Effectiveness of whooping cough vaccine declines: study

TORONTO — The current vaccine for pertussis, or whooping cough, is highly effective during the first three years after children get their shots, but immunity wanes over the next several years, leaving little protection from the disease, researchers say.

In an Ontario study published Monday in the Canadian Medical Association Journal, researchers found that immunity begins waning about four years following the last scheduled childhood immunization or booster shot given to an adolescent or adult. Within a few more years, the vaccine’s effectiveness has diminished significantly.

“The protection that the vaccine gives is very good in the first couple of years after the immunization is given,” said Dr. Natasha Crowcroft, chief of applied immunization research for Public Health Ontario. “So it’s good news for protecting babies.

“But that protection, it fades pretty quickly. So by the time you’re at seven or eight years [out], you’ve got very little protection left.”

Researchers found the odds of contracting whooping cough following vaccination increased by 27 per cent each year, although the overall risk remained small.

Whooping cough is a highly contagious bacterial infection that affects the respiratory tract and is often marked by a severe hacking cough, followed by a high-pitched intake of breath that sounds like a “whoop.” Some infants may not cough, but may struggle to breathe or even temporarily stop breathing. Pertussis is particularly dangerous for babies and can lead to hospitalization and, in rare cases, death.

Before the introduction of Canada’s public pertussis vaccine program, annual incidence of the disease averaged 156 cases per 100,000 people. Since the vaccination program came into effect, the number of new cases has ranged from two per 100,000 in 2011 to 13.9 cases per 100,000 in 2012.

Most cases occur in under-immunized populations, often among those in which parents oppose vaccination.

In 2012, there was an outbreak of pertussis in southwestern Ontario that was first identified in an under-immunized religious community, which spread to the general population and then to a second under-immunized religious community in the region. Almost 450 cases of whooping cough were confirmed, including 13 that required hospital admission. Among those who were fully immunized and yet contracted the disease, more than half were 10 to 14 years old, with a median of 5.6 years since their last immunization.

Last year, Manitoba and New Brunswick both experienced pertussis outbreaks, while B.C. and the Northwest Territories also saw above-normal spikes in cases.

The pertussis vaccination — which also includes components to protect against diphtheria, tetanus and sometimes other diseases — is recommended at two, four, six and 18 months of age, followed by a dose between four and six years old. Since 2011, a booster for 14- to 16-year-olds was introduced and a second booster during adulthood is also recommended.

“What we’re finding is that the baby vaccines are barely going to get you through to that booster,” Crowcroft said of the adolescent shot. “And after that, the question is how long are people going to be protected?”

Crowcroft said adults should consider getting a booster to help protect younger kids, since babies often catch pertussis from their parents. Older kids, whose vaccineinduced immunity wanes, can also contract the infection and pass it to infants.