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A nurse prepares a whooping cough vaccine at the Skagit County Public Health office in Mount Vernon, Wash., on May 10, 2012.

MATTHEW RYAN WILLIAMS/NYT

The protective benefits of the pertussis vaccine all but disappear after about seven years, according to a new study whose authors say Canada needs to adopt a new approach to prevent whooping cough outbreaks.

The current vaccine is highly effective at preventing pertussis during the first few years after immunization. But starting about four years after an individual's last vaccination, immunity starts to wane, with little protection after seven years, according to the study, published Monday in the Canadian Medical Association Journal.

Although the overall risk of whooping cough remains low across Canada, the odds of contracting the illness increased 27 per cent each year after vaccination, the study found.

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The pattern of decline is similar to what's been observed in other jurisdictions hit with whooping cough outbreaks in recent years, such as California, which experienced major outbreaks in 2010 and 2014 that were tied to several infant deaths.

Dr. Natasha Crowcroft, study author and chief of applied immunization research at Public Health Ontario, says the new study's findings are a concern because they suggest Canadian children could be vulnerable to a whooping cough outbreak.

"I worry about there being a big increase in disease in the future because it's been seen elsewhere and why would we be different?" Crowcroft said.

Until 1997, Canada used a "whole cell" pertussis vaccine. Reports of adverse events, including fever, seizures and pain at the injection site – as well as unproven links to more serious problems, including encephalopathy – prompted the change to an "acellular" pertussis vaccine. (The difference between the vaccines is the components of the bacteria that are used – whole cell vaccines contain the entire B. pertussis bacteria, while the acellular has only purified components of the bacteria.)

The new study looked at vaccination data for people in Ontario born from 1992 to 2013. It found that infants who received the new acellular vaccine were twice as likely to develop whooping cough as those who received the whole cell vaccine when it was still available in the early 1990s.

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But because of the heightened risk of adverse events, it's unlikely Canada will see a shift back to the whole cell vaccine, said Dr. Nicole Le Saux, an infectious disease physician at the Children's Hospital of Eastern Ontario and author of a commentary published with the study. Le Saux said what countries like Canada really need are more effective pertussis vaccines that give longer immunity.

In the meantime, public health programs may need to better target adults, pregnant women and other vulnerable groups who may not have any immunity, Crowcroft said. For instance, many people don't realize that one combination booster shot for tetanus, diphtheria and pertussis is publicly available to adults across Canada, she said.

She added that current recommendations might need to be updated to recommend all pregnant women receive a pertussis vaccine to ensure their babies are protected from the illness.

"There's really good data to show giving vaccines to women in pregnancy is safe and works at protecting the baby," she said. "Let's use the tools we have right now."

There have been outbreaks of pertussis in Canada in recent years, although not to the scale of what's been seen in California. For instance, a religious community in Ontario that was mainly unvaccinated against the illness experienced an outbreak beginning in 2011. It eventually spread to the vaccinated population, causing an additional 273 illnesses.

Whooping cough, a contagious bacterial infection of the lungs and airways, affects between 1,000 and 3,000 Canadians a year, according to the Public Health Agency of Canada. It's characterized by coughing fits that can cause a whooping sound as the individual affected tries to catch his or her breath. The illness is particularly dangerous for infants less than a year old because their immune systems are still developing and it can lead to serious complications and death.

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Across Canada, a series of pertussis shots are recommended at two, four, six and 18 months and four to six years of age. An adolescent booster is also recommended between 14 and 16 years of age. In 2011, a single adult pertussis booster was also introduced.

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