Throughout the past two decades, pertussis re-emergence in developed countries has been cause for public health concern. In Canada, there has been a significant increase in pertussis cases since the 2010s, with some areas experiencing outbreaks of this airway infection. Until the early 1990s, there had been a decline in incidence after the whole-cell (contains weakened or deactivated pathogens) pertussis vaccination had been introduced. However, unfavorable side effects following whole-cell immunization led to the introduction of the acellular (does not contain pathogen cells) pertussis vaccine in the late 1990s. Infant and young adolescents have the highest reported incidence rates of pertussis, with the highest rates of morbidity and mortality occurring in infants less than 12 months old.
The increase in pertussis cases is not completely understood, but is hypothesized to be caused by a combination of waning immunity after vaccination and low levels of vaccine uptake. Genetic mutations of the pertussis bacterium may also impact the effectiveness of natural and vaccine-derived immunity. Increased incidence could also be correlated with the conclusion of the “honeymoon effect”, a period of low disease prevalence in the population after a vaccine is introduced.
CEID’s Pejman Rohani worked with a team of researchers to develop a mathematical model of pertussis transmission in Alberta, Canada to examine the impact of immunization during pregnancy on pertussis incidence. Demographics, compliance with vaccine scheduling, social contacts, infection, and immunology were factored into this model.
The model was structured with three main components: population, social interactions, and vaccine coverage and attitudes. The population utilized in this model was reflective of household data collected in the 2016 census, such as household makeup, household ages, and fertility rate. Data from the Alberta Department of Education was used to simulate school capacity and grade levels offered.
In this model, individuals made contact with members of their household, those in the surrounding geographic area, and school classmates once reaching a certain age. Vaccine attitudes were selected randomly which impacted the family’s likelihood of complying with the recommended immunization. The parameters for assigning attitude towards vaccination were calibrated to reflect coverage rates in Alberta for each vaccine dose. In the model, 50% of mothers received the pertussis vaccination during pregnancy, and 50% of mothers did not get vaccinated during pregnancy.
Results showed that the annual pertussis incidence was 10.4 cases per 100,000 people. The annual incidence rate for infants under 1 was 55.7 cases per 100,000, and for one to four year olds there were 17.7 cases per 100,000 individuals. Mothers that received the pertussis vaccine during pregnancy represented 1123 cases recorded in infants under 12 months of age. For mothers that did not get vaccinated, there were 8404 infant cases recorded. Overall, the study showed strong protection against pertussis for infants up to 12 months whose mothers were vaccinated during pregnancy, and the level of pertussis protection was directly impacted by vaccine coverage (doses received).
Future research can expand this model, accounting for vaccination type (whole-cell versus acellular) along with natural immunity from previous infection. The cost-effectiveness of maternal pertussis vaccination should also be evaluated.
For more information about this research, click here.
By: Brenna Daly