FluMist is safe for healthy people aged 5 to 49, the FDA and CDC say.

This is the third flu season the live-virus flu vaccine — squirted in the nose by a health care provider — has been on the market. Safety results for the first two years now are in. And they look good, says CDC researcher Penina Haber, MPH.

"We did not find any unexpected serious risks," Haber tells WebMD. "We didn’t see anybody dying. Overall, the vaccine is very safe from what we see."

Haber, FDA researcher Hector S. Izurieta, MD, MPH, and colleagues report the findings in the Dec. 7 issue of The Journal of the American Medical Association.

"This is a safe vaccine if used as indicated," Izurieta tells WebMD.

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Traditional Flu Shot vs. FluMist

The traditional flu shot is made from killed flu viruses. It's been around for some 50 years and is very safe, says University of Washington flu researcher Kathleen M. Neuzil, MD, MPH, senior clinical advisor at the nonprofit Program for Appropriate Technology in Health in Seattle.

In an editorial accompanying the Izurieta team's report, Neuzil notes that FluMist promises broader flu protection than the old flu vaccine. But because it's a new product, the FDA is moving cautiously. FluMist is approved only for people aged 5 to 49 who don't have chronic diseases, asthma, egg allergies, or aren't pregnant.

"So when you have a second vaccine come out — for use in healthy people — we hold it to a higher standard for what is an acceptable level of adverse events," Neuzil tells WebMD.

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Adverse-Event Reports

The FDA and CDC researchers analyzed reports of adverse events associated with FluMist. These reports come to the federal agencies through their joint Vaccine Adverse Event Reporting System (VAERS). It collects reports from doctors, nurses, and patients who think they may have seen a health problem linked to a vaccine.

Soon after FluMist hit the market in 2003, VAERS received a flurry of adverse-event reports: 283 reports from the first 500,000 people vaccinated.

That's not unusual, Izurieta says. Newly approved drugs and vaccines often see a flurry of worries in the first year or so. It's called the Weber effect, after the researcher who first observed the phenomenon in the 1980s.

The good news from Izurieta and colleagues is that these reports dropped off sharply in the second year after FluMist approval. Even though four times more people got the vaccine — 2 million people — there were fewer adverse-event reports (277).

Izurieta and Haber note that though the FDA and CDC count every report, it does not mean that every reported event is actually caused by the vaccine.

"The fact that someone reported an adverse event does not mean the vaccine caused the adverse event," Izurieta says. "People say, 'The vaccine did this to me.' Yes, their problem occurred after they received the vaccine, but that does not mean the problem was caused by the vaccine. You could have a car accident after getting FluMist, but that does not mean FluMist caused the car accident."

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Allergic Reactions

But some adverse events clearly were linked to FluMist. The VAERS reports show that the most common serious problems likely due to FluMist were allergic reactions. Among the 2.5 million Americans who got the vaccine, there were 54 allergic reactions — including three reports of serious anaphylactic events.

"This is why VAERS is so important. Because when you have these larger numbers of people, you pick up these adverse events," Neuzil says. "Anaphylaxis is the most concerning one to us. But if you give any food or vaccine to enough people, you are going to pick up some with an allergy."

The VAERS reports included rare reports of other serious events. But where investigation was possible, Izurieta says, it seemed that these events — including nervous-system problems, heart problems, and eye problems — were not linked to proper use of the vaccine.

In clinical trials of FluMist, there was a case in which a vaccinated person seemed to have transmitted the flu virus to another person. That may have happened a few times: there were 22 reports of this so-called "secondary transmission." But in the single case that was investigated with lab testing, the report turned out to be a false alarm.

Vaccination Doesn't Cause Flu

If you ask enough people who got flu shots or FluMist, you'll find a few who say they got the flu from the vaccine. Since the flu shot is made from killed virus, it's impossible for it to cause the flu. FluMist is made from a live virus. There is a theoretical possibility it could cause the flu. But that doesn't really seem to happen.

"The virus in the live FluMist vaccine is very, very, very weak," Izurieta says. "The manufacturer made sure of that, and we at the FDA made sure they made sure of that. It can reproduce a little in the nose. So that is why there is some chance it could be passed to other people. But in none of the reports could we confirm that this vaccine caused the flu."

The flu, of course, usually involves a sudden fever followed by coughing and muscle soreness. A runny nose isn't the flu — but it's the most common FluMist side effect in children. A mild sore throat isn't the flu, either. But it's the most common FluMist side effect in adults, Neuzil says.

"It is a trade-off for many people: Get the flu shot and risk a sore arm, or get a flu spray up the nose and risk a minor sore throat for a day or two," Neuzil says. "But neither causes true flu-like illness."

We'll likely see more of FluMist in the future. The manufacturer is testing it against the flu shot to see whether it gives better flu protection. And if safety studies continue to look good, the manufacturer may succeed in getting the FDA to approve the vaccine for younger and older healthy people.

By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: Izurieta, H.S. The Journal of the American Medical Association, Dec. 7, 2005; vol 294: pp 2720-2725. Neuzil, K.M. Journal of the American Medical Association,Dec. 7, 2005; vol 294: pp 2763-2765. Hector S. Izurieta, MD, MPH, Center for Biologics Evaluation and Research, FDA, Rockville, Md. Penina Haber, MPH, National Immunization Office, CDC, Atlanta. Kathleen M. Neuzil, MD, MPH, senior clinical advisor, Program for Appropriate Technology in Health; associate professor, University of Washington, Seattle.