Dr. Michelle Barton worried when measles found a foothold in southwestern Ontario last fall.
The infectious disease specialist knew childhood vaccination rates hadn鈥檛 bounced back to pre-pandemic levels聽in Ontario, leaving some kids exposed to the virus.
There were also communities with high numbers of unvaccinated people near her hospital in London that were especially vulnerable to the extremely infectious disease. Anyone who wasn鈥檛 immune would likely get measles.
鈥淚t鈥檚 like a forest fire,鈥 said Barton, division head of Paediatric Infectious Diseases at Children鈥檚 Hospital, London Health Sciences Centre. 鈥淚t will continue to burn until all the trees are gone.鈥
At first, many of those sick enough to need hospital care were infants and young children.
As the outbreak spread, cases swelling from dozens to hundreds to now more than 1,600 provincewide, a new group of measles patients has emerged聽鈥 one that few doctors in Canada have treated before.
Pregnant people were getting sick with measles. And Barton, working at the outbreak鈥檚 epicentre, raced to figure out how to care for them and their babies.
鈥淏ecause we’re now talking about not just one patient, but two or even three,鈥 she said. 鈥淢easles is hitting the adult mother, and it鈥檚 hitting her baby in her womb.鈥
Ontario鈥檚 measles outbreak, the largest in Canada in almost 30 years, started last October and has since sickened hundreds of people, the majority unvaccinated infants, children and teens. As of May 13, the outbreak has led to at least 1,622 cases, including 119 who have been hospitalized, according to . Of those hospitalized, 89 were unimmunized children.
Though kids have borne the brunt, adults are also getting sick. At least 377 people 20 and older, about two-thirds of whom were not vaccinated, have now been infected with what is typically thought of as a childhood disease.聽
Before routine immunization programs slashed measles cases in Canada, few people would get to child-bearing age without natural immunity. Now, in the post-vaccine era, people who did not get vaccinated as children have grown up to become 鈥渧ulnerable hosts,鈥澛爏aid Barton.
鈥淚n a full-blown outbreak, with measles circulating in their community, (unvaccinated) pregnant women are at risk.鈥
Studies from other countries show unvaccinated pregnant women are more likely to be hospitalized, develop pneumonia and die from measles than those who are not pregnant. Measles also threatens the fetus or unborn baby. An infection during pregnancy can lead to miscarriage, premature birth and cause low birth weight.
鈥淚f there’s one group of people you really need to avoid getting serious infections, it’s pregnant women,鈥 said Dawn Bowdish, an immunologist and professor of medicine at McMaster University.
In mid-March, Public Health Ontario started tracking the number of pregnant people infected with measles during the outbreak. Seven such cases at the time have grown to 34 in two months.
In response to a request by the Star, the agency provided figures for the number of pregnant cases admitted to hospital between Oct. 18, 2024, and April 29.
During that period, the province recorded 25 pregnant measles cases. Among those, six were hospitalized, including one admitted to the ICU. Five of those cases were unimmunized, while one case had received two doses of measles-containing vaccine.
Some of the pregnant people infected with measles are likely part of specific populations that historically haven鈥檛 been vaccinated, Bowdish said. Health officials have confirmed Mennonite, Amish and other communities with low immunization rates have the highest rates of disease in this outbreak.
Unvaccinated pregnant women may also be part of a cohort of young adults whose parents refused childhood immunizations following a now-invalidated study that falsely linked the mumps-measles-rubella (MMR) vaccine to autism, Bowdish said. Research shows the study, published in 1998 and since retracted, was associated with a decline in uptake of the MMR vaccine.
鈥淭here’s a worry that there are quite a few people (of child-bearing age) who weren鈥檛 vaccinated or who don’t know their vaccine status, because you can鈥檛 remember what Mom and Dad did when you were a child,鈥 Bowdish said.
She noted people who are thinking of starting a family don鈥檛 typically check their MMR vaccination status. Up until this outbreak, there was a low risk of getting a measles infection in Ontario.
鈥淢y big concern is that we鈥檙e in this terrible situation, where we know people aren鈥檛 vaccinated, where we鈥檙e seeing measles spread into more and more communities, leading to more and more opportunities for pregnant people to get infected,鈥 said Bowdish.
Vaccinated individuals with compromised immune systems are also at risk of infection during pregnancy, said Barton.
鈥淭he new group that we’re seeing ...聽is women who have now come to their adult years and were never vaccinated,鈥 she said. 鈥淭his is distinctly different from the pre-vaccine era where those women would have acquired natural immunity in childhood.鈥
Dr. Darine El-Chaar, a maternal-fetal medicine specialist at The Ottawa Hospital, said changes in a person鈥檚 immune system during pregnancy make them generally more susceptible to infections, which is why it鈥檚 important to update vaccines before starting a family.
Once pregnant, individuals aren鈥檛 generally immunized against measles out of an abundance of caution because the shot, which is an attenuated (weakened) live virus vaccine, poses a theoretical risk to a fetus, El-Chaar said.
Unimmunized pregnant people who get exposed to measles are offered immunoglobulin (Ig), which consists of antibodies made from donated human blood and offers protection from infection, or reduces disease severity. Ig must be giving as early as possible after exposure and within a six-day window.
El-Chaar, a subject-matter expert for the Society of Obstetricians and Gynaecologists of Canada, said keeping up strong herd immunity is critical.
鈥淚 wish people would understand that when we refuse vaccines when we聽can take them, we’re making it worse for those who can’t be vaccinated,鈥 she said.
At the start of the outbreak, Barton said she scoured the medical literature for clues on how measles would affect pregnant patients and their unborn babies. Though there are few recent reports, Barton said a U.S. study from 1993 found聽that about 60 per cent of unimmunized pregnant women with measles needed hospital care, with 27 per cent getting sick with pneumonia.
Since measles increases the risk of pregnancy loss and premature birth, Barton said she looked for guidance on these complications, too. One 2014 study from Namibia showed that 30 per cent of unvaccinated pregnant women exposed to the virus would go into labour within seven days, and 50 per cent would deliver within three weeks.聽 聽
Barton, who confirmed London Health Sciences Centre was managing the bulk of complicated cases in the region, would not reveal how many pregnant measles patients and their babies the hospital has cared for during the outbreak due to patient privacy concerns. Instead, she stressed how hard she and her colleagues are working to care for these聽vulnerable patients.
鈥淲e’re still at the height of things, but we’re determined to find the answers.鈥
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