Lois Knight remembers when her mother was hospitalized with shingles years ago. Then one of her mother's best friends lost her eyesight when shingles attacked her optic nerves. So when Knight and her husband, William Schaffner, MD, a professor of preventive medicine at Vanderbilt University, were offered a chance to participate in the clinical trial of a shingles vaccine, they eagerly accepted.

Both ended up in the group that received the vaccine, not the placebo. (And neither has ever gotten shingles.) Now Knight tells anyone who will listen how important getting the vaccine is, hoping to spare them the misery of infection.

She meets plenty of resistance. "Some say, I've had chicken pox, so I don't need it," says Knight, a retired realtor in Nashville. Wrong, wrong, wrong, she tells them.

Related: 6 Things Adults Should Know About Chickenpox and Shingles

What is shingles?

If you’ve ever had the chicken pox, the virus, called herpes zoster, stays in your body, hiding in nerve tissue for the rest of your life. Decades later, when your immune system has weakened with age, the virus can spring back into action in the form of a painful blistering rash, often in a single row or stripe, like roof shingles. That’s shingles.

Shingles can take weeks or even months to resolve. Schaffner says he has had patients in so much pain from shingles that they can't let their shirt touch the rash. "They cut out the shirt so it doesn't touch [the skin]," he says. Eventually, the pain usually subsides, he says, but it can linger and make life miserable. ''There are people so troubled by it that it triggers thoughts of suicide.''

What the vaccine can do

The vaccine, Zostavax, can reduce the risk of developing shingles by about half, according to the Centers for Disease Control and Prevention (CDC). If you get shingles despite having been vaccinated, the vaccine can reduce your risk of a complication known as postherpetic neuralgia by about 67 percent. In postherpetic neuralgia, which affects both the skin and the nerve fibers, the pain lasts long after the rash and blisters go away. The burning pain can be so severe it affects sleep and appetite.

Zostavax, which contains a weakened form of the virus, is far from perfect at preventing shingles, says Schaffner, who is a former member of the board of directors for the Infectious Diseases Society of America and serves on data safety monitoring committees for Merck and Pfizer. But it's currently the only licensed shingles vaccine.

Certain people should not get it, the CDC warns, including people who have had a life-threatening reaction to gelatin, the antibiotic neomycin or other vaccine components. People with a weakened immune system, such as those with HIV, should not get it either. Nor should women who are pregnant or might be pregnant.

No serious problems from the vaccine have been reported, according to the CDC.

Related: Is It Mono or Something Else?

Getting the timing right

The vaccine is approved by the Food and Drug Administration (FDA) for people 50 and over — but you may want to wait until you’re older to get it.

The older you are when you develop shingles, the worse the outbreak usually is. And here’s the catch: The effects of the vaccine wear off over time. In people who get it at age 60, protection decreases within about the first five years. If you get vaccinated when you’re 50, you may not have much protection left at 60. That’s why the CDC recommends most people wait until age 60 or older to get it. (It also recommends getting it regardless of whether or not you remember having had chickenpox.)

"Think of this as a pistol with one bullet," Schaffner says. "The question is ,'When do you shoot the bullet?'''

The CDC tells people 50 to 60 to talk to their doctor about whether they should get the vaccine before age 60. Some people might have reason to get it earlier. According to the CDC, “factors that might be considered include particularly poor anticipated tolerance of herpes zoster or postherpetic neuralgia symptoms (e.g., attributable to preexisting chronic pain, severe depression, or other comorbid conditions; inability to tolerate treatment medications because of hypersensitivity or interactions with other chronic medications; and occupational considerations).”

What if you’re already had shingles?

If you've had shingles and don't want to repeat that nightmare? Yes, you can get the vaccine, Schaffner says. He advises waiting a year for the best protection from the vaccine. Getting the vaccine after you have had shingles ''won't hurt you and it might help you," Schaffner says.

Is it covered?

Most private health insurance plans cover the vaccine for people age 60 and above, the CDC says, and some plans cover it for people 50 to 59. Under Medicare, it's covered under Part D, though there may be a partial charge or a co-pay.

Related: 6 Heart Attack Symptoms Women Ignore

Is something better coming?

Another vaccine in the works may prove more effective, Schaffner says. "It looks to provide 97 percent protection." In a study of this new vaccine, researchers assigned more than 15,000 men and women to get either the experimental vaccine or placebo. During a three-year follow up, 6 people in the vaccine group got shingles compared to 210 people in the placebo group. The vaccine is in the final phases of testing, but it's difficult to predict when the FDA might approve it. Schaffner estimates it will take at least a couple of years to reach the market.

Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.