Newer Pertussis Vaccine Is Safer But Less Effective

A new study has revealed children immunized with the current whooping cough vaccine are three times more likely to contract the disease, than those who were given the old one.

Researchers from the University of Queensland’s Children’s Medical Research Unit in Queensland, Australia examined the incidence of whooping cough in children who were born in 1998, the year in which that province began phasing out whole-cell pertussis vaccine (known as DTwP) in favor of less-reactive acellular vaccine (known as DTaP).

Children who were born in that year and who received a complete series of infant pertussis shots (at 2, 4 and 6 months) might have received all-whole cell, all-acellular, or a mix.  Thanks to excellent record-keeping of the state-based healthcare system, researchers were able to confirm which children received what shots.

The researchers were prompted to investigate because, like the U.S., Australia is suffering a ferocious pertussis epidemic.

“At the time we thought the vaccine was probably going to be as good, or almost as good, as the old vaccine in terms of protection,” professor Stephen Lambert from Queensland Children’s Medical Research Institute said.

However, 40,000 children have been studied since and research has discovered while the current vaccine is safer, it’s not as effective.

When they examined the disease history for 40,694 children whose vaccine history could be verified, they found 267 pertussis cases between 1999 and 2011.

Apparently, children who received a 3-dose DTaP primary course had higher rates of pertussis than those who received a 3-dose DTwP primary course in the pre-epidemic and outbreak periods. Among those who received mixed courses, rates in the current epidemic were highest for children receiving DTaP as their first dose. This pattern remained when looking at subgroups with 1 or 2 DTwP doses in the first year of life, although it did not reach statistical significance. Children who received a mixed course with DTwP as the initial dose had incidence rates that were between rates for the pure course DTwP and DTaP cohorts.

“During the current outbreak which started in 2009, children who got the new vaccine are about three times more likely to get whooping cough than children who got the old vaccine,” Professor Lambert said.

That means around 100 more Queensland children a year have contracted the disease despite being vaccinated.

Acellular and whole-cell vaccines both have high efficacy during the first 2 years after vaccination, but recent changes in the epidemiology of pertussis in the United States strongly suggest diminished duration of protection afforded by childhood acellular vaccine (DTaP) compared with that of whole-cell pertussis (DTwP) vaccine.  Since the mid-2000s, the incidence of pertussis among children aged 7–10 years has increased. This increased risk translates to greater susceptibility as those children age.

Dr. Anne Schuchat, director of the CDC’s National Center for Immunization and Respiratory Diseases, explains why she believes there has been such an increase, “In young children, we think that within a couple of years of vaccination the Dtap series is 95 percent protection.  Five years later after the series, we think it wanes to 70 percent.  That going down from 95 percent effectiveness to 70 percent may be why we see this increase in the older children or young teens.”


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