Two Patients, One Shot: Vaccines in Pregnancy

A newborn is too young for most vaccines. For the first months of life, the best protection a baby has against whooping cough, RSV and flu comes from antibodies made by the mother during pregnancy and passed across the placenta. That is why these vaccines matter — and why anti-vaccine campaigners work so hard to frighten pregnant women away from them.

1. How it works

When a pregnant woman is vaccinated, her immune system makes antibodies, and in the last weeks of pregnancy those antibodies cross the placenta to the baby. The baby is born already protected, for the first two to six months, against diseases it cannot yet be vaccinated against. Whooping cough is most dangerous, and most often fatal, in babies under three months — before the first infant dose at two months has had time to work.

2. Which vaccines, and when

3. Whooping cough is back

In 2024 Canada had its worst whooping cough year in decades. The Public Health Agency of Canada reported more than 19,000 cases in Quebec and Ontario alone, which it called a tenfold increase over the 2015–2023 national annual average. New Brunswick declared an outbreak, and Prince Edward Island and Newfoundland and Labrador passed their pre-pandemic peaks (Global News, Sept 2024). Waterloo Region saw its own surge.

Whooping cough comes in waves every few years, protection from the current vaccine fades, and many children missed doses during the pandemic. All of that makes vaccination in pregnancy more important, not less: the mother’s antibodies protect the baby during exactly the weeks when whooping cough is most likely to kill. In Ontario, researchers studied infant whooping cough before and after NACI’s 2018 recommendation (population-based study), and countries that adopted vaccination in pregnancy earlier, including the United Kingdom, reported that it prevented the large majority of infant cases.

4. RSV: a new choice for parents

“We had huge numbers of children with RSV at almost every hospital where I trained and worked,” Dr. Polevoy recalls in Before the Vaccines. Ontario now offers two ways to protect a newborn (Ontario Ministry of Health, 2025–26):

• Vaccinate the mother with Abrysvo at 32 to 36 weeks, if the baby will be born during the RSV season; or
• Give the baby an antibody, Beyfortus (nirsevimab), after birth. NACI prefers this option, and in its first season it prevented 97% of RSV admissions to intensive care in a study at Ontario and Quebec children’s hospitals.

Most babies need only one or the other. Talk it over with your doctor or midwife before 32 weeks.

5. The fear campaign

Pregnant women are the anti-vaccine movement’s most effective target, because the fear is so easy to plant and the harm, when a baby gets whooping cough or measles, falls on someone who never had a say.

• “82% miscarriages.” The most widely shared claim came from a single arithmetic trick: dividing miscarriages by the wrong group of women. Proper analysis showed no increase. See The 82% Miscarriage Claim.
• “It hasn’t been tested in pregnancy.” Tdap and flu vaccines have been given to millions of pregnant women for years, with safety monitored in Canada, the United States and Europe.
• “Natural immunity is better.” A newborn has no immunity of its own to whooping cough, and catching measles in pregnancy is exactly what harmed the six Ontario babies born with it in 2024–25.

6. Meanwhile, in the United States

In May 2025 the U.S. Health Secretary, Robert F. Kennedy Jr., withdrew the federal recommendation of COVID-19 vaccination for healthy pregnant women. In September 2026 his department rewrote the CDC’s COVID-19 guidance, replacing its advice for pregnancy with package-insert language, and in October the CDC’s public pregnancy page began redirecting to that clinicians’ guidance (CIDRAP). Canada’s advice has not changed: NACI still recommends these vaccines in pregnancy. Canadians who search online will meet the American changes first, so it is worth knowing they do not apply here. See Rewriting the Schedule.

7. What to do