Is RSV vaccine safe during pregnancy?

What does the evidence say about RSV vaccine during pregnancy?

1. Abrysvo is a protein subunit vaccine that contains no live virus, and it prompts the body to make antibodies against RSV that then pass to the fetus. 1
2. It is recommended at 32 to 36 weeks of pregnancy, from September to January in most of the United States, and the transferred antibodies help shield the baby from severe RSV for about 6 months after birth. 1
3. Studies of the vaccine in pregnancy have not found a higher chance of birth defects, and ongoing studies have not found a higher chance of preterm birth at the recommended timing. 1
4. In the phase 3 MATISSE trial, efficacy against medically attended severe RSV lower respiratory tract illness in infants was 81.8 percent within 90 days of birth and 69.4 percent within 180 days. 2
5. Across 24 to 36 weeks the preterm birth rate was 5.7 percent with the vaccine versus 4.7 percent with placebo, narrowing to 4.2 versus 3.7 percent in the 32 to 36 week window; regulators judged the data insufficient to establish or exclude a causal link and required four postmarketing safety studies. 3

Is RSV vaccine safe in each trimester?

• First trimester. Not given now. The maternal RSV vaccine is timed for late pregnancy because the whole point is antibody transfer close to delivery, so there is nothing to decide in the first trimester.
• Second trimester. Still outside the window. Early trials did include 24 weeks onward, and it was the preterm birth imbalance in that wider range that led regulators to narrow the approved window later.
• Third trimester. This is the window: 32 to 36 weeks, and in most of the United States from September through January. It takes roughly two weeks after the dose for antibodies to transfer, which is why the timing is specific.