• Food and Drug Administration staff said Pfizer's vaccine that protects infants from respiratory syncytial virus has "generally favorable" safety data.
• The FDA staff made the conclusion ahead of a meeting on Thursday when an advisory panel will discuss whether to recommend full approval of the RSV shot.
• The agency is slated to make a decision on whether to clear the jab in August before RSV season in the fall. 
• Pfizer's shot is administered to expectant mothers.

U.S. Food and Drug Administration staff on Tuesday said Pfizer's vaccine that protects infants from respiratory syncytial virus has "generally favorable" safety data.

The FDA staff made the conclusion in briefing documents ahead of a meeting on Thursday when a panel of external advisors to the agency will discuss whether to recommend full approval of the RSV shot.

The advisors will vote on whether Pfizer's late-stage clinical trial data on the vaccine supports its safety and efficacy. The FDA typically follows the advice of its advisory committees, but is not required to do so.

The agency is slated to make a decision on whether to clear the shot in August before RSV season in the fall. If approved, Pfizer's jab would become the world's first vaccine that protects infants against RSV. 

RSV usually causes mild, cold-like symptoms. But infants and older adults are particularly vulnerable to having more severe cases.

Each year the virus kills 6,000 to 10,000 seniors and a few hundred children younger than 5, according to the Centers for Disease Control and Prevention.

The FDA earlier this month approved an RSV shot from GlaxoSmithKline for adults ages 60 and older.

The agency is expected to make a decision within weeks on Pfizer's other RSV shot for that same age group.

Pfizer's RSV vaccine that protects infants is administered to expectant mothers in the late second or third trimester of their pregnancy. 

The single-dose vaccine triggers antibodies that are passed to the fetus, which provides it with protection against RSV from birth through the first six months of life. 

Along with safety information, the staff also reviewed the vaccine's efficacy data.

Pfizer's trial found the shot was 82% effective at preventing severe disease from RSV in newborns during the first 90 days of life. The shot was about 70% effective during the first six months of the baby's life.

Most of the more than 3,000 mothers who received the shot in a phase three trial experienced mild to moderate adverse reactions, according to the FDA staff's review of data.

The most common reactions were fatigue, muscle pain, headache and pain at the injection site. Most reactions resolved within three to four days after vaccination, the staff review noted.

A higher number of premature births occurred among mothers who took the vaccine compared to those who received a placebo, according to the staff review.

But they said that difference does not appear to be statistically significant. 

Some of the participants' children also had low birth weights, the staff review said. 

The trial reported a total of 18 peripartum fetal deaths, which includes infants who survived briefly after birth and fetuses that died during pregnancy. 

But the FDA staff said it is unlikely that those deaths are related to Pfizer's shot.