As the country's flu outbreak becomes an epidemic, odds are that you've had a few sheepish feelings about not doing something you probably should have: Gotten a flu shot.

As of this November, the majority of American adults - 64.8 percent, to be exact - had not received a flu immunization. This wasn't a surprise to researchers: Flu is a disease with one the lowest vaccination rates.

Though more than 95 percent of students entered kindergarten last year immunized against measles, mumps and rubella, the Centers for Disease Control and Prevention estimates that 36.5 percent of all Americans received flu vaccinations by November 2012.

It's worth noting that the percentage does tend to rise over the course of flu season: By March 2012 vaccination rates had grown to 47.6 percent from 36.3 percent in November 2011.

Why are flu vaccination rates so low? Lori Uscher-Pines, a policy researcher at the RAND Corp., estimates that part of the issue has to do with no consequences for not getting vaccinated (well, except for coming down with the flu). Unlike childhood vaccines, which are generally required to start a school year, employers don't stop their workers' from coming to work if they cannot prove flu immunization. 

"Children have regular encounters like well child visits where they get vaccinated," she said. "There's a constant contact with the health-care system."

Americans also tend to have negative perceptions about the flu vaccine. A study Uscher-Pines did in 2011 found that about half of those who did not get vaccinated agreed with statements such as "I don't need it" or "I don't believe in flu vaccines."

Some of these complaints do have a kernel of truth to them: Flu vaccines tend to have a lower efficacy rate than other immunizations. Patients given the MMR vaccination (measles, mumps and rubella), for example, have a 95 percent chance of developing immunity to those diseases (97 percent for mumps).

This year's flu vaccine is 62 percent effective, meaning that those who receive the vaccination are 62 percent less likely to develop the flu than those who don't. That does leave space for someone who receives the vaccine to become sick but, as public health officials would argue, gives them better odds than an individual without any protection at all.

Flu vaccines are tricky, in no small part because the disease "mutates often," The Washington Post's Lena H. Sun explains, "and the antibodies that people produce only protect them through one flu season." As one CDC official told her, "The nature of flu viruses and the complexity of the human immune response makes it very difficult to develop a 100-percent effective vaccine."

What can convince Americans to get immunized? Researchers at Wake Forest University recently conducted an experiment on this subject, and their findings weren't exactly optimistic.

The team -- led by economists Fred Chen, Allin Cottrell and Amanda Griffith -- had participants play a not-so-fun video game that simulated the spread of the flu.

Players could earn points (later converted into gift cards) by staying healthy. They could also spend some of those points to buy a vaccine.

In other words, they faced a choice: Should they pony up on a preventive measure early or roll the dice, save a few bucks and not buy the vaccine?

Researchers found that a few external factors made participants more likely to buy a vaccine. If the immunization was cheaper, vaccination rates went up. As the number of infections went up, too, players became more likely to decide to take the plunge and pay for immunization.

That research gels with what we're seeing now: As the flu epidemic has spread, so has desire for flu vaccines -- and so has flu vaccine shortages. It's just what the economists expected to happen.