NEW YORK — The nation’s infant mortality rate dropped last year after two years of hovering at a late-pandemic plateau.

Some experts think one reason for the drop could be a vaccination campaign against RSV, or respiratory syncytial virus, which is a common cause of cold-like symptoms that can be dangerous for infants.

The infant mortality national rate dropped to about 5.5 infant deaths per 1,000 live births in 2024, according to provisional data from the Centers for Disease Control and Prevention posted Thursday. That’s down from about 5.6 per 1,000 live births, where it had been the previous two years.

CDC officials believe the findings will not change much when the final numbers come out later this year.

Infant mortality is the measure of how many babies die before they reach their first birthday. Because the number of babies born in the U.S. varies from year to year, researchers instead calculate rates to better compare infant mortality over time.

U.S. infant deaths fell to about 19,900 last year, according to CDC data, compared with about 20,150 in 2023.

The U.S. infant mortality rate has been worse than other high-income countries, which experts have attributed to poverty, inadequate prenatal care and other things. Even so, the U.S. rate generally has improved over the decades because of medical advances and public health efforts.

The 2022 and 2023 levels were up from 5.44 per 1,000 in 2021 — the first statistically significant jump in the rate in about two decades.