(HealthDay News) — In what state health officials are calling the worst outbreak in 70 years, 9,935 cases of pertussis were diagnosed between January 1 and November 26. That translated into 26 cases per 100,000 people, according to research published in the December 5 issue of the U.S. Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.

While infants under the age of 1 year were the most susceptible to both the disease and its serious complications because they were too young to be vaccinated, Hispanic infants were 70% more likely to contract pertussis. Overall, the disease rate among infants under the age of 1 year was 175 per 100,000 people. Infants were also much more likely to be hospitalized after developing pertussis. Of the 347 hospitalizations that were reported across the state, 275 were under the age of 1 year, including 214 under the age of 4 months, according to the authors of the report.

When the researchers looked more closely at the available data, they found that most of the mothers of the infants who contracted pertussis had not been vaccinated. There was also a spike in the number of teens with milder cases of pertussis, even though 87% of them had been vaccinated against the disease as children. That suggests that their immunity was waning and that this population might need booster shots with the newer, more effective version of the vaccine, the report authors said.

“Very few mothers of infants with pertussis had received Tdap [tetanus, diphtheria, and acellular pertussis vaccine] during pregnancy; many more were vaccinated after delivery, which does not confer any direct protection to the infant and is no longer a preferred strategy,” the researchers write. “Recently published data indicate that Tdap vaccination coverage among pregnant women [in California] was only 19.5% in 2012.” Recent research has shown that vaccinating women in the third trimester of pregnancy confers protection to the fetus, and health care providers need to encourage pregnant women to get the vaccine, the authors added.

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