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 in the Canadian Medical Association Journal, 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 old, you’ve got very little protection left.”
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. The odds of contracting whooping cough following vaccination increased by 27 per cent each year, although the overall risk was still small, the researchers found.
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 in 2012.
Most cases occur in under-immunized populations, often among those in which parents oppose vaccination, usually on philosophical or religious grounds.
The current whooping cough vaccine, known as an “acellular” vaccine, has been used in Canada since 1997 and is also used in the rest of North America, Australia, New Zealand and much of Europe. The whole-cell vaccine was discontinued in North America because of 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 newer 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 authors suggest that the province’s current pertussis immunization strategy needs to be changed. Potential strategies to improve pertussis control include immunizing women during pregnancy, reintroducing the whole-cell vaccine for infants, or introducing additional acellular vaccine boosters for children and adults.
However, the ultimate goal should be a new, more effective and longer-lasting vaccine against the disease, they say.
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