It's that time of year again — flu season.

Between August and November, 2,704 people got a flu vaccination at the Hall County Health Department. More are being given daily there and at private practices, said David Palmer, public information officer for Georgia Public Health District 2.

The Centers for Disease Control and Prevention reported more than 1.2 million flu shots have been given in Georgia this year, an important statistic given that influenza contributes to between 3,000 and 49,000 deaths nationwide each year.

There are two types of flu vaccines: a shot and a nasal spray.

"The shot itself is made from a killed virus. It has enough of the properties of the (live virus) that when it's injected into the body, the body believes it's being infected with a germ," Palmer said.

"The flu shot tricks the body into thinking it's been infected, so the body builds immunity against that strain of flu."

The nasal spray is a weakened live virus only given to healthy people between 18 and 49, Palmer said.

The flu vaccine covers three different strains: H3N2, H1N1 and the B strain. The first two are A strains of flu.

The "H" and "N" in the names of flu strains are proteins. They stand for "hemagglutinin" and "neuraminidase," respectively.

"That's what determines the type of flu," Palmer said. "There are 16 different H and nine different N subtypes."

For influenza B, however, the virus cannot be broken into H and N subtypes.

A third type, influenza C, has about the same severity as a cold. It's less common than the other two, Palmer said.

And Palmer said people shouldn't discount novel H1N1 - commonly referred to as swine flu - and avian influenza.

"They are still of concern because flu viruses are unpredictable and they can change at any time," Palmer said. "That's what we saw with the H1N1 strain, was a new flu became present and started to spread. At any time the flu virus could shift and we could have a new strain."

There are two different ways flu viruses can change.

The first is called an "antigenic shift," when one strain changes enough to totally displace another one, said Dr. Michael Shaw, associate director for laboratory science in the influenza division of the National Center for Immunization and Respiratory Diseases in Atlanta.

In 1957, he said, the main flu virus around was from the 1918 influenza epidemic. Then that changed to H2N2. The H3N2 today's flu shots vaccinate against did not come about until 1968.

This occurs because influenza A viruses from different species are able to mix, according to the CDC.

Another type of change is called "antigenic drift," which happens constantly and naturally as the flu virus grows, Shaw said.

"If you've been exposed to a certain strain or you've had a flu shot, your body has built up antibodies for that specific strain," Palmer said. "If a new strain comes up, you may not have immunity against that new strain. That was the problem with H1N1."

Influenza A typically infects and transmits among one species, but sometimes it can cross over and infect another species, as happened with novel H1N1.

Shaw said novel H1N1 was "an odd case."

"Way back in 1918 when that pandemic hit, it went into both humans and pigs at about the same time. The viruses evolved separately in the two different species," Shaw said. "Human viruses are constantly going into pigs and not as often coming back out. A pig is more likely to get a virus from a human than vice-versa."

Novel H1N1 went through both antigenic drift and shift and picked up two new genes along the way, which allow it to spread not just from an infected pig to a human, but also between humans.

It's a shift in the sense that it's a totally new virus, but it's a drift in that it originally came from humans and is now a human virus again, Shaw said.

Palmer said the CDC and the World Health Organization monitor influenza-like illnesses worldwide. They determine what strains to vaccinate for depending on which appear most often.

Though health professionals contend annual flu vaccines are necessary, there are some people who cannot have the flu vaccine, including those who are allergic to egg or egg products as eggs are used in the manufacture of flu vaccines.

Others include people who have had a reaction to a previous flu shot, children younger than 6 months and people who suffer from Guillain-Barré syndrome, a disease that causes muscle weakness and paralysis.

In addition, anyone with a moderate to severe illness with a fever should wait until they get better before they get a flu shot, Palmer said.