FDA Approves Abrysvo Vaccine During Pregnancy to Protect Babies

In clinical trials, Pfizer's new maternal vaccine significantly reduced the risk of RSV in infants. However, the vaccine's prescribing information warns of potential risks.

On August 21, the FDA approved Abrysvo, a respiratory syncytial virus (RSV) vaccine designed for use in pregnant individuals at 32 through 36 weeks gestation to prevent RSV in infants from birth to six months of age.

The FDA approved the vaccine based on two clinical studies. In one clinical trial, funded by Pfizer, about 3,500 pregnant individuals received the vaccine, and 3,500 were given a placebo. The results showed that the vaccine reduced the risk of RSV-related severe lower respiratory disease in infants by 81.8% within 90 days and 69.4% within 180 days after birth.

In a subgroup of approximately 3000 participants who were 32 through 36 weeks pregnant, half received the maternal RSV vaccine, and half received a placebo. In this group, the vaccine reduced the risk of RSV-related lower respiratory disease by 34.7%. It reduced the risk of severe disease by 91.1% in infants within 90 days after birth compared to the placebo.

In addition, Pfizer's maternal RSV vaccine reduced the risk of lower respiratory disease by 57.3% and severe forms of the illness by 76.5% within 180 days after birth.

Though most RSV infections are mild, according to the CDC, an estimated 58,000 to 80,000 children under five years are hospitalized due to the virus. The hope is that the new vaccine will reduce the incidence of the disease among those most at risk.

In an FDA news release, Peter Marks, M.D., Ph.D., director of the FDA's Center for Biologics Evaluation and Research, said, "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. This approval provides an option for healthcare providers and pregnant individuals to protect infants from this potentially life-threatening disease."

Is the vaccine safe?

Due to the urgent need to prevent RSV in infants, the FDA gave Pfizer's new maternal RSV vaccine Priority Review status and Fast Track and Breakthrough designations before the agency approved it on August 21.

However, earlier this year, experts voiced concerns about the safety profile of a similar maternal RSV vaccine developed by GSK, a biotech company. During clinical trials, data showed potential links between the GSK vaccine and an increased risk of preterm birth.

According to the FDA, reported side effects of Pfizer's maternal RSV vaccine included headache, injection site pain, muscle pain, and nausea. However, pre-eclampsia occurred in 1.8% of pregnant individuals who received Abrysvo versus 1.4% of those receiving a placebo.

In addition, infants born to pregnant individuals who received Pfizer's maternal RSV vaccine had a higher rate of low birth weight and jaundice.

The agency also says Abrysvo's prescribing information includes a warning about the potential risks of preterm birth. The trial data showed that 5.7% of recipients experienced preterm birth versus 4.7% receiving the placebo.

The warning specifically advises healthcare providers to administer the RSV vaccine to individuals 32 through 36 weeks pregnant to avoid the potential risks of premature birth associated with the use of the vaccine before 32 weeks gestation.

The FDA notes that clinical studies did not include pregnant individuals with an increased risk of premature birth. The agency also says the available data is insufficient to establish a causal link between preterm birth and the new maternal vaccine.

Moving forward, the FDA is requiring Pfizer to conduct studies after pregnant individuals begin receiving the vaccine to evaluate the risk of preterm birth and pregnancy-related hypertensive disorders, including pre-eclampsia, signaled in the clinical trials.

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