If you’re over 60, you probably know that federal officials recommend getting a shingles vaccine for every American your age and older. But what you might not know is that some people who have received the shot may not be fully protected.

The effectiveness of the Zostavax vaccine against herpes zoster, the virus that causes shingles, gradually fades with time, but a booster vaccine is not yet approved, The New York Times reports.

A study of Zostavax, published in 2005, followed more than 38,000 people and found that it reduced the incidence of disease by 51 percent. It also offered protection against post-herpetic neuralgia — the intense nerve pain that can follow shingles — by more than 66 percent. But that study said nothing about efficacy beyond three years.

A study published in March 2015 that tracked 6,043 people for 11 years after their vaccination also found the vaccine’s effectiveness declined with time. In fact, after eight years no longer worked to prevent disease.

The vaccine is given in a single shot, and although the Food and Drug Administration approved it in 2011 for people over 50, the Centers for Disease Control and Prevention recommends it only for those over 60.

The CDC notes if a person gets vaccinated in his or her 50s, the vaccine may provide peak protection at a time when shingles is less likely to occur, since the risk of shingles increases with age. Therefore, with a booster vaccine not approved, it may be better to wait.

But a study published online in October in The Journal of Infectious Diseases found that a booster dose of Zostavax was safe and effective in people over 70 who had first been vaccinated more than 10 years earlier.

Physicians can legally and ethically prescribe approved drugs off-label, including vaccines, and this may happen with Zostavax.

“Based on the encouraging boost in immunity seen in people who get a booster, it’s a reasonable expectation that they would be protected for some time after the second dose,” said the lead author of the new study, Dr. Myron J. Levin, a professor of pediatrics and medicine at the University of Colorado Denver School of Medicine. “But it’s not yet proven. Some physicians may think the evidence is strong enough to give the second dose now.”