Cold, Flu, & Cough Health Center

Flu Shot: Influenza Vaccine and Side Effects

Thinking about getting a flu shot or influenza vaccine this year? The influenza vaccine is the best way to prevent the flu, and flu prevention should be a goal for everyone.

According to the CDC, up to 20% of Americans get the flu each year. More than 200,000 people in the U.S. are hospitalized each year, and about 3,000 to 49,000 deaths are flu-related. These statistics would decrease if more people took advantage of the opportunity to prevent flu with an influenza vaccine or flu shot.

When should I get a flu shot?

Because flu season may begin as early as October and run through May, the best time to get a flu shot is in September or October. It takes about two weeks for the flu shot to be effective. You can still get an influenza vaccine in December or later -- flu season lasts well into spring -- but the earlier in flu season you get it, the better your odds of staying flu free.

What types of flu shots or influenza vaccines are available?

There are four types of flu vaccine:

• The traditional flu shot. It's an injection into the muscle. It contains flu-virus particles that stimulate anti-flu immunity, but which cannot cause the flu.
• The high-dose flu shot approved for people age 65 and older. The ingredients are the same as the regular flu shot, but the dose is higher, as the aging immune system needs more help.
• An intradermal flu shot is approved for people age 18 to 64. This shot uses a tiny needle that only goes skin deep. It contains the same flu-virus particles as the traditional flu shot.
• Nasal-spray flu vaccine. It's approved for people age 2 to 49. This vaccine, called LAIV for live attenuated influenza vaccine, contains a live, weakened flu virus. Clinical trials show that it cannot cause the flu. For in-depth information on this vaccine, see WebMD's What is FluMist?

All four flu vaccines protect against three different flu strains: the two type A flu bugs most likely to circulate, and one type B flu bug. 

The FDA has approved flu vaccines that protect against four flu strains -- two type A bugs and both of the type B bugs. But these vaccines aren't likely to become available until the 2013-2014 flu season.

How does the flu shot or influenza vaccine work to prevent flu?

Flu shots and the nasal flu vaccine work by causing antibodies to develop in your body. These antibodies provide protection against infection from the flu virus. This antibody reaction may cause fatigue and muscle aches in some people.

Remember that the flu vaccine cannot cause the flu. Sometimes people who get vaccinated during flu season catch the flu in the two weeks before the vaccine has a chance to work. While it's human nature to see a link between the two events, there's no medical evidence that flu vaccines cause flu or make people susceptible to flu. And even though flu vaccines are not 100% effective -- vaccinated people sometimes get flu infections -- vaccinated people almost always have milder flu than people who weren't vaccinated.

Each year, the flu vaccine contains several different kinds of the virus. The strains chosen are the ones that researchers say are most likely to show up that year.