Flu Shot 101: Answering the Important Questions


The first rounds of flu shots are now being administered, but some Americans, even those who’ve read up on the basics of the flu still have questions about both the seasonal and swine flu shots.

For instance, how effective are they? Who develops them? How can the vaccine I get be different from what other people might get? And what kind of side effects might I experience? Here are some answers.

1. If vaccines are made based on guesses of what influenza strains will be around each flu season, how effective can they really be?

If results from decades past are any indication, flu vaccines have been effective.

“Past studies have shown in years when the vaccine viruses and circulating viruses are well-matched, the vaccine can reduce the chances of getting the flu by 70% to 90% in healthy adults,” the CDC says.

“If match is very suboptimal then effectiveness in preventing illness among healthy persons would typically be more like 40% to 50%,” says CDC spokesman Anthony Fiore.  While the CDC wouldn’t say how often strain selection wasn’t so good, “we think there is a good match in most years over past 20,” Fiore noted. If you’re one of those people who have gotten a flu shot and then the flu in the same year, that statement probably offers little comfort.

Effectiveness can also differ for the very young and the elderly, who may be more susceptible to illness, but they can still prevent major complications and death.  “The seasonal flu shot is 30% to 70% effective in preventing hospitalizations due to lung infections in people with chronic conditions like asthma or heart disease, notes the CDC.  And in nursing home residents, the vaccine might be “50% to 60% effective in preventing hospitalization or pneumonia, and 80% effective in preventing death from the flu,” the agency says.

It’s also important to note, however, that the flu vaccine can’t prevent colds and other illnesses with similar symptoms, and it won’t protect you from the flu virus if it’s already in your body because it takes about two weeks for the shots to work, according to the CDC.

2. Who decides which strains to target?

Strains of the flu virus that will be target by the flu vaccine are chosen based on circulating strains of the flu infecting people in previous years and information on how they change.  The Food and Drug Administration, Centers for Disease Control and Prevention and the World Health Organization collect samples of the influenza virus and choose the most prevalent and virulent strains that could cause the most illness in a given flu season, the CDC explains.

The CDC conducts lab tests on thousands of strains of the flu virus to see how antibodies fighting the flu react to the strain that’s already out, and to new flu strains.  The agency decides what strains should be targeted, then talks to the FDA.  Soon after, vaccine makers start the production process.

3. Is it safe to get both the seasonal flu and the swine flu shots at the same time?

Yes, according to the Department of Health and Human Services.  “Both the seasonal and … H1N1 flu shots can be given on the same day, but should be given at different sites,” different arms for example, HHS says.

Remember, two H1N doses are needed in children younger than 9 the first year they are vaccinated in order to maximize the protective benefit from vaccination, but only one is needed for anyone older. 

4. How was H1N1 “swine” flu vaccine development approached differently from regular flu vaccine development, and what is the vaccine development process like?

One difference between the regular season flu shot and the H1N1 shot is that the regular shot is a trivalent vaccine (it targets three different strains of the flu virus) and the H1N1 vaccine targets only one strain, says Jeff McLaughlin, spokesman for GlaxoSmithKline (Stock Quote: GSK), a vaccine maker.

“Otherwise, the H1N1 vaccines are produced by the same U.S.-licensed manufacturers who produce the seasonal flu and they are produced in the same way,” says FDA spokeswoman Pat El-Hinnawy. Companies develop vaccines to match the influenza strains expected to circulate that year and submit applications to the Food and Drug Administration for approval, she adds.  Both seasonal and H1N1 vaccines are manufactured using the same approval and manufacturing processes, according to HHS.

For shots, “the virus is inactivated, using the same processes the manufacturers use for seasonal influenza vaccines,” HHS explains, using eggs to develop the shot.  “The vaccine administered via nasal spray contains a live, attenuated virus,” which means that it’s weakened, according to the HHS.

“Influenza vaccine production begins as early as six to nine 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,” according to the CDC. The vaccines are distributed in phases over several months.

With that lead time in mind, swine flu shot development seems particularly rushed, with the first H1N1 vaccine trial only beginning in August, according to Medscape.  But since it takes about two weeks for a vaccine to take effect and prompt the body to develop disease-fighting antibodies, there was enough time to observe its effectiveness and determine that younger kids, those less than 9 years old, might need two shots in order to develop enough flu-fighting antibodies.

Vaccine developer Sanofi Pasteur (Stock Quote: SNY) collected trial data 21 days after vaccinations and found that “in adults 18 years of age through 64 years of age, 98% of participants achieved seroprotective antibody titers at 21 days,” meaning they developed the antibodies needed to fight the swine flu.  “In adults 65 years of age and older who received the same formulation, 93% achieved seroprotective titers,” the company reported. Results from trials in children will  be available later this month.  As far as safety goes, since swine flu vaccines are so similar to seasonal flu vaccines, they’re expected to be safe.

5. Are vaccines made by different companies different from each other?

While the targeted strains of the flu virus are the same, other flu shot ingredients can vary.  On its Web site, the FDA lists information on vaccines made by each manufacturer, including CSL Limited, Novartis Vaccines and Diagnostics (Stock Quote: NVS), Sanofi Pasteur and MedImmune (Stock Quote: AZN).

MedImmune makes FluMist, the nasal vaccine made of a weaker version of the live virus. Sanofi’s injectible version of the vaccine contains thimerosal, a chemical preservative derived from mercury that some have blamed for autism in children.  The Novartis version of the flu shot contains thimerosal as well.  CLS’s version of the vaccine does not include thimerosal, and unlike the other injectibles which are relatively clear, this version contains some sediments that can be mixed in by shaking the syringe, according to package inserts.

6. What kind of side effects are there?

The most common side effects following flu vaccinations are mild, such as soreness, redness, tenderness or swelling where the shot was given, according to McLaughlin of GlaxoSmithKline.  And the CDC and FDA will be watching closely for any “adverse” or “unusual events,” says FDA spokeswoman El-Hinnawy.

“Seasonal influenza vaccines are highly effective in preventing influenza disease. The expectation is that a vaccine against 2009 H1N1 influenza would probably work in a similar fashion to the seasonal influenza vaccines. CDC and FDA believe that the benefits of vaccination with the 2009 H1N1 influenza vaccine will far outweigh the risks,” the CDC says.

“CDC and its partners will use multiple systems to monitor the safety of 2009 H1N1 influenza vaccine. Two of the primary systems that will be used to monitor the safety of these vaccines after they are in widespread use are the Vaccine Adverse Event Reporting System, which is jointly operated with FDA, and the Vaccine Safety Datalink Project,” the CDC says.  The agency adds that it will also work with other federal agencies, local health departments, health professionals and academics monitor people who have been vaccinated for side effects.

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