COVID-19 caused an alarming surge in premature births, but vaccines were key to returning the early birth rate to pre-pandemic levels, according to a new analysis of California birth records.

“The effect of maternal COVID infection from the onset of the pandemic into 2023 is large, increasing the risk of preterm births over that time by 1.2 percentage points,” says Jenna Nobles, a University of Wisconsin–Madison sociology professor. “To move the needle on preterm birth that much is akin to a disastrous environmental exposure, like weeks of breathing intense wildfire smoke.”