Q. As a new grandparent, how crucial is it that I get the whooping cough vaccine?

A. “On a scale from 1 to 10? Probably 9,” says Marvin M. Lipman, M.D., Consumer Reports’ chief medical adviser. That goes for anyone about to have regular contact with an infant. Whooping cough, or pertussis, is a highly contagious bacterial respiratory tract infection that can be serious in adults, but even more so for infants. In fact, half of U.S. babies younger than 12 months who contract pertussis end up in the hospital and, of those, one out of every 100 dies. In some cases, “you can have pertussis and not know it,” Lipman says, so an unprotected adult could pass it on. (More than 20,000 U.S. cases were reported in 2015.)

Children are most vulnerable to whooping cough before they receive their first pertussis shot at 2 months, but they aren’t well-protected until age 6, when they receive the last of five such shots. Even if the mother got a pertussis vaccine while pregnant (babies can inherit some immunity this way), relatives should still get the whooping cough vaccine.

If you’re younger than 70, it’s likely you were vaccinated against whooping cough as a kid, but because protection fades over time, the Centers for Disease Control and Prevention advises that adults get revaccinated. It takes about two weeks for the whooping cough vaccine to be effective, so time your shot well before you meet the baby. Tell your doctor if you got the shot more than five years ago, or if you’ve had any allergic reactions to vaccines in the past.

For more about how to protect yourself and those around you—especially infants—find out why you need a whooping cough vaccine.

Editor’s Note: This article also appeared in the October 2017 issue of Consumer Reports magazine.