Confluence of Factors Contribute to California's Worsening Pertussis Outbreak

December 31, 2014 01:18 pm News Staff

No doubt, many family physicians who care for young children -- especially those in California -- recall the pertussis epidemic that state experienced in 2010. According to an article( published earlier this month in Morbidity and Mortality Weekly, about 9,000 cases were reported that year, including 808 hospitalizations and 10 deaths among infants.

In some ways, this year's numbers bode even worse.

This child has broken blood vessels in his eyes and bruising on his face from coughing associated with pertussis.

Between Jan. 1 and Nov. 26, a total of 9,935 pertussis cases -- the most in nearly 70 years -- were reported to the California Department of Public Health (CDPH), yielding an overall statewide incidence of 26 cases per 100,000 population. Further analysis of cases for which demographic and other key data were available revealed a disease incidence among infants age 12 months or younger of 174.6 cases per 100,000 for that time period.

When AAFP News first reported on this outbreak, case numbers from Jan. 1 through June 10 stood at 3,458. Even that figure represented a considerable increase from the 2,530 cases reported to the CDPH in all of 2013.

This year, of 6,790 case reports for which data were available, 347 patients had been hospitalized, including 275 infants younger than 12 months. Only one in four of those hospitalized infants had received any doses of the diphtheria, tetanus and acellular pertussis (DTaP) vaccine series, which is typically started at age 2 months.

Story Highlights
  • Between Jan. 1 and Nov. 26, a total of 9,935 pertussis cases -- the most in nearly 70 years -- were reported to the California Department of Public Health.
  • Among factors contributing to the outbreak are a lack of appropriate vaccination of pregnant women and waning immunity associated with the currently recommended acellular version of the pertussis vaccine.
  • In addition, new research suggests that acellular vaccine antigen encoding genes are evolving at higher rates than other surface protein encoding genes.

Finally, of 211 hospitalized infants younger than 4 months whose mothers' tetanus, diphtheria and acellular pertussis (Tdap) vaccine histories were known, fewer than one in five of those mothers had received the recommended single dose of Tdap at 27-36 weeks' gestation.

According to the MMWR article, disease incidence has also been high among older children and adolescents this year, peaking at 137.8 cases per 100,000 among 15-year-olds. Of the 83 percent of cases in adolescents ages 14-16 who had known vaccination histories, only 2.2 percent reported never receiving any doses of pertussis-containing vaccine. And of vaccinated adolescents for whom complete data were available, 87 percent had received the Tdap booster vaccine, with the median length of time since having received Tdap being 3 years.

Two separate issues appear to be reflected in these data.

Because infants younger than 12 months bear the highest risk for hospitalization and death from pertussis, public health strategies historically have prioritized disease prevention in this age group. During the 2010 pertussis epidemic in California, for example, the main strategy used to protect these youngest patients was "cocooning" (i.e., vaccinating contacts of infants). However, this strategy proved difficult to implement, and even when such contacts could be identified and vaccinated, the degree of protection conferred was suboptimal because infants could still be exposed while out in the community.

The following year, data demonstrating efficient transplacental transfer of antipertussis antibodies became available, and public health experts theorized that vaccinating pregnant women might protect their infants until they were old enough to begin receiving the primary DTaP series. Accordingly, the CDC's Advisory Committee on Immunization Practices (ACIP) recommended( that pregnant women who had never received Tdap be given a dose after 20 weeks' gestation.

The ACIP fine-tuned that recommendation in 2012( after reviewing data that showed antipertussis antibody concentrations markedly declined one year after vaccination. The revised recommendation called for administering a dose of Tdap during the third trimester of every pregnancy. Because the immune response to Tdap peaks about 2 weeks after the vaccine is given, and most maternal antibodies are acquired by the fetus at 36-40 weeks' gestation, current recommendations specifically call for Tdap to be given at 27-36 weeks' gestation, thus optimizing antibody transfer to the fetus.

According to the MMWR article, the fallout of having less than 20 percent of pregnant women vaccinated according to that recommendation likely is reflected in the current California outbreak. Also evident is what is now recognized as a waning immunity associated with the acellular pertussis vaccine component, which, in 1997, completely replaced whole-cell pertussis vaccines in the United States.

"Notably, the peak age of disease incidence beyond infancy increased to age 14-16 years in 2014 compared with the peak among children aged 10 years during the 2010 pertussis epidemic," the MMWR article observes. "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.

"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."

Finally, new research suggests another factor may be at work. A recent article( in the Journal of Infectious Diseases reports on a study conducted in the United Kingdom (UK) in which researchers performed genomic analyses of multiple strains from a 2012 pertussis outbreak there, demonstrating that acellular vaccine antigen encoding genes are evolving at higher rates than other surface protein encoding genes.

"This was true even prior to the introduction of pertussis vaccines," the study authors note, "but has become more pronounced since the introduction of the current acellular vaccines.

"The fast evolution of vaccine antigen genes has serious consequences for the ability of
current vaccines to continue to control pertussis," they conclude.