Among those who received mixed vaccines, the rates of whooping cough were highest among children receiving DTaP as their first dose, they noted.
"Our findings go some way to explaining why we are currently seeing a resurgence of pertussis in Australia and the U.S., and in particular why rates are higher in some age groups compared to others," Lambert said.
In switching to a different vaccine, "we may have traded off some of the protection whole-cell vaccines provided for the better safety profile, with many fewer adverse events, seen with acellular vaccines," Lambert suggested.
The earlier vaccine caused pain and swelling at the injection site. Also, rare adverse brain effects occurred in some children, although it's not known if the vaccine was the direct cause.
Dr. Thomas Clark, a medical epidemiologist at the CDC, said the acellular vaccine was introduced because of those concerns. "What we know now is that the whole-cell vaccines were probably a little better than the acellular vaccines," he said.
The open question is how long the vaccine remains effective, Clark added.
While most older children and adults have few problems with whooping cough, infants are at great risk because their airway is underdeveloped. Vaccination can't take place until a child is 2 months old and they're not well protected until the third dose at 6 months of age, which is why the CDC urges pregnant women and anyone who comes into contact with infants to be vaccinated. For women who get vaccinated during pregnancy, the mother's immunity may protect her baby before he's old enough to get his own vaccines, Clark added.
The vaccine's immunity wears off over time so children should get a booster at age 11 or 12, and adults need one dose, too, Clark said.
So far this year almost 19,000 cases of pertussis have been reported in the United
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