What’s the risk of pertussis in kids who are undervaccinated or delayed in vaccination?


For children who have either experienced a delay in pertussis vaccination or who haven’t received the recommended number of doses, what is their risk of pertussis? A new study provides answers.

With the pandemic raging for nearly a year and a half, many children have missed routine childhood vaccinations. What is the risk of pertussis in infants and young children who are either undervaccinated or have experienced a delay in their diphtheria-tetanus–acellular pertussis vaccine, which is administered as a 3-dose series in infancy with a booster at age 5 years?1

The population-based, retrospective cohort study utilized Washington State Immunization Information System data and pertussis surveillance data from Public Health Seattle and King County, Washington. It included participants who were aged 3 months to 9 years and were born or living in King County, Washington, between January 2008 and December 2017. Being undervaccinated was described as receiving fewer doses than recommended at a given age. Delayed vaccination was defined as not being administered the vaccine doses in the time frames recommended by the Centers for Disease Control and Prevention.

There were a total of 316,404 children, representing 17.4 million person-months of follow-up included in the study. No vaccines were recorded in the Immunization Information System for 19,943 (6.3%) of children. A delay in vaccine was noted for 116,193 children (36.7%) and 180,268 (56.9%) were fully vaccinated without a delay. Both undervaccination and delayed vaccination were higher for older children (17.6% delayed or undervaccinated at age 2 months for dose 1 at 3 months vs 41.6% at age 5 years for dose 5), but this rate improved within successive birth cohorts (52.2% for 2008 birth cohort vs 32.3% for 2017 birth cohort). The investigators found that undervaccination was significantly linked to a higher risk of pertussis (adjusted relative risk [aRR], 4.8; 95% CI, 3.1-7.6), the first booster (aRR, 3.2; 95% CI, 2.3-4.5), and the second booster (aRR, 4.6; 95% CI, 2.6-8.2). A delay in vaccination among children who did end up receiving the recommended number of doses was not tied to a risk of pertussis.

According to the investigators, undervaccination was tied to a higher risk of pertussis. However, short delays in vaccination appear to be less important than previously thought, as long as the child received the age-appropriate number of doses, although delays are not recommended. For clinicians facing a number of undervaccinated or delayed vaccines in their patients, the key takeaway is that ensuring all doses of pertussis are administered is imperative.


1. Rane M, Rohani P, Halloran M. Association of diphtheria-tetanus–acellular pertussis vaccine timeliness and number of doses with age-specific pertussis risk in infants and young children. JAMA Netw Open. 2021;4(8):e2119118. doi:10.1001/jamanetworkopen.2021.19118

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