Whooping cough is a disease that I take somewhat personally, because I managed to contract it as an adult — in the line of pediatric duty, of course — and I am here to tell you that it was extremely unpleasant. No, I didn’t cough so hard that I broke any ribs (a well-known complication), but I certainly understood, as never before, what it might mean to experience coughing spasms so convulsive and severe that a broken rib seemed perfectly possible.

The diagnosis of whooping cough, also known as pertussis, is difficult in adults because they don’t tend to have the characteristic “whoop” that makes the disease identifiable in young children. The sound is produced by the inrush of air into a small pediatric airway after a coughing paroxysm, and it can be truly terrifying to hear. Young children with pertussis can stop breathing or have seizures; they often require hospitalization and even intensive care, while in an adult, pertussis can seem like just another persistent cough.

Nowadays, there is a pertussis booster shot available for adults, something that didn’t exist when I got sick about 15 years ago; the vaccine came out in 2005. But it turns out that the immunity that shot confers is not long-lasting; we give it to children when they are around 11, and by two or three years later, their immunity may already be waning, though they are no longer at risk of the kind of serious and even life-threatening pertussis symptoms that babies can have.

“We’ve really had a shift in thinking about pertussis management and control in recent years,” said Kathleen Winter, an assistant professor of epidemiology at the University of Kentucky. “We need to focus on targeting those at highest risk. We can’t do a lot to prevent widespread transmission in the community.”

Most hospitalizations occur in young babies, she said, especially in the window between birth and the beginning of routine immunization at 2 months. And almost all deaths are in children under 2 months.

The Centers for Disease Control and Prevention recommends that pregnant women should be vaccinated, and a large study done in California and published in April in the journal Pediatrics showed that vaccinating the mothers was highly protective for the infants during the first months of life, and continued to offer additional protection even after the childhood vaccine schedule began. And a study by Dr. Winter and her colleagues published earlier this year showed that when infants of vaccinated mothers do develop pertussis, they are less likely to be hospitalized than the babies born to unvaccinated mothers, their hospital stays are shorter, and they are less likely to need intensive care.

So when pregnant women get that adult booster shot, the immunity is passed on to the newborn, and offers protection during those first two months, and furthermore, the mother is protected from infection during the weeks around the birth when the baby is most at risk of serious disease, and even death.

“In the U.S., the highest reported rates of pertussis illnesses and deaths occur in infants, so it’s important to understand what the specific risk factors are for infants,” said Dr. Robin Curtis of the C.D.C., who spent over a decade working at the National Center for Immunization and Respiratory Diseases.

Dr. Curtis was the first author on a study published this year in The Pediatric Infectious Disease Journal, in which researchers looked at 115 babies up to 4 months old, from four different sites, all of whom had pertussis. Each baby was matched with two control infants (same age, same site), and the researchers interviewed all the caregivers regarding symptoms of all contacts.

“Nearly three-quarters of the infants who became ill with pertussis had at least one household contact with a prolonged cough of more than five days,” Dr. Curtis said. Infants who were mostly breast-fed had less chance of contracting pertussis compared with those who had two or more formula feedings a day. And if the mother had a new cough that lasted for five days or longer in the peripartum period (from four weeks before delivery to four weeks after), the risk to the baby was more than seven times higher.

“Vaccinating pregnant women with Tdap is the most important thing we can do to help protect babies during the first few months of life,” Dr. Curtis said. That protects babies until they themselves begin their series of vaccinations to protect them against pertussis. The babies are given DTaP, a slightly different vaccine than the one given to older children and adults. Today’s shots, using a purer acellular protein unit, are much less likely to cause fever, redness, pain or more serious reactions than the older formulations, which used whole killed bacterial cells — but the acellular vaccine also produces a more short-lived immunity.

“Tdap vaccination is effective but protection is not likely to be long-lasting, which is one reason it’s so important to vaccinate mothers in the third trimester of each pregnancy,” Dr. Curtis said. “That allows for protective antibodies to peak at two weeks after vaccination, and also allows protective antibodies to pass through the placenta to the baby.” And then even if someone in the household should become infected, the most vulnerable member is protected.

It’s important, Dr. Curtis said, for doctors to keep pertussis in mind, and to remember that the immunity wears off. So if there is a household contact with a cough, you have to worry about pertussis even if that household contact got the pertussis vaccine sometime in the past. And that’s why pregnant women should get that additional dose in the third trimester — so they’re newly protected, and able to pass the protection on to the vulnerable newborn.

“There’s always been pertussis, it’s never been eliminated,” Dr. Winter said. There was a low incidence of reported disease in the 1960s and ’70s, she said, but that was partly because there was not much awareness of what the disease looked like in adolescents and adults, and there were not good diagnostic tests available.

And now there has been some true increase in the disease, she said, with the adoption of the acellular pertussis vaccine and the consequent waning immunity, but there is also more awareness, and better diagnostic tests. “Pertussis is complicated; it’s been the most poorly controlled vaccine-preventable disease for many years, and that’s still true today,” she said. But thanks to the vaccine, we no longer have thousands of deaths every year.

“We have vaccines that work,” Dr. Winter said. “Though they’re limited, they do a good job in preventing severe disease.”