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 have found.
In an Ontario study published Monday in the , researchers found that by the time a child is seven or eight years old, 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, usually on philosophical or religious grounds.
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. No deaths were reported. Among those who were fully immunized and yet contracted the disease, more than half were aged 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?"
Part of the answer seems to lie in the type of whooping cough vaccine given to kids.
Since 1997, Canada has employed what's known as an "acellular" vaccine, which is also used in the rest of North America, Australia, New Zealand and much of Europe. The original whole-cell vaccine was discontinued in North America because it "didn't work very well" and caused adverse reactions in children, which included soreness at the injection site and fevers.
But Crowcroft said the study, which analyzed provincial health data for almost 6,000 people born between 1992 and 2013, turned up an interesting finding.
"We found something that's quite new, which is that people in our study who had their very first dose of vaccine that was the whole-cell vaccine -- the old-fashioned style of vaccine -- as a baby, they seemed to get longer-lasting protection," she said, noting that subsequent doses would have been the acellular type of immunization.
People who received the acellular version of the vaccine for their first three infant vaccine doses were twice as likely to contract pertussis compared with people who had received the older whole-cell vaccine when they were infants, the researchers showed.
Scientists aren't sure what's behind the difference, but they agree a better type of pertussis vaccine needs to be developed to prolong immunity throughout childhood, the teen years and into adulthood.
"There's a couple of things we can do now while we're waiting for a better vaccine to come along, and one of them is that adults can go and get a booster, which helps protect younger kids," said Crowcroft, adding that studies have shown that babies often catch pertussis from their parents.
Older kids, whose vaccine-induced immunity wanes over time, can also contract the infection and pass it to infants, who are most at risk for the disease's adverse effects.
Women should be immunized against whooping cough during pregnancy, she said, because the vaccine has been shown to be safe and provides early protection to the newborn, as the mother's antibodies are passed to the baby in-utero.
"The first couple of months of life is a really vulnerable time. Babies are the important targets of the immunization program and most of the deaths from whooping cough happen in the first six months or even two months of life."
In a related commentary, Dr. Nicole Le Saux of the Children's Hospital of Eastern Ontario and Kingston, Ont.-area Medical Officer of Health Dr. Ian Gemmill suggest that until newer vaccines with a longer duration of protection are available, "a rethinking of the optimal use of the current pertussis vaccine is needed."
"Perhaps a booster at 10 years of age should be recommended to reduce the incidence among pre-adolescents, or perhaps regular boosting throughout life is needed," they write. "We don't yet have a clear answer."
