Here are the facts about flu vaccines

Flu season is here, and most of us have probably considered getting a flu shot. But what is in that tube? And should we get the flu shot now, or wait until later in the season? Dr. Aimee Zaas, an infectious disease specialist and associate professor of medicine at Duke University Medical Center, fills us in. Questions and answers have been edited.

Q: What is in a flu vaccine?

Vaccines contain either killed or weakened influenza virus. According to the CDC (Centers for Disease Control and Prevention) website, this year's seasonal influenza protects against influenza H3N2, influenza B and the 2009 H1N1 virus that got so many people sick.

Flu vaccines come in two forms: the injectable "trivalent influenza vaccine" (TIV) and the nasal spray "live attenuated influenza vaccine" (LAIV). Both vaccines are made from influenza virus that is raised in an egg-based culture, so should not be given to persons with a history of severe egg allergy. The LAIV is only for healthy people, ages 2 to 49, who are not pregnant. A higher dose of the TIV can be used in individuals 65 and older.

Q: How does the flu vaccine work?

The vaccine works by showing our immune system the flu virus. This allows our immune system to recognize the flu and attack when we are exposed to the live virus.

Q: How long does a flu vaccine last, and when does it make the most sense to get vaccinated?

Generally, a vaccine lasts for the entire flu season. It makes the most sense to get vaccinated as soon as vaccine is available (late summer to early fall), however it is recommended that vaccine efforts continue as long as there is influenza circulating in the community.

Q: Why do some people get the flu vaccine and still catch the flu?

There are a few parts to this answer.

First, no vaccine is 100 percent protective. Second, the manufacturers must make vaccine each year before we know what the prevalent strain of flu will be. Sometimes, the prediction is wrong and the vaccine does not work.

Last, many respiratory illnesses cause similar symptoms as influenza (the virus that causes the flu). Therefore, you may get the flu vaccine, and get another viral illness and think it is influenza.

Q: Why is it that some people skip the shot and yet never appear to get sick?

Not everyone gets influenza, so it is possible to skip the flu shot and get lucky enough not to get influenza. Seasonal influenza vaccines do reduce the likelihood that you will get the flu or transmit it to others.