Flu Shot Failure? Questions & Answers

The annual flu vaccine may not be as effective this year as in past years, the CDC has announced. Here's why.

Medically Reviewed by Louise Chang, MD on February 27, 2008

The CDC reports this year's flu shot may not protect against a strain of influenza that's hitting the U.S.

Q. Does this mean the flu shot is useless?

Not at all. Although this year's flu vaccine doesn't match two of the three main types of flu strains now in circulation, people who did get a flu shot and catch the flu get a much milder disease. This can make a life-or-death difference to people who are at high risk of flu complications, such as pregnant women, young children, the elderly, and those with chronic medical conditions such as diabetes, asthma, and heart disease.

At the Feb. 27 meeting of the CDC's Advisory Committee on Immunization Practices, University of Virginia professor James C. Turner, MD, said a UVA study found this year's vaccine to be 41% protective against serious flu illness. A similar study in 2003 found that year's vaccine to be 69% protective.

Turner, executive director for student health at UVA, said the school this year had a "robust" flu outbreak. Because UVA has a unique program that tracks students' flu vaccination status and uses rapid flu tests to detect flu outbreaks, CDC flu experts tell WebMD that this estimate is probably accurate, although the vaccine may be more or less protective in different regions of the country.

Q. Why was there a mismatch in this year's flu shot?

Flu vaccines contain a mixture of two influenza "A" virus strains -- this year they are types H3N2 and H1N1 -- and a "B" virus strain. These viruses mutate or "drift" a little each year, which is why the annual flu vaccine must be reformulated each year.

Most years, experts manage to match their flu vaccine selections in the spring to the strains that circulate later in the year throughout the U.S. and globally. In winter 2007, experts considered including a particular H3N2 strain of influenza A, but dropped the plan when they couldn't find samples that would grow properly in the manufacturing process.

Influenza A typically makes up about 85% of all flu cases. However, this year the particular H3N2 strain that was left out of the vaccine wound up being the dominant influenza A virus, which accounts for more than 60% of flu cases.

Q. Who determines which influenza strains go into each year's vaccine?

The CDC collaborates with the World Health Organization (WHO) and other groups to track the flu virus throughout the world. These organizations also monitor influenza activity and virus isolates throughout the world to monitor disease activity and forecast the appropriate components for each year's flu vaccine.

Because of this year's vaccine mismatch, a government scientific advisory board recommended for the first time this week a complete overhaul of the makeup of the flu vaccine for next year. This move would replace all three flu virus strains in this year's vaccine with three new strains for next year flu season.

Q. Who should get a flu shot?

An annual flu shot is recommended for anyone who wants to reduce his or her chances of getting the flu. The flu shot is highly recommended for certain high-risk individuals who are more prone to flu complications, such as pneumonia. The CDC defines higher-risk people as the following:

  • People 50 years or older
  • People who live in nursing homes or long-term care facilities
  • People with certain chronic medical conditions
  • Women who are pregnant
  • All children age 6 months to 18 years.

The CDC also recommends that persons in close contact with someone in a high-risk group, people who care for children less than 6 months old, and health care workers be vaccinated.

Q. Do children need to get two doses of the flu vaccine the first time they are vaccinated?

Yes. A CDC study presented to the Advisory Committee on Immunization Practices panel showed that the flu vaccine in the past two flu seasons (2005-2006 and 2006-2007) was 75% effective -- but only if children got two doses the first time they were vaccinated.

Q. Can the flu shot cause the flu?

No, the flu shot cannot cause the flu because it does not contain a live virus. This flu shot is an inactivated (killed) virus that is given with a needle. The nasal influenza vaccine called FluMist contains weakened viruses, which do not cause flu illness but sometimes can cause reactions such as runny nose, nasal congestion, and sore throat.

Both the flu shot and FluMist work by causing antibodies to develop in your body. These antibodies provide protection against infection from the flu virus. The flu vaccine may cause low-grade fever, fatigue, and muscle aches in some people.

Q. How can I catch flu?

Flu is spread when you inhale droplets in the air that contain the flu virus, make direct contact with respiratory secretions through sharing drinks or utensils, or handle items contaminated by an infected person. In the latter case, the flu virus on your skin infects you when you touch or rub your eyes, nose, or mouth. That's why frequent and thorough hand washing is crucial to limit the spread of influenza.

Q. How do I know if I have a cold or flu?

Both cold and flu bring coughing, headache, and chest discomfort. With the flu, though, you are likely to run a high fever for several days and have headache, muscle aches, fatigue, and weakness. Usually, complications from colds are relatively minor, but a severe case of flu can lead to a life-threatening illness such as pneumonia.

Q. How is stomach flu different from flu?

It's not uncommon to mistake an intestinal bug or stomach flu (called gastroenteritis) for the flu. Gastroenteritis refers to irritation of the gastrointestinal tract (stomach and intestines). With gastroenteritis, you will have symptoms such as abdominal cramps, stomach pain, vomiting, nausea, and diarrhea. With flu, you will have symptoms such as high fever, headache, fatigue, respiratory congestion, and weakness.

Q. Do antibiotics treat flu?

No, antibiotics cannot conquer the flu virus because antibiotics only treat bacterial infections. Antibiotics may be helpful if there is a secondary bacterial infection. A few antiviral drugs are available to treat flu. Some antivirals may also reduce the severity and the duration of flu if started within the first 48 hours of flu symptoms.

Show Sources


American Lung Association: "Cold and Flu Guidelines: Influenza." 

CDC: "Seasonal Flu." 

FDA: "Influenza: Vaccination Still the Best Protection."

WebMD Medical News: "Panel: Overhaul Next Year’s Flu Vaccine." 

WebMD Medical News: "Most Flu Bugs Don't Match Flu Vaccine."

WebMD Medical Reference: "What Is Flu?"

WebMD Medical Reference: "Flu Shot: Influenza Vaccine and Side Effects."

WebMD Medical Reference: "Pregnancy and Flu."

WebMD Medical Reference: "Stomach Flu or Influenza?"

WebMD Medical Reference: "Flu Prevention Strategies."

David Shay, MD, CDC researcher.

James C. Turner, MD, executive director for student health, University of Virginia.

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