Tdap is a booster vaccine against tetanus, diphtheria, and pertussis. Since 2005, when it was licensed, the CDC has recommended the shot for adolescents and adults, including pregnant women, because protection from childhood vaccination wanes.
But surveys show only about 8 percent of adults have gotten Tdap. (There are two brands: Boostrix retails for about $40; Adacel is $128.)
Pediatrician Paul Offit, head of the Vaccine Education Center at the Children's Hospital of Philadelphia, said doctors don't inform adult patients about Tdap, and those patients don't know to ask.
"We think of vaccination as a child thing," he said.
In babies, especially newborns, the whooping cough bacteria - spread by a cough or sneeze - can lead to pneumonia, brain damage, even death. The name comes from the sound children make as they gasp for breath during convulsive coughing fits. Antibiotics can't cure it.
Infected adults, in contrast, may have no symptoms, or an illness that is mistaken for bronchitis, the flu or a cold.
Thus, adults - new mothers, grandparents, babysitters, even health-care workers - can be an unwitting lethal threat to the tots they love.
That's not the only factor behind the pertussis resurgence. The childhood vaccine used for decades was reformulated in the 1990s to reduce side effects. The new version's protection starts to fade within five years, studies show.
"The current vaccine is definitely safer, but we traded some efficacy for safety," Offit said.
A smaller but troubling factor, the CDC says, is parents who purposely opt not to vaccinate their children.
But the effect of these other problems could be offset if more adults rolled up their sleeves.
"For the most part," Offit said, "children catch pertussis from young adults and adults. They are the reservoir for the bacteria."
- Marie McCullough