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Questions & Answers
Seasonal Flu Vaccine
What kind of flu vaccines are there?
There are two types of vaccines that protect against the flu. The "flu shot" is an inactivated vaccine (containing killed virus) that is given with a needle, usually in the arm. The flu shot is approved for use among people 6 months of age or older, including healthy people and those with chronic medical conditions (such as asthma, diabetes, or heart disease). A different kind of vaccine, called the nasal-spray flu vaccine (sometimes referred to as LAIV for Live Attenuated Influenza Vaccine or FluMist®), was approved in 2003. The nasal-spray flu vaccine contains attenuated (weakened) live viruses, and is administered by nasal sprayer. It is approved for use only among healthy* people 2-49 years of age who are not pregnant.
Each of the two types of vaccine contains three influenza viruses, which are chosen based on information about recently circulating strains. Each of the three vaccine strains in both vaccines – one A (H3N2) virus, one A (H1N1) virus, and one B virus – are representative of the influenza vaccine strains recommended for that year. Viruses for both vaccines are grown in eggs.
How do flu vaccines work?
Both flu vaccines (the flu shot and the nasal-spray flu vaccine (LAIV)) cause antibodies to develop in the body. These antibodies provide protection against influenza virus infection.
Why should people get vaccinated against the flu?
Influenza is a serious disease, and people of any age can get it. In an average year, the flu causes 36,000 deaths (mostly among those aged 65 years or older) and more than 200,000 hospitalizations in the United States. The "flu season" in the United States is usually from November through April each year. During this time, flu viruses are circulating in the population. An annual flu vaccine (either the flu shot or the nasal-spray flu vaccine) is the best way to reduce the chances that you will get the flu and lessen the chance that you will transmit to others.
When should I get a flu vaccination?
CDC recommends that people get their flu vaccine as soon as vaccine becomes available in their community. Vaccination before December is best since this timing ensures that protective antibodies are in place before flu activity is typically at its highest. CDC continues to encourage people to get vaccinated throughout the flu season, which can begin as early as October and last as late as May. Over the course of the flu season, many different influenza viruses can circulate at different times and in different places. As long as flu viruses are still spreading in the community, vaccination can provide protective benefit.
In addition, there are other people who may benefit from vaccination as late or April or May, even if influenza viruses are no longer circulating in the United States. This includes:
- Persons likely to be traveling to the Southern Hemisphere where influenza may be circulating before the 2009-10 vaccine is available, and
- Children younger than 9 being vaccinated for the first time who still have not received their second recommended dose of vaccine. (If they get their second dose, then they will only need one dose of vaccine next season. If they do not get their second dose, they will still need to get two doses of vaccine the next season in order to best be protected by the vaccine. This is because studies have shown that two doses are needed in children younger than 9 the first year they are vaccinated in order to maximize the protective benefit from vaccination.)
Once you get vaccinated, your body makes protective antibodies in about two weeks. However, children younger than 9 years old who are being vaccinated for the first time still need a second dose 4 or more weeks later in order to be protected.
Does getting vaccinated against flu early in the season pose a risk that immunity may wane before the end of the season?
Flu vaccination provides protection against the influenza strains contained in the vaccine through one influenza season. Vaccination can begin as soon as vaccine is available. Studies have not demonstrated a benefit of receiving more than one dose during an influenza season, even among elderly persons with weakened immune systems.
Is it too late to get vaccinated after Thanksgiving (or the end of November)?
No. CDC recommends that providers begin to offer influenza vaccination as soon as vaccine becomes available in the fall, but if you have not been vaccinated by Thanksgiving (or the end of November), it can still be protective to get vaccinated in December or later because influenza disease usually peaks in January or February most years, and disease can occur as late as May.
Does flu vaccine work right away?
No. It takes about two weeks after vaccination for antibodies to develop in the body and provide protection against influenza virus infection. In the meantime, you are still at risk for getting the flu. That's why it's better to get vaccinated early in the fall, before the flu season really gets under way.
Can I get the flu even though I got a flu vaccine this year?
Yes. The ability of flu vaccine to protect a person depends on two things: 1) the age and health status of the person getting vaccinated, and 2) the similarity or "match" between the virus strains in the vaccine and those circulating in the community. If the viruses in the vaccine and the influenza viruses circulating in the community are closely matched, vaccine effectiveness is higher. If they are not closely matched, vaccine effectiveness can be reduced. However, it's important to remember that even when the viruses are not closely matched, the vaccine can still protect many people and prevent flu-related complications. Such protection is possible because antibodies made in response to the vaccine can provide some protection (called cross-protection) against different, but related strains of influenza viruses. For more information about vaccine effectiveness, visit How Well Does the Seasonal Flu Vaccine Work?
Why do I need to get vaccinated against the flu every year?
Flu viruses change from year to year, which means two things. First, you can get the flu more than once during your lifetime. The immunity (natural protection that develops against a disease after a person has had that disease) that is built up from having the flu caused by one virus strain doesn't always provide protection when a new strain is circulating. Second, a vaccine made against flu viruses circulating last year may not protect against the newer viruses. That is why the influenza vaccine is updated to include current viruses every year.
Another reason to get flu vaccine every year is that after you get vaccinated your immunity declines over time and may be too low to provide protection after a year.
How are the viruses for flu vaccine selected?
Each year, many laboratories throughout the world, including in the United States, collect flu viruses. Some of these flu viruses are sent to one of four World Health Organization (WHO) reference laboratories, one of which is at the Centers for Disease Control and Prevention (CDC) in Atlanta, for detailed testing. These laboratories also test how well antibodies made to the current vaccine react to the circulating virus and new flu viruses. This information, along with information about flu activity, is summarized and presented to an advisory committee of the U.S. Food and Drug Administration (FDA) and at a WHO meeting. These meetings result in the selection of three viruses (two subtypes of influenza A viruses and one influenza B virus) to go into flu vaccines for the following fall and winter. Usually, one or two of the three virus strains in the vaccine are changed each year.
Why do manufacturers and distributors take a phased approach to vaccine distribution?
Influenza vaccine production begins as early as 6-9 months before the beginning of vaccine distribution. Even with this early start, it isn't possible to complete the entire production and distribution process prior to the vaccination season, particularly given the limited number of influenza vaccine manufacturing plants in the United States and the large number of doses that are produced each year. Instead, influenza vaccine distribution takes place in a phased fashion over a number of months. It begins in late summer for some manufacturers and vaccine products and usually completes near the end of November or early in December. This system can leave doctors and other vaccine providers with uncertainty about when they can expect to receive their full order of vaccine and can make it difficult for them to plan their vaccination activities. Manufacturers and distributors work to try to get some vaccine to as many providers as possible as soon as possible so that they can begin vaccinating their patients. Getting some vaccine to all providers early in the season is important, because all providers serve at least some high-risk patients (such as people 50 years of age and older or those with chronic health conditions such as asthma, kidney disease, diabetes, lung disease and weakened immune system) and their household contacts.
What role does the Department of Health and Human Services play in the supply and distribution of the seasonal influenza vaccine?
Influenza vaccine production and distribution are primarily private sector endeavors. The Department of Health and Human Services and CDC do not have the authority to control influenza vaccine distribution nor the resources to manage such an effort. However, the Department has made significant efforts to enhance production capacity of seasonal influenza vaccines, including supporting manufacturers as they invest in processes to stabilize and increase their production capacity and improving guidance about the approval process at the Food and Drug Administration.
For More Information
For information about the 2008-09 influenza vaccine including vaccine supply, visit "2008-2009 Flu Season" at http://www.cdc.gov/flu/about/season/index.htm
For general information about flu vaccine side effects, visit http://www.cdc.gov/flu/protect/keyfacts.htm
For specific information related to the flu shot, visit http://www.cdc.gov/flu/about/qa/flushot.htm
For information specific to the nasal spray vaccine, visit http://www.cdc.gov/flu/about/qa/nasalspray.htm
For information about the efficacy of flu vaccine, visit http://www.cdc.gov/flu/about/qa/vaccineeffect.htm
* "Healthy" indicates persons who do not have an underlying medical condition that predisposes them to influenza complications.
- Page last modified September 8, 2008
- Content Source: Coordinating Center for Infectious Diseases (CCID)
- National Center for Immunization and Respiratory Diseases (NCIRD)
