flu seasons can last as late as April or May. By early October 2009, extensive 2009 H1N1 flu activity was being reported in the United States. It’s possible that there may be waves of 2009 H1N1 activity during the 2009-2010 flu season that hit communities more than once over the course of the season. While 2009 H1N1 influenza virus is the predominant influenza virus in circulation, CDC still expects that seasonal influenza viruses will circulate and continues to recommend that people get a seasonal flu vaccine to protect against seasonal flu viruses. The ACIP has issued separate recommendations on who should get the 2009-10 seasonal vaccine.

Vaccine Supply

Vaccines to protect against 2009 H1N1 are widely available. More doses are expected for shipment each week. We ask members of the public who want to receive this vaccine to be patient as this program expands and more vaccine continues to become available.

Who Should Get Vaccinated

CDC’s Advisory Committee on Immunization Practices (ACIP), a panel made up of medical and public health experts, met July 29, 2009, to make recommendations on who should receive the 2009 H1N1 vaccine when it becomes available. While the federal government has purchased enough vaccine so that anyone who wants to get vaccinated can, ACIP's statement on the "Use of Influenza A (H1N1) 2009 Monovalent Vaccine" recommends that vaccination efforts should focus first on people in five target groups who are at higher risk for 2009 H1N1 influenza or related complications, like children, especially those younger than 5 years of age and those who have high risk medical conditions, healthcare and emergency medical personnel who are likely to come in contact with influenza viruses as part of their occupation and could transmit influenza viruses to others in medical care settings, or those who are close contacts of infants younger than 6 months (who are too young to be vaccinated). These five target groups make up an estimated 159 million people in the United States.

For children younger than 10 years of age, CDC recommends that the two doses of 2009 H1N1 vaccine be separated by 4 weeks. However, if the second dose is separated from the first dose by at least 21 days, the second dose can be considered valid.

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Late Season Recommendations

Why should flu be taken seriously?

Flu should never be dismissed as “just the flu.” It’s a serious disease, and while most cases are mild, some can be deadly.  So far this flu season, most flu activity has been caused by the 2009 H1N1 virus, which was first identified in April 2009 and caused the first flu pandemic in 40 years.

Because many people with influenza illness are not tested for flu or are tested late in their illness, methods have been developed to estimate the numbers of people with influenza illness and with influenza-related complications, including hospitalizations and deaths.  CDC estimates that from April to January 16, 2010, approximately 57 million cases of 2009 H1N1 occurred in the United States, including 257,000 H1N1-related hospitalizations and about 11,690 deaths. 

With 2009 H1N1, approximately 90% of estimated hospitalizations and 87% of estimated deaths from April through January 16, 2010 occurred in people younger than 65 years old.  In contrast, with seasonal influenza, about 60% of seasonal flu-related hospitalizations and 90% of flu-related deaths occur in people 65 years and older.   This data confirms that the 2009 H1N1 impacted younger adults and children more than older adults compared to seasonal flu.  However, people in all age groups can develop severe illness from either seasonal flu or from 2009 H1N1.

Flu season seems to be dying down—Why do people still need a vaccination against 2009 H1N1?

The flu season is not over yet. As recently as January 15, 7 states were still reporting regional flu activity, so flu is still out there. Also, seasonal flu typically peaks in February and March and influenza activity can occur as late as May. So, increased activity from either seasonal flu, 2009 H1N1 or both are still possible this season.
For example, during the 1957-58 pandemic, flu activity dropped in December and January. Public health officials assumed the worst was over, and stopped encouraging people to get vaccinated. Then flu activity increased abruptly in February and March, and hospitalizations and deaths increased as well.

That was an important lesson—even if flu activity dies down in January, as it has this year, the season is not over. It’s still important to get vaccinated against the flu and be as prepared as possible if activity increases again this season.

Is there anyone who should take priority when it comes to getting a flu vaccination?

Right now there is enough vaccine for everyone who hasn’t been vaccinated and wants to be. Children who are 9-years old or younger need two doses of vaccine about a month apart. There is enough vaccine for them as well.

But it is especially important for certain groups of people to get vaccinated. If you have certain underlying health conditions—such as asthma, heart disease, or diabetes—or if you’re pregnant, you’re at greater risk of complications from flu. If you’re over 65, you’re also at risk of complications.

If you’re a health care worker, you should get vaccinated. If you care for or live with an infant under 6 months of age (babies less than 6 months old are too young to be vaccinated), you should get vaccinated, too, to prevent spreading the flu to people who are vulnerable.

Where should I go to get vaccinated against 2009 H1N1?

This is a very good time to get vaccinated. Right now not many people are sick, and there is enough vaccine for everyone who hasn’t been vaccinated yet—including kids 9 years old and younger who need a second dose.

An easy way to find out where vaccine is available in your area is by going to and clicking on the vaccine locator. In addition, has a new Facebook application—the “Flu Fighter.” It’s one of the site’s many “viral” communication tools to help spread the word that getting a vaccine against 2009 H1N1 is the best way to protect yourself and your friends. When you go to please check it out.

If the second dose of 2009 H1N1 flu vaccine for children under 10 years of age is given over a month after the first dose, is it still considered effective?

Yes. CDC recommends that the two doses of vaccine against 2009 H1N1 influenza virus be separated by 4 weeks. However, if the second dose is separated from the first dose by at least 21 days, the second dose can be considered valid.

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Who Should Not Be Vaccinated

There are some people who should not get any flu vaccine without first consulting a physician. These include:

  • People who have a severe allergy to chicken eggs.
  • People who have had a severe reaction to an influenza vaccination.
  • People who developed Guillain-Barré syndrome (GBS) within 6 weeks of getting an influenza vaccine previously. (For information, see General Questions and Answers on Guillain-Barré syndrome (GBS).
  • Children younger than 6 months of age (influenza vaccine is not approved for this age group), and
  • People who have a moderate-to-severe illness with a fever (they should wait until they recover to get vaccinated.)

Vaccine Effectiveness

The ability of a flu vaccine to protect a person depends on the age and health status of the person getting the vaccine, and the similarity or "match" between the viruses or virus in the vaccine and those in circulation. CDC analyzes circulating inflluenza viruses on an ongoing basis to determine how closely matched they are to vaccine viruses and publishes the information weekly in FluView. In addition, every year CDC monitors vaccine effectiveness. For more information about flu vaccine effectiveness, see How Well Does the Seasonal Vaccine Work? 

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Vaccine Side Effects (What to Expect)

The same side effects typically associated with the seasonal flu shot and the seasonal nasal spray vaccine are expected with the 2009 H1N1 flu shot and 2009 H1N1 nasal spray vaccine.

These are:

The flu shot: The viruses in the flu shot are killed (inactivated), so you cannot get the flu from a flu shot. Some minor side effects that could occur are:

  • Soreness, redness, or swelling where the shot was given
  • Fever (low grade)
  • Aches

If these problems occur, they begin soon after the shot, are usually mild, and usually last 1 to 2 days. Almost all people who receive influenza vaccine have no serious problems from it. However, on rare occasions, flu vaccination can cause serious problems, such as severe allergic reactions.

The nasal spray: The viruses in the nasal-spray vaccine are weakened and do not cause severe symptoms often associated with influenza illness. (In clinical studies, transmission of vaccine viruses to close contacts has occurred only rarely.)
In children, side effects from LAIV can include:

  • runny nose
  • wheezing
  • headache
  • vomiting
  • muscle aches
  • fever

In adults, side effects from LAIV can include

  • runny nose
  • headache
  • sore throat
  • cough

For more information about vaccine side effects and safety see General Questions and Answers on 2009 H1N1 Influenza Vaccine Safety.

For More Information

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