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Is the quadrivalent vaccine better than the trivalent? How long does the flu shot last? And how effective is it? Find the answers below. For more information, visit the Centers for Disease Control web page, "What You Should Know for the 2013-2014 Influenza Season," at www.cdc.gov/flu/about/season/flu-season-2013-2014.htm.

What vaccines are available this year?

There are several flu vaccine options for the 2013-2014 flu season. Traditional flu vaccines made to protect against three different flu viruses (called "trivalent" vaccines) are available. In addition, this season flu vaccines made to protect against four different flu viruses (called "quadrivalent" vaccines) also are available.

The trivalent flu vaccine protects against two influenza A viruses and an influenza B virus. The following trivalent flu vaccines are available:

Standard dose trivalent shots that are manufactured using virus grown in eggs. These are approved for people ages 6 months and older. There are different brands of this type of vaccine, and each is approved for different ages. However, there is a brand that is approved for children as young as 6 months old and up.

A standard dose trivalent shot containing virus grown in cell culture, which is approved for people 18 and older.

A standard dose trivalent shot that is egg-free, approved for people 18 through 49 years of age.

A high-dose trivalent shot, approved for people 65 and older.

A standard dose intradermal trivalent shot, which is injected into the skin instead of the muscle and uses a much smaller needle than the regular flu shot, approved for people 18 through 64 years of age.

The quadrivalent flu vaccine protects against two influenza A viruses and two influenza B viruses. The following quadrivalent flu vaccines are available: A standard dose quadrivalent shot and a standard dose quadrivalent flu vaccine, given as a nasal spray, approved for healthy people 2 through 49 years of age.

A person receives a flu shot at Kaiser Park Shadelands in Walnut Creek, Calif., on Friday, Oct. 11, 2013.
A person receives a flu shot at Kaiser Park Shadelands in Walnut Creek, Calif., on Friday, Oct. 11, 2013. (Dan Rosenstrauch/Bay Area News Group)

Is the quadrivalent vaccine better than a trivalent?

CDC does not recommend one flu vaccine over the other. The important thing is to get a flu vaccine every year. But, some infectious disease experts, including Jeffrey Silvers of Eden Medical Center in San Leandro, and Randy Bergen of Walnut Creek's Kaiser Permanente, say if you qualify for the quadrivalent vaccine, it may provide greater protection, especially among children, and is worth the slightly higher sticker price.

What flu viruses does this season's vaccine protect against?

Flu vaccines are designed to protect against the influenza viruses that experts predict will be the most common during the upcoming season. Three kinds of influenza viruses commonly circulate among people today: Influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses. Each year, these viruses are used to produce seasonal influenza vaccine.

The 2013-2014 trivalent influenza vaccine is made from the following three viruses:

an A/California/7/2009 (H1N1)pdm09-like virus;

an A(H3N2) virus antigenically like the cell-propagated prototype virus A/Victoria/361/2011;

a B/Massachusetts/2/2012-like virus.

It is recommended that the quadrivalent vaccine containing two influenza B viruses include the above three viruses and a B/Brisbane/60/2008-like virus.

How do we know if there is a good match between the vaccine viruses and those causing illness?

Over the course of a flu season, CDC studies samples of flu viruses circulating during that season to evaluate how close a match there is between viruses used to make the vaccine and circulating viruses. Data are published in the weekly FluView at www.cdc.gov/flu/weekly. In addition, CDC conducts studies each year to determine how well the vaccine protects against illness.

How long does a flu vaccine protect me from getting the flu?

According to the CDC, multiple studies conducted over different seasons and across vaccine types and influenza virus subtypes have shown that the body's immunity to influenza viruses (acquired either through natural infection or vaccination) declines over time. The decline in antibodies is influenced by several factors, including the antigen used in the vaccine, age of the person being vaccinated, and the person's general health (for example, certain chronic health conditions may have an impact on immunity).

When most healthy people with regular immune systems are vaccinated, their bodies produce antibodies and they are protected throughout the flu season, even as antibody levels decline over time. People with weakened immune systems may not generate the same amount of antibodies after vaccination; further, their antibody levels may drop more quickly when compared to healthy people.

If you live in the Bay Area, where influenza hits hardest in December and January, Silvers recommends getting the flu shot in late October or early November.

Source: The CDC