Children's risk of contracting whooping cough increases over the years following their final scheduled vaccination, a new study says.

While the vaccine protects 98 out of 100 children in the first year after the final shot in the five-injection series, protection drops to 71 out of 100 children five years later, according to the study, which included cases from the 2010 California outbreak of whooping cough, also called pertussis. In other words, the vaccine's effectiveness declines by about 30 percent within five years of the final dose, the researchers said.

The United States is on track for more cases of whooping cough this year than in any other year since 1959.

Children receive the last dose of the vaccine, known as DTaP (which also protects against diphtheria and tetanus), between ages 4 and 6. They get a booster shot in adolescence.

The new study joins several others in the last few years in suggesting that children ages 7 to 10 have less immune protection against whooping cough. But this the first study to estimate how much the vaccine's effectiveness declines after the final dose, the researchers said.

Findings from this study and others may prompt health officials to change the age at which children receive one of the DTaP shots or to add another booster shot.

However, health officials should make sure that the changes won't “be creating new pockets of disease in other age groups,” said study researcher Lara Misegades, of the Centers for Disease Control and Prevention. For instance, moving the booster shot up from the teen years to age 10 could mean fewer kids get the shot, since they do not routinely visit the doctor at that age, Misegades said.

Whooping cough outbreak

In 2010, more than 9,000 cases of whooping cough occurred in California, including 10 deaths — the state's worst outbreak in 60 years.

Misegades and colleagues analyzed information from 682 children ages 4 to 10 who had whooping cough during that outbreak and, as a control group, about 2,000 children who visited the doctor for other reasons during the same time period. Close to 70 percent of children in both groups had received their fifth dose of DTaP at age 4.

The researchers used the unvaccinated children as a reference group to help determine the vaccine's effectiveness.

About 8 percent of children who had whooping cough, and 1 percent of those who did not get whooping cough, never got vaccinated against the disease, researchers noted.

New vaccine schedule?

An older version of the whooping cough vaccine, known as the whole-cell pertussis vaccine, was thought to protect people against the disease for most of their lives. But there were concerns that this vaccine caused unwanted side effects, such as fever and swelling at the injection site. So a new, more purified version of the vaccine, called the acelluar vaccine, was introduced in 1997.

When health officials made the switch, they were, “to some extent, trading efficacy for safety,” said Dr. Paul Offit, chief of the Division of Infectious Diseases at the Children's Hospital of Philadelphia. “I just don't think that people had a sense that the trade would be this big,” Offit said, referring to new vaccine's waning protection.

The CDC's Advisory Committee on Immunization Practices (ACIP), which makes vaccine schedule recommendations, has not discussed changes to the whooping cough vaccine schedule for children. And it will likely be some time before the committee votes on the issue, said ACIP member Dr. Mark Sawyer, a professor of pediatrics at the University of California, San Diego.

Before the committee could recommend a booster shot for children younger than 11, there would need to be studies showing it is safe to give the vaccine at that age, Sawyer said.

Experts agreed that the United States is unlikely to return to the whole-cell pertussis vaccine, despite the shot's long-lasting immunity. So, doctors may have to wait for the creation of a new, longer-lasting vaccine that lacks the side effects of the old one, Sawyer said.

Pass it on: The effectiveness of the whooping cough vaccine in children declines by about 30 percent five years after the final shot.

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