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.
âIn the U.S., the highest reported rates of pertussis illnesses and deaths occur in infants, so itâs important to understand what the specific risk factors are for infants,â 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.
âNearly 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,â 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.
âVaccinating pregnant women with Tdap is the most important thing we can do to help protect babies during the first few months of life,â 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. Todayâs 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 â but the acellular vaccine also produces a more short-lived immunity.
âTdap vaccination is effective but protection is not likely to be long-lasting, which is one reason itâs so important to vaccinate mothers in the third trimester of each pregnancy,â Dr. Curtis said. âThat allows for protective antibodies to peak at two weeks after vaccination, and also allows protective antibodies to pass through the placenta to the baby.â And then even if someone in the household should become infected, the most vulnerable member is protected.
Itâs 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 thatâs why pregnant women should get that additional dose in the third trimester â so theyâre newly protected, and able to pass the protection on to the vulnerable newborn.
âThereâs always been pertussis, itâs never been eliminated,â Dr. Winter said. There was a low incidence of reported disease in the 1960s and â70s, 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. âPertussis is complicated; itâs been the most poorly controlled vaccine-preventable disease for many years, and thatâs still true today,â she said. But thanks to the vaccine, we no longer have thousands of deaths every year.
âWe have vaccines that work,â Dr. Winter said. âThough theyâre limited, they do a good job in preventing severe disease.â