viruses that are relatively stable, influenza viruses are always changing.

Influenza genes drift to create new viruses.

This means that the infection-fighting antibodies that your body makes in

response to the flu vaccine may only protect you for one flu season.

After that, circulating flu strains may change and make the old vaccine

ineffective. A new vaccine is needed to match the new viruses.

The heart of a flu virus has eight spring-like segments called RNA.

These are the genes that encode parts of the virus.

Around the RNA are different proteins.

The proteins involved in drift are called Hemagglutinin and Neuraminidase.

Scientists call them HA and NA.

There are 16 subtypes of HA proteins and nine of NA.

From these protein combinations, scientists name influenza A viruses. For example,

an H5N1 virus contains hemagglutinin subtype 5 and

neuraminidase subtype 1.

If a person has been vaccinated or has a natural immunity against the virus,

antibodies will attach to the HA protein,

blocking it from infecting human cells and stopping the virus from spreading.

Without natural immunity or immunity provided by the correct vaccine,

the HA proteins on the virus interact with sugars on the outside of those cells.

This creates a bond that allows the virus to enter targeted cells where it can

cause infection.

When the influenza viruses replicate, they can mutate slightly.

Scientists have shown that this leaves small changes in the HA and NA

proteins.

Over time, as the virus spreads from person to person,

these changes accumulate, and the virus drifts from its original form.

Eventually, the immune system no longer recognizes the HA proteins from the

virus, and the flu vaccine becomes ineffective.

Each year, flu vaccine is made to protect against the most common flu viruses

predicted to circulate among the population.

Historically, scientists have done a remarkable job of matching the vaccine to

the circulating viruses.

Although there is an effective vaccine,

not enough people choose to get it.

As a result, influenza sickens between 5 and 20 percent of the U.S.

population each year.

More than 200,000 people may be hospitalized,

and up to 49,000 people may die.

That is why health officials recommend flu vaccination before every flu season.