Three or more doses of DTaP were 96.2% effective in Alaska’s whooping cough outbreak

Children who had received three or more doses of the DTaP vaccine were 96.2% less likely to catch whooping cough than comparable children who had not, according to a new analysis of Alaska’s 2024 pertussis outbreak. The 95% confidence interval ran from 89.6% to 98.6%.

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Unvaccinated children in the same outbreak had 26.1 times the adjusted odds of pertussis. The findings came from a matched study of children aged 6 months to 6 years and were published October 15 by the Centers for Disease Control and Prevention.

How the Alaska study measured protection

The report, in CDC’s Morbidity and Mortality Weekly Report, was written by Katherine Newell, Elizabeth Ohlsen, Kristen Moore, Sarah Aho, Louisa Castrodale and Joseph McLaughlin of the Alaska Section of Epidemiology and CDC’s field epidemiology program. It covers cases with symptom onset between May 1 and November 30, 2024.

Alaska recorded 113 laboratory-confirmed cases in children aged 6 months to 6 years during that window, an incidence of 193.0 per 100,000 children in that age group. Statewide, the authors write, 2024 incidence was a record high of 83 per 100,000 population.

The team used a matched case-control design. After excluding children born outside Alaska and those with no age-matched controls at the reporting facility, 84 case-patients remained, paired with 229 controls from 17 facilities. Controls were matched on birth year and health care facility, and the analysis adjusted for race, sex and calendar month.

The dose-by-dose breakdown

Of the 313 children in the analysis, 198 (63%) were fully vaccinated, meaning at least three valid DTaP doses. That group held 21 cases and 177 controls. At the other end, 80 children (26%) were unvaccinated: 52 cases and 28 controls.

The partially vaccinated, with one or two doses, numbered 35 (11%), including 11 cases. Their adjusted vaccine effectiveness was 85.3%, with a confidence interval of 64.4% to 94.0%, lower than the full-series figure but still substantial.

Turned around, the unvaccinated-versus-fully-vaccinated comparison gives the 26.1 figure, with a confidence interval of 9.6 to 70.9. The authors flag that interval as wide, so 26 is better read as a strong signal than a precise multiplier. Partially vaccinated children had 3.8 times the adjusted odds of fully vaccinated ones.

Limits the authors put on the number

The study could not look at how severe the illnesses were, because symptom severity and outcomes were unavailable. It also did not assess maternal pertussis vaccination, and vaccinations given in other states may have been misclassified. Controls could have included undiagnosed pertussis, which the authors expect to be random enough not to skew the result in one direction.

The results apply to laboratory-confirmed disease in children aged 6 months to 6 years, and the authors caution that they may not generalize to older children or milder cases. The sample was also too small to break results down by number of doses or time since the last shot.

On waning protection, the data are suggestive but inconclusive. Among fully vaccinated children, the median time since the last dose was 15.3 months for cases and 10.2 months for controls, a gap that did not reach statistical significance (p = 0.23). The authors say waning immunity may have played a role and call for longitudinal studies.

A wider look at the same outbreak came out of the state itself. In Bulletin No. 3, issued February 12, 2025, the Section of Epidemiology analyzed children from 2 months to 17 years, comparing 120 first-time laboratory-confirmed cases with 344 matched controls. Full vaccination there showed an adjusted effectiveness of 92.7% (95% CI 86.8% to 95.9%), and unvaccinated children were more than 13 times as likely to get pertussis. The 96.2% figure belongs to the narrower group of children aged 6 months to 6 years.

The outbreak’s growth and the authors’ recommendation

The authors say declining childhood vaccination coverage in Alaska may have contributed to the size of the outbreak. The state health department had already sounded the alarm in a Public Health Alert dated August 5, 2024, which reported 91 confirmed and probable cases since June and described a marked increase in pertussis activity compared with recent years.

That alert identified unvaccinated and under-vaccinated infants and young children as priorities for vaccination, along with pregnant women at 27 to 36 weeks, who should receive a Tdap dose in each pregnancy. The new report says its findings reinforce the 2018 recommendations of the Advisory Committee on Immunization Practices for vaccinating all infants against pertussis beginning at 2 months of age.

The study is also a reminder that effectiveness figures like 96.2% describe a specific outbreak and a specific age band. The full citation is MMWR 75(40):601-606. The open question the authors leave for later work is how quickly protection fades in the years after the third dose.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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