The bad news is, flu season officially begins Friday.

The good news is, unlike last year’s season when shortages lead to rationing, there will be plenty of vaccine on hand for everyone who wants a flu shot, according to Loyola University Chicago Stritch School of Medicine.

“There are seven companies making flu vaccine this year and more than 160 million doses are expected for the United States,” said Dr. Michael Koller, associate professor at Stritch. “That’s more than ever. At Loyola we expect the complete delivery of all our flu vaccine supply by mid-October.”

Every flu season is different and there is no way to know in advance how mild or serious any particular season will be, Koller said.

“You learn lessons from past seasons but you always have to have a healthy respect for the flu because in one week’s time, the situation can change dramatically for the worse,” Koller said.

This season the H1N1 strain, also known as “swine flu,” is still around but it’s not as widespread. To account for its presence, this year’s flu vaccine will contain the H1N1 strain as well as two others – the Perth H3N2 virus and the B Brisbane virus.

“So only one vaccine is required, unlike the two that were recommended last year,” Koller said.

Also, a new high-dose vaccine is available for people ages 65 and older, Koller said. The vaccine for seniors contains four times the amount of flu antigen (the active ingredient) as the standard flu shot.

“An older person’s immune system is not as robust as a younger person’s, so when seniors get a standard flu shot, they don’t generate as great an immune response,” Koller said. “The immune response is what protects everybody two weeks after they are vaccinated.”

Also, because flu vaccine supplies are plentiful, the Centers for Disease Control (CDC) recommends universal vaccination for all people ages 6 months and older this year. Health-care workers and those caring for people in an at-risk group should also be vaccinated, since a person can be infected and contagious for a short period of time before coming down with the classic symptoms of the flu.

The flu (or influenza) is a highly contagious viral infection that attacks the respiratory system. Doctors used to advise getting a flu shot only in October and November. Now doctors vaccinate through February because it takes about two weeks to develop an antibody response after the flu shot. For the last 30 years in the United States, February has been the peak month for illness, though infections can occur through April.

“Flu is primarily spread by respiratory droplets,” Koller said. “When somebody with influenza coughs or sneezes, out shoots this spray of flu virus that can infect anyone nearby. In addition to covering your mouth when you cough and covering your nose when you sneeze, it’s really important to wash your hands to decrease the spread of the flu.”

Each year in the U.S. between 5 and 20 percent of the population contracts the flu. Symptoms include an abrupt onset of fever, chills, headaches, exhaustion, aching muscles and a constant, unproductive cough, Koller said.

Most people recover from the flu in a few days, although they may experience some fatigue for several weeks after, Koller said. However, for some people flu is a much more serious illness that requires hospitalization. In extreme cases, the flu can lead to pneumonia or death. About 36,000 Americans die and 200,000 are hospitalized from the flu each year.

“Once you have the flu, you never forget it,” Koller said. “Usually those are the people you don’t have to convince to get a flu shot because they never want to get it again.”

For parents of young children who are worried about vaccines containing thimerosal, a preservative that contains mercury, Koller said that some of the flu shots being distributed in the Chicago area no longer contain thimerosal.

“The product that Loyola has doesn’t have any thimerosal at all,” Koller said.

Koller said, however, that any child under the age 9 who is getting a flu shot for the first time will need a second or “booster” shot four weeks later. Parents often are unaware that their young child may need a second flu shot in the first year of vaccination.

Koller said that it’s impossible to get the flu from getting a flu shot, which is a common misperception. However, he added that some would experience some side effects.

“Some people get soreness or pain at the site of the injection. A smaller number of people will feel achy and tired,” Koller said. “But all of those side effects are usually gone after two days. If it’s the first year that you’ve gotten the flu shot, you’re more likely to get the side effects. In the subsequent years, you’re much less likely to get them.”

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