New research shows vaccinating pregnant women helped protect their newborns from the common but scary respiratory virus called RSV that fills hospitals with wheezing babies each fall.

The preliminary results buoy hope that after decades of failure and frustration, vaccines against RSV may finally be getting close.

Pfizer announced Tuesday that a large international study found vaccinating moms-to-be was nearly 82% effective at preventing severe cases of RSV in their babies' most vulnerable first 90 days of life. At age 6 months, the vaccine still was proving 69% effective against serious illne...