Vaccinating mothers during the third trimester of pregnancy would be the most effective way to protect infants against pertussis, health officials from the California Department of Public Health have said.
Infants are among the hardest hit in California’s ongoing pertussis epidemic. Because they are too young to be effectively vaccinated and are therefore dependent on maternal antibodies for protection, the most effective strategy to protect infants is to make sure all pregnant women receive the tetanus, diphtheria, and acellular pertussis vaccine (Tdap) in the third trimester of pregnancy. This timing will ensure the maximum transfer of protective maternal antibodies to the infant, wrote lead author Kathleen Winter and her colleagues in the US Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.
“Since the immune response to Tdap peaks about two weeks after administration and the majority of maternal antibodies are acquired by the fetus at 36-40 weeks’ gestation, Tdap is currently recommended at 27-36 weeks’ gestation to optimize antibody transfer and protection at birth,” they said. “Preliminary data indicate that infants born to vaccinated mothers have a lower risk for pertussis early in life.”
Pertussis epidemics in the United States are cyclical, peaking every three to five years as the number of susceptible people in the population increases. In its most recent epidemic in 2010, California saw approximately 9000 cases, including more than 800 admissions to hospital and 10 infant deaths. The state-wide incidence in that epidemic was 24.6 cases per 100 000 population.
Between 1 January and 26 November this year, the state saw 9935 cases, for a state-wide incidence of 26.0 cases per 100 000, Winter and her colleagues reported. The incidence among infants aged under 12 months, the hardest hit group, was 174.6 cases per 100 000.
Of the 6790 patients for whom data were available, 347 were admitted to hospital, 275 (79%) of whom were under 12 months of age. Of these, 214 were under 4 months of age. One third of the infants admitted to hospital required intensive care. One death was reported of an infant who had been 5 weeks old at the time of illness onset. Two additional infants died, who had become ill in 2013. They were also <5 weeks old at the time of illness onset.
The Center for Disease Control and Prevention’s Advisory Committee on Immunization Practices recommends all prenatal care providers provide Tdap to pregnant women during each pregnancy, ideally at 27-36 weeks’ gestation. However, of the 211 infants aged under 4 months who fell ill and whose mothers’ Tdap immunization records were available, only 35 (17%) had mothers who had received the vaccine at 27-36 weeks of gestation. Among those mothers who were not vaccinated during pregnancy, 56 (36%) received Tdap within seven days after delivery, which does not confer any direct protection to the infant, the authors said.
Winter and colleagues said that when pertussis was prevalent Tdap can be administered to infants on an accelerated schedule, with the first dose being administered as early as 6 weeks of age. Even one dose of Tdap “might offer some protection against serious pertussis disease in infants,” they wrote.
Another group being hard hit in the current epidemic were adolescents aged 14-16 years, among whom there were 2006 cases, the report said. Only 2.2% of those for whom vaccination histories were known (83%) reported never having received any doses of the vaccine. And among those for whom complete data were available, 87% had received the Tdap booster. The median time since the last Tdap doses was three years (range 0-7 years).
Because of safety concerns with whole cell pertussis vaccines, children and teens born in the US since 1997 have received an acellular pertussis vaccine. This vaccine is less immunogenic than whole cell formulations and confers an immunity that wanes rapidly, the researchers noted.
“Most of the cases among adolescents aged 14-16 years were among those who had previously received Tdap ≥3 years earlier, suggesting that their illness was the result of waning immunity,” they wrote. “It is likely that increased incidence will continue to be observed among this cohort in the absence of a new vaccine or more effective vaccination strategy.”