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Fact Sheet

  January 15, 2004

Influenza Vaccine Effectiveness Studies

About Vaccine Effectiveness Studies

The Centers for Disease Control and Prevention routinely conducts studies to estimate the effectiveness of vaccines for the diseases they are designed to protect against. The early onset of the 2003-2004 flu season and the presence of a new, or “drifted” influenza A strain caused CDC to pursue a series of studies to answer questions about how effective this year’s influenza vaccine is against the circulating viruses. The first study in the series was designed to obtain a rapid estimate of the effectiveness of the vaccine against influenza like illness while the flu season was still underway. Assessing influenza vaccine effectiveness presents unique challenges because circulating influenza viruses and influenza vaccine vary from year to year.

Ideally, for flu vaccine effectiveness studies, researchers plan well in advance. Study participants are enrolled before the flu season begins; those who are going to be vaccinated receive their vaccinations before influenza arrives in the area; all suspected cases of influenza are confirmed by laboratory testing; and both vaccinated and unvaccinated persons are followed throughout the influenza season.

This is the first time CDC has conducted such a study in the midst of a flu season. CDC decided to undertake this rapid study to see if it would reveal any information that would help the public, physicians and public health officials make decisions about the use of influenza vaccine, antiviral medications, or other preventive measures.

Results of the study

This initial study showed that the 2003-2004 influenza vaccine was not effective or had very low effectiveness against “influenza-like illness” in a group of healthcare workers in Colorado. However, the study does not provide sufficient information to conclude that the vaccine is not effective at all against the dominant circulating influenza strain.

“Influenza-like illness” is a broad definition that includes people with influenza, but it may also include people with colds and people who are suffering from respiratory illness not caused by influenza.

This study provides us some limited information about the effectiveness of this year’s influenza vaccine. The tradeoff for immediate information is the inability to obtain complete information. Results from additional studies, now underway, will be necessary to further gauge the effectiveness of this season’s vaccine. Additional studies to determine the effectiveness of the vaccine against laboratory-confirmed influenza and influenza-related hospitalization are underway. In previous years with drifted influenza virus strains, such studies have demonstrated the effectiveness of the vaccine against laboratory confirmed influenza and influenza-related complications, even when no effectiveness is demonstrated against influenza-like-illness. Results from other studies are expected in late spring or early summer 2004.

CDC Vaccine Recommendation

CDC continues to recommend that people, particularly those who are at highest risk of complications from influenza, and their contacts, get vaccinated if they have not yet done so because: 1. The study was limited 2. The vaccine likely still offers protection against lab-confirmed influenza and flu-related complications and 3. The influenza A (H1N1) and influenza B strains may cause illness later in the flu season. 

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CDC protects people's health and safety by preventing and controlling diseases and injuries; enhances health decisions by providing credible information on critical health issues; and promotes healthy living through strong partnerships with local, national, and international organizations.


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