Did You Get a Flu Shot and Still Get Sick? Here's Why

You did your due diligence and received a flu vaccine this year. Congratulations, you're one of the estimated 36 percent of people to have gotten the shot or nasal spray so far this flu season. So why are you feeling achy, exhausted, congested, feverish and generally lousy?

There are many factors at play here, including that the flu shot is no where close to 100 percent effective at preventing the virus.

Vials of flu vaccine are displayed at the Whittier Street Health Center in Boston, Mass. (Photo: AP/Charles Krupa)

"There are any number of reasons why people could have done all the right things and still get the flu," Massachusetts' head of the bureau of infectious diseases, Kevin Cranston, said, according to the Associated Press.

This season the flu is already considered "widespread" in nearly all states. Some cites, like Boston, have officially declared it a public health emergency. Boston has seen 700 confirmed cases of the flu, compared with 70 all of last season, with 18 flu-related deaths so far.

A sign in the entry of the the Mayo Clinic Health System hospital in Mankato, Minn., spells out visitor restrictions that have been implemented to curb the spread of influenza. (Photo: AP/Mankato Free Press, John Cross)

The CDC said the proportion of people visiting health care providers with flu-like symptoms climbed from 2.8 percent to 5.6 percent in four weeks. By contrast, the rate peaked at only 2.2 percent during the relatively mild 2011-12 flu season. The estimated rate of flu-related hospitalizations in the U.S. was 8.1 per 100,000 people, which is high for this time of year, according to Dr. Joe Bresee, chief of the epidemiology and prevention branch of the CDC's influenza division.

(Image: CDC)

Watch this report from CBS regarding Boston's situation with the flu outbreak:

But why does it seem to be so virulent this year? The most popular strain identified this season is Type A influenza H3N2, which is historically associated with more serious illnesses. It is also among the strains covered by the flu vaccine.

But as Cranston said "no vaccine is 100 percent effective." One reason is the lag time between vaccination and active immunity. A person who received a flu shot but comes in contact with the virus before the vaccine becomes effective -- between 10 days and two weeks -- they are still susceptible to becoming ill.

Recent studies have shown that the flu vaccine as a whole is only about 59 percent effective at preventing the illness. NRP pointed out that the vaccine appears to be less effective for the elderly, which is a population often highly encouraged to receive the shot in the first place.

Children and the elderly are among those most recommended for the flu shot. Four-year-old Gabriella Diaz sits as registered nurse Charlene Luxcin, right, administers a flu shot at the Whittier Street Health Center in Boston. (Photo: AP/Charles Krupa)

Another reason someone might contract the flu even after receiving vaccination is if the strain they were infected with wasn't included in the vaccine in the first place. Each year, virus experts select the strains most likely to cause illness that year to include in the flu shot. This year the World Health Organization recommended including the three following strains in vaccines for use in the northern hemisphere:

• an A/California/7/2009 (H1N1)pdm09-like virus;
• an A/Victoria/361/2011 (H3N2)-like virus;
• a B/Wisconsin/1/2010-like virus (from the B/Yamagata lineage of viruses).

Even though the flu vaccine might have its flaws, the CDC says it's still the most effective method at preventing the flu and recommends people receive it either in shot or nasal spray form. Whether or not you receive the vaccine, the CDC also points to a few other ways to prevent and/or stop the spread of the virus:

• Cover your nose and mouth with a tissue when you cough or sneeze. Throw the tissue in the trash after you use it.
• Wash your hands often with soap and water. If soap and water are not available, use an alcohol-based hand rub.
• Avoid touching your eyes, nose and mouth. Germs spread this way.
• Try to avoid close contact with sick people.
• If you are sick with flu-like illness, CDC recommends that you stay home for at least 24 hours after your fever is gone except to get medical care or for other necessities. (Your fever should be gone without the use of a fever-reducing medicine.)
• While sick, limit contact with others as much as possible to keep from infecting them.

The CDC said 18 children have died from the flu so far this season. While it doesn't keep a tab of deaths overall from the flu, the CDC estimates that 24,000 Americans die each year.

The Associated Press contributed to this report.