Thursday, November 29, 2012

Study Shows that the Whooping Cough Vaccine May Become Less Effective Over Time


Vaccination is a hot topic in today’s world.  It is often a debated and changing subject.  Vaccination is proven to safeguard children against diseases such as whooping cough, but new research has shown that the vaccine may become less effective over time. 

A recent outbreak of whooping cough, also known as pertussis, was in California in 2010.  The outbreak sickened more than 9,000 people and left 10 infants dead.  This outbreak is what compelled an examination into the effectiveness of the vaccine.  The study found that the vaccine is effective, but it does become less effective over time, especially leaving children 7 to 10 years old vulnerable.

Lara Misegades, an epidemiologist at the US Centers for Disease Control and Prevention in Atlanta said, “The pertussis vaccine is our best protection against disease.”  Misegades also said, “We found that unvaccinated children were eight times more likely to be a pertussis case than vaccinated children.  Parents should ensure children complete the childhood series and make sure your children get the adolescent booster too.”

Just this year alone, the US has had more than 36,000 whooping cough cases have been reported.  There were 16 deaths, mainly in infants younger than 3 months old, according to the CDC.

"We're continuing to evaluate the changing epidemiology [of pertussis], but it's too early to speculate if there's a need for an additional booster," Misegades said.

The results of the study are available in the November 28th issue of the Journal of American Medical Association.

The vaccine is commonly referred to as the DTaP vaccine.  It also includes immunizations for diphtheria and tetanus.  The vaccine is a 5 dose series that are administered at age 2 months, 2 months, 6 months, 15 months and between 4 and 6 years old. The CDC recommends a booster at the age of 11 or 12.

The research study was intended to evaluate how much time had elapsed since the last DTaP vaccination and the development of whooping cough in the patients in the California outbreak, which was the largest state outbreak in 60 years.

Included in the data for the study were 15 counties in California with a high incidence of pertussis cases.  A total of 682 children with ages ranging from 4 to 10 years old who developed pertussis were included and for each child in the this group, three children of the same age that did not have pertussis were also included for comparison.

The study found that in comparison to the controls, the children who had contracted pertussis were 89% less likely to have received all the recommended doses of the vaccine. Those children that developed the disease were more likely to be unvaccinated than those that did not have the disease 7.8% versus 0.9%.  The researchers also found that the vaccine became less effective with the more time that had passed.  

"Within the first few years, the vaccine's efficacy was around 98 percent," Misegades said. "Five or more years out, the vaccine effectiveness had dropped to about 71 percent."

The study also showed that the highest incidence of disease occurred in 7 to 10 year old children and that the incidence increased with age.  For example, about 3% of the cases occurred in 4 year olds and 31% of the cases were found in 10 year olds.  The current vaccine available is an acellular vaccine; it does not contain whole cells of the bacterium that is responsible for whooping cough infections.  The previous version of the vaccine did contain whole cells of the bacterium; however it was more likely to cause side effects, according to researchers.

The switch to the newer vaccine may explain the reemergence of pertussis, according to the editorial by Dr. Eugene Shapiro, accompanying the journal article.  Dr. Kenneth Bromberg, director of the Vaccine Study Center at the Brooklyn Hospital in New York City, agreed that the previous whole-cell vaccine might have been more effective than the current vaccine, but he pointed out that the whole-call vaccine did have production issues, including batch effectiveness variance. He also said that the components of the whole-cellular vaccine may have been the reason for the risk of side-effects.

The take-home message for parents, according to Bromberg is, “"the vaccine works. It's effective, so make sure everyone is appropriately vaccinated according to the current schedule." "We'll have to wait for further study to [determine if the current dosing schedule needs change]," he said. "Parents should stay tuned as to whether we recommend additional vaccinations for pertussis."

To learn more about pertussis, visit http://www.cdc.gov/pertussis/vaccines.html.