A nationwide flu vaccine shortage is prompting health service providers to offer the shot only to individuals in high-priority groups.

University Health Services (UHS) will administer the vaccine only to students at high risk according to the Center for Disease Control (CDC) recommendations.

Individuals with weakened or compromised immune systems, pregnant women, workers in the health care industry, children and individuals over the age of 65 are eligible for the vaccine.

Students with lung problems (such as asthma and emphysema), chronic kidney disease, diabetes, heart disease and other long-term health problems are also eligible for the vaccine.

Margaret Spear, director of UHS, said she expects to treat hundreds, rather than thousands, of people this year.

The lessened availability of flu vaccine will not necessarily result in a flu season that is worse than normal, some say.

There is "no indication that this is going to be a particularly bad flu season," said Christopher Norbury, assistant professor of microbiology and immunology.

Many people believe they have the flu when in fact they have a bad cold, Norbury said.

"That's one of the problems," he said. "There's really no diagnosis for it because there's really no cure."

Most people with colds will experience a runny nose and a cough; the flu is characterized by muscle aches, said Joseph Geskey, assistant professor of pediatrics and internal medicine at the Penn State College of Medicine and director of in-patient pediatrics at Penn State Children's Hospital in Hershey.

Students should make sure to take precautions to avoid catching the flu.

"If you get a bad flu infection, you're going to be in bed for three weeks," Norbury said. "The potential of a nastier flu virus is scarier than anything you can possibly imagine."

To minimize the risk of contracting the virus, people should cover their mouths and noses when sneezing and clean their hands frequently, Geskey said.

"The best thing that people can do is [maintain] good health habits," he said.

There are medicines available that can potentially alleviate flu symptoms, though they cannot kill the virus.

Geskey said if you are sick, it is unethical to go to work or class where you may infect others.

"Students who do get sick are better off staying at home," Spear said.

The CDC formulates the flu vaccine based on strains of the virus making people sick in the countries bordering the Pacific Ocean, Norbury said.

"It's basically an educated guess. And they put three strains into the vaccine every year," he said.

The CDC chooses three strains because usually, two or three particular strains of flu infect the population at a time, he said. Different strains of flu form because of either genetic mutations that occur spontaneously in the virus or when an antigenic shift gives rise to a different virus altogether, Norbury said.

When the virus unexpectedly mutates, the antivirus cannot recognize it and people may become sick. In an antigenic shift, two different viruses infect the same host. The viruses can then recombine to create a new strain and a different virus, he said.

The flu vaccine itself cannot make people sick, Norbury said. It may cause individuals to feel poorly but because the viruses in the vaccine are dead, patients will not get the flu from the shot, he said.

The vaccine acts as an alarm signal for the immune system, which has to perceive the vaccine as dangerous for it to work, Norbury said. Then, when the immune system is presented with the actual flu virus, it will recognize it and attack it.