Safety data for Pfizer Inc's experimental respiratory syncytial virus (RSV) vaccine in pregnant women was "generally favorable," the U.S. Food and Drug Administration staff reviewers said on Tuesday.

The drugmaker is seeking approval for use of the shot in pregnant women to protect newborns to six-month-old babies from complications of the RSV disease. If approved, it could become the first maternal vaccine against the virus.

While a higher number of pre-term births occurred among participants taking Pfizer's vaccine compared to those in the placebo group, the FDA staff, in the briefing documents, said the difference did not appear to be statistically significant.

Comments by the staff reviewers come ahead of a meeting of the FDA's outside advisers on Thursday, where they will provide non-binding recommendations to the health regulator.

If approved by the FDA, the U.S. Centers for Disease Control and Prevention will then need to sign off on the use of the vaccine to make it widely available to pregnant mothers.

A late-stage study of Pfizer's vaccine showed it was nearly 82% effective in preventing severe lower respiratory tract illness due to RSV, a leading cause of pneumonia in infants, as well as the elderly.

The FDA staff also said the safety and efficacy findings in the vaccine's development program could be applicable to trans-males who may be pregnant.

Development of effective RSV shots has picked up pace in recent years after researchers were able to better understand the virus' structure and then stabilize a type of protein found on its surface that could activate the body's immune response.

Rival GSK recently became the first company to receive U.S. approval for its RSV vaccine for older adults. Pfizer is expecting a decision for its RSV shot for the elderly later this month.

Sanofi and AstraZeneca's single-dose antibody nirsevimab, already approved by the European Commission, is being reviewed by the FDA for prevention of RSV in infants.