DTaP 96% effective against pertussis in Alaska outbreak

Children aged 6 months to 6 years who had completed the 3-dose primary diphtheria and tetanus toxoids and acellular pertussis (DTaP) series were strongly protected against laboratory-confirmed pertussis during Alaska's record 2024 outbreak, according to a report published October 15, 2026, in Morbidity and Mortality Weekly Report (MMWR). Adjusted vaccine effectiveness (VE) was 96.2% among fully vaccinated children and 85.3% among those who were partially vaccinated.¹

"These real-world findings support childhood vaccination recommendations to protect young children against pertussis," wrote Katherine Newell, DPhil, of the Section of Epidemiology, State of Alaska Department of Health, and the CDC Career Epidemiology Field Officer Program, and colleagues.¹

Case-control design during Alaska's 2024 pertussis outbreak

Pertussis incidence in Alaska fell to 0.27 cases per 100,000 population in 2022 before surging to a record 83 cases per 100,000 in 2024.¹ Among children aged 6 months to 6 years, 113 laboratory-confirmed cases were reported from May 1 to November 30, 2024, an incidence of 193.0 per 100,000 children.¹

Investigators conducted a matched case-control analysis using state surveillance data and VacTrAK, Alaska's immunization information system. Case patients had first-time pertussis confirmed by culture or polymerase chain reaction. Each case was matched by birth year and health care facility with up to 3 control patients who had an acute cough and were tested for respiratory viruses but had a negative pertussis test or were not tested for pertussis.¹ Children born outside Alaska or with a history of pertussis were excluded. The final analysis included 84 case patients and 229 control patients from 17 facilities, with a median age of 2.2 years.¹

Full vaccination was defined as at least 3 valid DTaP doses given at least 2 weeks before specimen collection; 1 or 2 valid doses counted as partial vaccination.¹˒²

DTaP vaccine effectiveness against laboratory-confirmed pertussis

Case patients were far more likely than control patients to be unvaccinated (61.9% vs 12.2%) and less likely to be fully vaccinated (25.0% vs 77.3%; P < .001).¹

After adjustment for race, sex, and calendar month, unvaccinated children had 26.1 times the odds of pertussis compared with fully vaccinated children (95% CI, 9.6-70.9), and partially vaccinated children had 3.8 times the odds (95% CI, 1.7-8.6).¹ Adjusted VE was 96.2% (95% CI, 89.6%-98.6%) for full vaccination and 85.3% (95% CI, 64.4%-94.0%) for partial vaccination. A sensitivity analysis without facility matching produced similar estimates.¹

Declining DTaP coverage and the pertussis resurgence in children

The Advisory Committee on Immunization Practices (ACIP) recommends DTaP at ages 2, 4, and 6 months, with boosters at 15 to 18 months and 4 to 6 years.²

DTaP coverage among young children in Alaska was below the national average during 2022-2023, and national coverage by age 24 months declined among children born in 2020 and 2021.¹˒³ The authors cited waning immunity, falling coverage, greater clinician awareness and testing, and improved diagnostics as possible contributors to the resurgence.¹

Waning pertussis immunity and study limitations

Among fully vaccinated children, case patients had a longer median interval since their last dose than control patients (15.3 vs 10.2 months), although the difference was not statistically significant (P = .23).¹ The pattern is consistent with prior evidence that acellular pertussis protection wanes over time,⁴ and the authors noted that children who had not yet received their 15- to 18-month or 4- to 6-year boosters may have been more susceptible.¹

The VE estimates exceeded those from prelicensure studies but matched other real-world analyses, which the authors attributed to the focus on laboratory-confirmed disease in young children.¹

Limitations included unavailable symptom severity data, possible inclusion of untested pertussis cases among control patients, no assessment of maternal pertussis vaccination, a wide CI around the primary odds ratio, and a sample too small for subgroup analysis by booster dose. Findings may not apply to older children or milder cases that did not reach care.¹

The authors concluded that declining coverage may have contributed to the size of the outbreak and that the results reinforce ACIP's recommendation to begin pertussis vaccination at age 2 months.¹

References

1. Newell K, Ohlsen E, Moore K, Aho S, Castrodale L, McLaughlin J. Effectiveness of diphtheria and tetanus toxoids and acellular pertussis vaccine in children aged 6 months–6 years during a statewide pertussis outbreak — Alaska, May–November 2024. MMWR Morb Mortal Wkly Rep. 2026;75(40):601-606. doi:10.15585/mmwr.mm7540a1

2. Liang JL, Tiwari T, Moro P, et al. Prevention of pertussis, tetanus, and diphtheria with vaccines in the United States: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep. 2018;67(2):1-44. doi:10.15585/mmwr.rr6702a1

3. Hill HA, Yankey D, Elam-Evans LD, et al. Decline in vaccination coverage by age 24 months and vaccination inequities among children born in 2020 and 2021—National Immunization Survey-Child, United States, 2021–2023. MMWR Morb Mortal Wkly Rep. 2024;73(38):844-853. doi:10.15585/mmwr.mm7338a3

4. Zerbo O, Bartlett J, Goddard K, Fireman B, Lewis E, Klein NP. Acellular pertussis vaccine effectiveness over time. Pediatrics. 2019;144:e20183466. doi:10.1542/peds.2018-3466