Even though high numbers of the vaccine for whooping cough have been administered throughout the United States, incidence of pertussis is still at its highest in years. One reason could be that after the fifth dose is administered, immunity starts to wane.

(Pertussis is the medical term for whooping cough.)

Research from the U.S. Centers for Disease Control and Prevention (CDC) confirms previous studies which revealed that protection against whooping cough wanes greatly between the fifth dose of the vaccine (when the patient is 4 to 6 years old) and the booster vaccine, which is administered at the age of 11 or 12.

Study lead author Sara Tartof, who was at the CDC at the time of the study and is now a researcher at Kaiser Permanente in Los Angeles, commented:

“This study provides fairly strong evidence that the trends we’re seeing are real — and a couple of other studies with similar findings have recently come out.”

Pertussis – commonly known as whooping cough – is an extremely contagious bacterial illness that affects the respiratory system.

In 2012, the U.S. reported the highest number of whooping cough cases since 1959 – 41,000 cases and 18 deaths – mostly in infants. In fact, the state of Washington even declared a state of emergency as pertussis became an epidemic in April of 2012.

Despite these numbers, the authors emphasized that the vaccine is still the best tool for protection. Children who receive the vaccine and develop pertussis end up with a much milder form of the disease.

The vaccine for whooping cough also consists of immunizations for tetanus and diptheria. According to the CDC, it is a five-dose series given at:

  • 2, 4, and 6 months
  • then 15 to 18 months
  • followed by between 4 and 6 years
  • During adolescence a booster shot is recommended between the age of 11 and 12.

In this study, which was published in the journal Pediatrics, the team analyzed kids born between 1998 and 2003 in Oregon or Minnesota and who had gotten all five doses of the vaccine.

By exploring immunization records and comparing them with whooping cough data for six years after the fifth dose of vaccine – the investigators were able to efficiently monitor how many vaccinated kids still contracted the illness and how much protection the vaccine offered each year.

In Minnesota, close to 225,000 kids born during the study were fully vaccinated. In Oregon, nearly 180,000 were fully vaccinated.

In Oregon, 89 cases of whooping cough were reported and in Minnesota, 458 cases. Each year of the follow-up period, rates of whooping cough increased.

In the first year after the last vaccination, the incidence of whooping cough was 15.6 per 100,000 in the population of Minnesota. After the sixth year, that number increased to 138.4 per 100,000.

Researchers saw a similar trend in Oregon with a rate of 6.2 per 100,000 in the first year of follow-up, then after the last year of follow-up an increase to 24.4 per 100,000.

There are two possible reasons why whooping cough cases have increased:

  • Doctors may be more aware of the illness and are reporting it more often
  • The vaccine itself – the current one has fewer side effects but is not quite as effective as the old one.

The whooping vaccine that is used now is acellular – meaning it does not have whole cells of the bacterium that causes pertussis infections. However, the previous vaccine, which was linked to more potential side effects, consisted of whole cells of the bacterium.

A downside to developing a vaccine that has a decreased number of side effects is that it is slightly less effective.

A previous study carried out by Kaiser Permanente Medical Center, San Rafael, CA, USA, revealed that acellular vaccines might not be as long-lasting as had been previously reported – especially with whooping cough cases.

The authors pointed out that currently there are no new whooping cough vaccines being developed – therefore its crucial for everyone to be vaccinated.

Whooping cough, like measles, chicken pox and other diseases for which there are routine vaccines, depends on the “herd protection” to keep it in check. If more people are vaccinated, its ability to spread decreases.

Written by Kelly Fitzgerald