The Food and Drug Administration (FDA) on Monday approved the first vaccine meant to prevent infections in infants, with the new drug designed to be administered to pregnant mothers in their third trimester so their newborn infants can have protection from the moment they’re born.

The single-dose injections called Abrysvo is made by Pfizer and has been approved for use between 32 through 36 weeks of pregnancy. The FDA’s Vaccines and Related Biological Products Advisory Committee voted in favor of recommending the drug in May.

Abrysvo has already been approved for use in preventing respiratory syncytial virus (RSV) in older adults.

“RSV is a common cause of illness in children, and infants are among those at highest risk for severe disease, which can lead to hospitalization,” Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said in a statement.

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“This approval provides an option for healthcare providers and pregnant individuals to protect infants from this potentially life-threatening disease,” Marks said.

When advisory committee reviewed Abrysvo in May, Pfizer presented data that found neutralizing antibodies against RSV were higher in infants whose mothers received the vaccine through 6 months of age. Infants under the age of 6 months are at a particularly high risk of developing severe cases of RSV.

The FDA noted in its announcement Monday that in one clincial study Abrysvo reduced the risk of lower respiratory tract disease by about 82 percent in the first 90 days after birth and about 70 percent in the first 180 days.

“We are incredibly grateful to the clinical trial participants and study investigator teams around the world, as well as our Pfizer colleagues, for their commitment to making this vaccine available,” Pfizer Senior Vice President Annaliesa Anderson said in a statement.

“Today, a long-sought-after goal to deliver a maternal vaccine that will help protect infants six months of age or younger – when they are at greatest risk of possible serious consequences from RSV – has been achieved.”

Like with other respiratory viruses, RSV cases usually start to rise in the fall, typically peaking in the winter.

High numbers of infants and toddlers were hospitalized last winter due to RSV, with many hospitals reaching capacity and some running low on ventilators as facilities also sought to take care of patients with COVID-19 and the flu.

The majority of children will be exposed to RSV — known as a common “daycare disease” — before the age of 2. COVID-19 quarantines are thought to have possibly limited exposure, contributing to a harsher RSV season, though clinicians have noted that higher than normal case rates occurred even before the pandemic.

Apart from a monoclonal antibody given to extremely high-risk cases, there were no effective treatments specifically geared toward RSV until this year. Two RSV vaccines for older adults were approved by the FDA this year and a preventive monoclonal antibody against the virus was approved for infants and toddlers last month.

When the FDA approved Abrysvo for older adults earlier this year, Pfizer said it anticipated the drug to be available some time during the third quarter of this year, before the expected RSV season.

The most common side effects that were reported were injection site pain, headache, muscle pain and nausea. One issue that came up during the FDA panel’s review of the drug was the observation that preterm births were numerically higher in the group that received the vaccine compared to the placebo group, though this difference was not found to be “statistically meaningful.”

Preeclampsia also occurred at a slightly higher rate among those who received the vaccine in studies and the FDA noted that “birth weight and jaundice in infants” occurred at a higher rate among the vaccine recipients.

The FDA said Monday that the available data was “insufficient to establish or exclude a causal relationship between preterm birth and Abrysvo.” The agency said it will be requiring Pfizer to conduct postmarketing studies to evaluate the serious risk of preterm birth and other conditions like preeclampsia.

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