Calling the shots
Why do you only get some shots once, while others need to be given over and over again?
Adam’s journal
Not long ago, my son came home from work with a cut on his leg. When his mother asked him how it happened, he said he’d scraped it on a rusty nail. Alarm bells immediately went off.
Will had last received a tetanus shot eight years ago. When I talked to his doctor, he recommended a new tetanus shot for Will immediately. So, of course, we followed doctor’s orders. (And he even administered it on a Sunday so that we didn’t have to go to an urgent care clinic; how lucky are we to have him?)
So, why do you only get some shots once, while others need to be given over and over again?
Dr. Prescott prescribes
Vaccines work by introducing a weakened or inactivated form of a pathogen — typically a virus or bacteria — to our body. This trains our immune system to create a defense to the pathogen. That way, if we’re ever exposed to the real pathogen, we’ll have an army of immune cells ready to repel the attack.
To know with certainty how long that army is still functional and capable of turning away infection, we’d have to conduct what are known as “challenge” studies. That would mean vaccinating subjects, waiting a certain period of time after vaccination — say, five years — and then exposing those people to live forms of viruses or bacteria. If the immunity was still effective, the subjects wouldn’t get sick.
I think you can see, though, why researchers would never conduct studies like this. It would be both dangerous and unethical.
Instead, we rely on measuring something called antibodies. These are proteins that the body produces as part of its immune response. If these antibodies are present at certain levels, we believe that the body remains capable of staving off an infection.
Different vaccines produce different antibodies. And the rate at which these antibodies decrease is far from uniform.
In a comprehensive study conducted by the University of Oregon, researchers looked at antibodies’ half-lives, the time required for antibody levels to decrease by 50 percent. Once they drop below this level, the body is likely no longer immune.
For mumps and measles, it looked like this number was 200 years or more. In other words, once you’ve been vaccinated against these viruses, you’ll never need it again.
For varicella, the virus that causes chickenpox and shingles, the researchers found antibody half-life of about 50 years. For diphtheria, it dropped to 19 years. And for tetanus, it was 11 years.
That’s why health authorities recommend a tetanus booster every 10 years.
In Will’s case, his cut increased the chances that he’d been exposed to the bacteria that cause tetanus. His doctor wisely decided to play it safe and revaccinate him at this moment to ensure that his body was protected against infection.
This story is a good reminder to all of us to ensure we keep our vaccinations current. No matter our age, we need to keep our immune systems primed. Because we never know when the next bug will show up.
Prescott, a physician and medical researcher, is president of the Oklahoma Medical Research Foundation. Cohen is a marathoner and OMRF’s senior vice president and general counsel.