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Selecting the Viruses in the Seasonal Influenza (Flu) Vaccine

Questions & Answers

What does the seasonal influenza vaccine protect against?

While there are many different flu viruses, each season a flu vaccine protects against the three or four viruses that research suggests will be most common. Three kinds of flu viruses commonly circulate among people today: Influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses.

All of the 2015-2016 influenza vaccine is made to protect against the following:

  • A/California/7/2009 (H1N1)pdm09-like viruses
  • A/Switzerland/9715293/2013 (H3N2)-like viruses
  • B/Phuket/3073/2013-like viruses. (This is a B/Yamagata lineage virus)

Some of the 2015-2016 flu vaccine is quadrivalent vaccine, which also protects against an additional lineage of B virus. For this season that will be a B/Brisbane/60/2008-like virus. This is a B/Victoria lineage virus.

Vaccines that give protection against three kinds of viruses are called trivalent vaccines. Vaccines that give protection against four viruses are called quadrivalent vaccines.

More information about influenza vaccines is available at Preventing Seasonal Flu With Vaccination.

How are the viruses selected to make flu vaccine?

The influenza (flu) viruses selected for inclusion in the seasonal flu vaccines are updated each year based on which influenza viruses are circulating, how they are spreading, and how well current vaccine components protect against newly identified viruses. Currently, 142 national influenza centers in 113 countries conduct year-round surveillance for influenza and study influenza disease trends. These laboratories also send influenza viruses to the five World Health Organization (WHO) Collaborating Centers for Reference and Research on Influenza located in Atlanta, Georgia, USA (Centers for Disease Control and Prevention, CDC); London, United Kingdom (National Institute for Medical Research); Melbourne, Australia (Victoria Infectious Diseases Reference Laboratory); Tokyo, Japan (National Institute for Infectious Diseases); and Beijing, China (National Institute for Viral Disease Control and Prevention) for additional analyses.

The influenza viruses in the seasonal flu vaccine are selected each year based on surveillance on which viruses are circulating and forecasts about which viruses are the most likely to circulate during the coming season. WHO recommends specific vaccine viruses for inclusion in influenza vaccines, but then each individual country makes their own decision about which viruses should be included in influenza vaccines licensed in their country. In the United States, the United States Food and Drug Administration (FDA) determines which vaccine viruses will be used in U.S.-licensed vaccines.

What other factors can influence which viruses are chosen to go into the seasonal flu vaccine?

An important practical factor in the recommendation about what viruses to include in a flu vaccine is whether or not there is a good vaccine virus available; that is, a virus that could be used in vaccine production and which would likely protect against the viruses likely to circulate in the upcoming season. Vaccine viruses must be similar to other influenza viruses that are forecast as being the most likely to circulate during the upcoming year and must be grown from a clinical specimen in special pathogen-free chicken eggs or canine kidney cells, but not in any other cell lines. Vaccine viruses must be tested and available in time to allow for production of the large amount of vaccine virus needed to make the vaccine. Occasionally, a suitable new vaccine virus cannot be identified in time for inclusion in the upcoming year’s vaccine.

How are influenza data used to select vaccine viruses?

Twice a year, the World Health Organization (WHO) organizes a consultation with the Directors of the WHO Collaborating Centers and representatives of key national laboratories. They review the results of surveillance, laboratory and clinical studies, and the availability of vaccine viruses and make recommendations on the composition of the influenza vaccine. These meetings take place in February for selection of the upcoming Northern Hemisphere’s seasonal influenza vaccine and in September for the Southern Hemisphere’s vaccine. WHO recommends specific vaccine viruses for inclusion in influenza vaccines, but then each individual country makes their own decision about which viruses should be included in influenza vaccines licensed in their country. In the United States, the Food and Drug Administration (FDA) makes the final decision about vaccine viruses for influenza vaccines to be sold in the U.S. Information about circulation of influenza viruses and available vaccine viruses is summarized and presented to an advisory committee of the FDA in February each year for the U.S. decision about which viruses to include in the upcoming season’s vaccine.

What is CDC’s Influenza Division’s role in vaccine virus selection?

As one of five WHO Collaborating Centers, CDC’s Influenza Division receives and tests thousands of influenza viruses from around the world each year and collaborates with other WHO Collaborating Centers and National Influenza Centers in the bi-annual seasonal vaccine virus selection process for the Southern and Northern Hemispheres. CDC plays a major role in testing and identifying new strains of influenza viruses through their global surveillance activities and then preparing candidate vaccine viruses. The Influenza Division provides this information to other directors of WHO Collaborating Centers and representatives of key national laboratories and participates in discussions regarding which viruses will be recommended for inclusion in flu vaccines. CDC also presents information to FDA’s advisory committee for their decision making and helps to identify vaccine viruses.

What happens after a recommendation has been made about which viruses should be included in the seasonal flu vaccine?

As soon as a recommendation has been issued about what viruses should be included in the vaccine, private sector manufacturers begin the process of producing vaccine. In fact, some manufacturers may start growing one or more viruses for the vaccine even before a WHO or FDA decision is made based on what they think may be the recommended vaccine viruses. This allows manufacturers more time to make vaccine for the fall; the more time a manufacturer has to make vaccine, the greater the number of doses that can be produced.

How long does it take to manufacture seasonal influenza vaccine?

It takes at least six months to produce large quantities of influenza vaccine. For vaccine to be delivered in time for vaccination to begin in October and November, manufacturers may begin to grow one or more of the vaccine viruses in January based on their best guess as to what viruses are most likely to be included in the vaccine. For information about flu vaccine production, see How Influenza (Flu) Vaccines Are Made.

What determines the effectiveness of the seasonal influenza vaccine each year?

How well the vaccine works each season, often called “vaccine effectiveness” varies. For more information see Vaccine Effectiveness - How Well Does the Flu Vaccine Work?

What is meant by a “good match” between viruses in the vaccine and circulating influenza viruses?

A good match is said to occur when the viruses in the vaccine and the viruses circulating among people during a given influenza season are closely related and the antibodies produced by the vaccine are able to provide protection against infection.

What if circulating viruses and the vaccine viruses are different?

During seasons when one or more of the vaccine viruses are different from circulating viruses, vaccine effectiveness can be reduced. However, even when the viruses in the vaccine and circulating viruses are not well matched, a vaccine may still offer some protection against circulating viruses. For more information, see Vaccine Effectiveness - How Well Does the Flu Vaccine Work?

How does CDC determine if the vaccine virus is like a circulating virus?

CDC’s Influenza Division collects and reports information on influenza activity in the United States each week. Laboratory studies of circulating influenza viruses allow CDC to evaluate how close a match there is between viruses in the vaccine and circulating viruses each season. Antigenic characterization is one process that can give an indication of the flu vaccine’s ability to produce an immune response against the influenza viruses circulating in people. CDC also conducts studies to determine the effectiveness of the seasonal vaccine against circulating viruses. For more information, see Vaccine Effectiveness - How Well Does the Flu Vaccine Work? For more information about CDC’s surveillance and to access the weekly reports, visit Flu Activity and Surveillance.

Can the seasonal flu vaccine provide protection even if the vaccine is not a “good” match?

Yes, antibodies made in response to vaccination with one flu virus can sometimes provide protection against different but related viruses. A less than ideal match may result in reduced vaccine effectiveness against the virus that is different from what is in the vaccine, but it can still provide some protection against flu illness.

In addition, it's important to remember that the flu vaccine contains three or four flu viruses (depending on the type of vaccine you receive) so that even when there is a less than ideal match or lower effectiveness against one virus, the vaccine may protect against the other viruses.

For these reasons, even during seasons when there is a less than ideal match, CDC continues to recommend flu vaccination for everyone 6 months and older. Vaccination is particularly important for people at high risk for serious flu complications, and their close contacts.

Why is it sometimes difficult to get a good vaccine virus for vaccine production?

There are a number of factors that can make getting a good vaccine virus for vaccine production challenging, including both scientific issues and issues of timing. Currently, only viruses grown in special pathogen-free chicken eggs can be used as vaccine viruses. However, some influenza viruses, like H3N2 viruses, grow poorly in eggs, making it difficult to obtain candidate vaccine viruses.

In terms of timing, in some years certain influenza viruses may not appear and spread until later in the influenza season, or a virus can change late in the season or from one season to the next. This can make vaccine virus selection very challenging.

Besides vaccination, what can I do to protect myself against influenza?

The single best way to protect against seasonal flu is to get vaccinated each year, but good health habits like covering your cough and washing your hands often can help stop the spread of germs and prevent respiratory illnesses like the flu. Flu antiviral drugs are an important second line of defense that can be used to treat flu.

These medications must be prescribed by a doctor. Visit What You Should Know About Flu Antiviral Drugs for more information about antiviral drugs.

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