Three or more doses of DTaP vaccine were 96.2% effective at preventing laboratory-confirmed pertussis in Alaska children aged 6 months to 6 years during the 2024 outbreak, according to a CDC Morbidity and Mortality Weekly Report published Oct. 15.
The authors, from the Alaska Department of Health and CDC’s Career Epidemiology Field Officer Program, compared 84 sick children with 229 children who had a cough but no pertussis diagnosis. Unvaccinated children had about 26 times the odds of getting pertussis as fully vaccinated children.
How the Alaska Study was Built
The report is a matched case-control study. Katherine Newell, Elizabeth Ohlsen, Kristen Moore, Sarah Aho, Louisa Castrodale and Joseph McLaughlin used Alaska’s disease surveillance system and the state immunization registry, called VacTrAK.
A case was a child aged 6 months to 6 years with a first laboratory-confirmed pertussis infection and symptom onset between May 1 and Nov. 30, 2024. Controls were children with an acute cough who tested negative for pertussis or were never tested for it. The team picked up to three controls per case, matched on birth year and health care facility.
Alaska reported 113 qualifying cases in that age group. Seven were born outside Alaska, so the state registry could not show their shots. Twenty-two had no matched control at their facility. That left 84 cases, or 79% of those reported.
The authors defined “fully vaccinated” as at least three valid DTaP doses, the last one given at least two weeks before the specimen was collected. One or two doses counted as partially vaccinated. No recorded doses counted as unvaccinated. Doses given closer together than the recommended interval did not count.
What the Numbers Show
Among the 84 cases, 52 children (61.9%) were unvaccinated, 11 (13.1%) were partially vaccinated and 21 (25.0%) were fully vaccinated. Among the 229 controls, 28 (12.2%) were unvaccinated, 24 (10.5%) were partially vaccinated and 177 (77.3%) were fully vaccinated.
After adjusting for race, sex and calendar month, the report puts vaccine effectiveness at 96.2% for full vaccination (95% confidence interval 89.6% to 98.6%) and 85.3% for partial vaccination (64.4% to 94.0%). The adjusted odds ratio for unvaccinated children compared with fully vaccinated children was 26.1, with an interval of 9.6 to 70.9.
The authors wrote that “pertussis vaccination was highly effective at preventing laboratory-confirmed pertussis” during the outbreak, according to the MMWR report. Children with one or two doses still did better than unvaccinated children but worse than those who finished the series. Their adjusted odds were 3.8 times those of fully vaccinated children.
Why Alaska was a Test Case
Alaska’s pertussis rate was 0.27 cases per 100,000 people in 2022, the report says. It reached a record 83 per 100,000 in 2024. The authors write that Alaska’s DTaP coverage among young children was below the national average in 2022 and 2023. The CDC’s provisional 2024 surveillance report shows how far Alaska stood apart.
The agency lists Alaska at 81.11 cases per 100,000 people, or 595 cases, the highest of any state. The national rate was 10.63 per 100,000, on 35,435 cases, up from 7,063 in 2023. The figures differ by source. The MMWR gives 83 per 100,000 for 2024. CDC’s provisional report gives 81.11 for the same year.
The state’s own October 2025 bulletin uses a different window, April 2024 through April 2025, and counts 679 confirmed and probable cases, or 92 per 100,000. The reports use different case windows and counting rules, and the documents do not say which factor accounts for each gap. A calculation from the named figures: Alaska’s 2024 rate of 81.11 per 100,000 is about 7.6 times the national rate of 10.63. The jump from 0.27 per 100,000 in 2022 to 83 in 2024 is a roughly 307-fold increase.
Who Got Sick, How Badly
The study group skewed young and female. Cases were 59.5% female, compared with 42.4% of controls, a difference the report puts at p = 0.005. The median age was 2.2 years.
American Indian or Alaska Native children made up 38.1% of cases and 41.0% of controls. The authors note that some large Alaska facilities serve almost entirely Alaska Native patients, who have higher vaccination rates. Matching controls by facility kept those groups comparable.
The state’s bulletin describes the youngest patients. Of its 679 cases in its window, 74 were under 1 year old, and 50 of them (68%) were not up to date on pertussis immunization. Twenty-nine patients were hospitalized, 18 of them under 1 year old. One infant died, and the infant was unvaccinated.
A calculation from the bulletin’s figures: infants were about 11% of cases (74 of 679) but 62% of hospitalizations (18 of 29). About one in four infant patients (18 of 74, or 24%) was hospitalized. CDC’s national data show the same pattern. The agency reports hospitalization for 33.4% of cases under 6 months old with known status, compared with 4.9% across all ages.
What the Authors Say Limits the Findings
The report lists seven limitations. It had no data on how severe each child’s illness was. Cases without a matched control were excluded, which may under-represent the smallest facilities. Some controls may have had pertussis that no one diagnosed. The study did not check whether mothers received Tdap during pregnancy. Children vaccinated outside Alaska might have been misclassified.
The confidence intervals are wide, especially for unvaccinated children. The authors note that even the low end of the interval for the unvaccinated comparison reflects a tenfold difference in odds.
The study also found a hint of waning protection. Among fully vaccinated children, the median time since the last dose was 15.3 months for cases and 10.2 months for controls. The difference was not statistically significant (p = 0.23). The sample was too small to compare children by number of doses or time since vaccination. The authors recommend longitudinal studies to test waning immunity.
The state’s October 2025 bulletin reported a different comparison. It found that during summer and fall 2024, unvaccinated children were more than 13 times as likely to get pertussis as children up to date on immunizations. The bulletin does not describe the method behind that figure, and the 13-fold and 26-fold numbers come from separate analyses.
What Health Officials Recommend
The authors say the findings reinforce the 2018 recommendation from the Advisory Committee on Immunization Practices to vaccinate all infants against pertussis starting at 2 months. They support following the full schedule, including booster doses.
Alaska’s health department recommends Tdap in weeks 27 through 36 of each pregnancy, according to its bulletin. The bulletin says that practice reduces infant hospitalizations under 3 months by up to 90%. Twelve of the hospitalized patients were under 3 months old, and the bulletin says nine of them (75%) had mothers who did not receive Tdap during pregnancy.
CDC says case reports in 2025 stayed elevated compared with the years just before the pandemic but trended down after a peak in November 2024, according to its pertussis surveillance page. Fewer cases have been reported so far in 2026 than at the same point in 2025. The Alaska authors call for further study of how long DTaP protection lasts in children.
