The infectious illness can cause a violent and uncontrollable cough.
Last week, the family of one-month-old Harper Whitehead, who died of whooping cough, pleaded with Albertans to get their children immunized.
In the U.S., the Centers for Disease Control and Prevention in Atlanta called this the worst year for whooping cough in five decades.
Mostly, the outbreaks are from the cycles of disease, said Dr. Allison McGeer, director of infection control at Toronto's Mount Sinai Hospital.
"All diseases have cycles and we're coming up to a peak in pertussis," McGeer said.
Whooping cough, also called pertussis, peaks at two- to five-year intervals, according to the Public Health Agency of Canada.
"There's another piece of it," McGeer said. "About 30 years ago, we went to a less strong vaccine for children to try to reduce the number of fevers and other adverse events with the vaccine. Turns out that the vaccine worked really well for kids, but it's wearing off in adulthood and so we now have a cohort of young adults who aren't as well-protected as we'd like them to be."
In young adults, whooping cough typically isn't dangerous, but occasionally it is, she noted.
Booster shots for young adults
The issue with waning immunity arises when adults pass on the infection to infants and young children at home because they can get seriously ill, McGeer said. Infants under the age of three months are too young to be fully vaccinated.
"I think what we're seeing is the direct result of people not getting the vaccine," Health Minister Leona Aglukkaq said in an interview from Washington. "I would encourage parents to go out and get the vaccine for themselves as well as their children and their families."
Like other upper respiratory infections, whooping cough starts with a stuffy, runny nose, a bit of a sore throat, aches and sometimes a fever. It takes a few days for the cough to start, McGeer said.
"It's not until days later when the cough isn't gone and is getting worse and is keeping you awake night after night that somebody thinks, 'Oh, gee maybe this isn't just some virus. Maybe this is whooping cough.'"
As of July 17, 1,024 confirmed cases of pertussis have been reported in New Brunswick, which is experiencing an outbreak, PHAC said.
Public Health Ontario reported 170 cases between January and the end of April, a rise from 29 cases during the same time period last year and in 2011.
"The continued increase can be attributed in part to an ongoing outbreak in seven southwestern Ontario health units which includes cases with symptom onset since November 2011," PHAC said in email.
Last month, the Middlesex-London health unit said London, Ont., hasn't noted a significant increase in cases but nearby regions of southwestern Ontario have.
"In Ontario, cases were initially found among unvaccinated groups, but there has recently been spread outside of these groups."
A different type of vaccine for those aged 15 to 19 was introduced in 2004. Since then, the incidence in that age group has decreased from 18.7 cases per 100,000 in 2003 to 2.8 cases per 100,000 in 2009, the federal agency said.