Abstract
In most countries, the diphtheria-tetanus-acellular pertussis (DTaP) vaccine is administered as a 3-dose infant series followed by additional booster doses in the first 5 years of life. Short-term immunity from the DTaP vaccine can depend on the number, timing, and interval between doses. Not receiving doses in a timely manner might be associated with a higher pertussis risk. To examine the association between number and timeliness of vaccine doses and age-specific pertussis risk. This population-based, retrospective cohort study used Washington State Immunization Information System data and pertussis surveillance data from Public Health Seattle and King County, Washington. Included participants were children aged 3 months to 9 years born or living in King County, Washington, between January 1, 2008, and December 31, 2017. Data were analyzed from June 30 to December 1, 2019. Being undervaccinated (receiving fewer than recommended doses at a given age) or delayed vaccination (not receiving doses within time frames recommended by Centers for Disease Control and Prevention). Suspected, probable, and confirmed pertussis diagnosis. A total of 316 404 children (median age, 65.2 months [interquartile range, 35.3-94.1 months]; 162 025 boys [51.2%]) as of December 31, 2017, with 17.4 million person-months of follow-up were included in the analysis. A total of 19 943 children (6.3%) had no vaccines recorded in the Immunization Information System, 116 193 (36.7%) received a vaccine with a delay, and 180 268 (56.9%) were fully vaccinated with no delay. Delayed vaccination and undervaccination rates 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 improved for successive birth cohorts (52.2% for 2008 birth cohort vs 32.3% for 2017 birth cohort). Undervaccination was significantly associated with higher risk of pertussis for the 3-dose primary series (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). However, delay in vaccination among children who received the recommended number of vaccine doses was not associated with pertussis risk. The results of this cohort study suggest that undervaccination is associated with higher pertussis risk. Short delays in vaccine receipt may be less important if the age-appropriate number of doses is administered, but delaying doses is not recommended. Ensuring that children receive all doses of pertussis vaccine, even if there is some delay, is important.
Highlights
Widespread rollout of the diphtheria-tetanus–whole-cell pertussis (DTwP) vaccines in the 1940s resulted in a dramatic decrease in pediatric pertussis incidence until the 1970s and 1980s in the US.[1,2] Owing to concerns surrounding the safety and reactogenicity of DTwP vaccines, a less reactogenic diphtheria-tetanus–acellular pertussis (DTaP) vaccine was developed.[1,3] The DTaP vaccines are safe and efficacious, and most developed countries recommend them for their infant primary series in national immunization programs.[4]
Undervaccination was significantly associated with higher risk of pertussis for the 3-dose primary series, the first booster, and the second booster
Short delays in vaccine receipt may be less important if the age-appropriate number of doses is administered, but delaying doses is not recommended
Summary
Widespread rollout of the diphtheria-tetanus–whole-cell pertussis (DTwP) vaccines in the 1940s resulted in a dramatic decrease in pediatric pertussis incidence until the 1970s and 1980s in the US.[1,2] Owing to concerns surrounding the safety and reactogenicity of DTwP vaccines, a less reactogenic diphtheria-tetanus–acellular pertussis (DTaP) vaccine was developed.[1,3] The DTaP vaccines are safe and efficacious, and most developed countries recommend them for their infant primary series in national immunization programs.[4]. Waning of DTaP-induced immunity has been widely cited as one of the main drivers of pertussis resurgence in countries with high vaccination coverage.[7,8,9,10] Short-term protection afforded by pertussis vaccines depends on the number, timing, and interval between doses.[11] strategic scheduling and timely uptake of boosters is crucial. Longer intervals between doses due to delays or missed immunizations could increase pertussis risk even in partially vaccinated children. This increased risk could lead to sustained transmission of pertussis and periodic outbreaks.[12,13] Observational studies in the US and Taiwan have suggested that undervaccination or delay in vaccination results in higher pertussis risk.[14,15,16,17] Current methods of estimating DTaP vaccination coverage at specific ages without estimating timeliness of each dose can mask delays in vaccination while showing high vaccination coverage at the national level.[12,13,18]
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