Pfizer's vaccine to protect newborns from respiratory syncytial virus, or RSV, by vaccinating their moms late in pregnancy cuts the risk that infants will need to see a doctor or be admitted to the hospital with a moderate to severe infection before 6 months of age, according to a new analysis by government regulators.

Many parents have been eagerly anticipating this news, particularly after last year's long and severe RSV season.

RSV is a major cause of hospitalization in infants and the elderly each year, and it typically hits hardest during the winter months.

Scientists have been working on an effective RSV for young children for roughly 60 years, so there's plenty of excitement around the prospect of having a candidate get so close to the finish line.

But the news isn't all rosy. Safety data published in an agency analysis Tuesday also showed a slightly higher proportion of preterm birth in babies whose moms got the experimental RSV vaccine compared with those who got a placebo: 5.7% vs. 4.7%, respectively.

The difference between the groups was not statistically significant, meaning it could have been due to chance. Rates of preterm births in both groups were lower than the rates of early birth reported in the general population, which is about 10%. And most infants, even when born prematurely, were delivered after 34 weeks of pregnancy, just a few weeks shy of their due dates.

Still, this finding will probably spur discussion this week at meeting of a group of independent experts who advise the US Food and Drug Administration on its vaccine decisions.

Late-stage trials of a similar maternal vaccine for RSV being tested by the drug company GSK were stopped last year over safety concerns.

GSK later reported that there was a higher risk of preterm births among vaccinated mothers in its study compared with those who got a placebo. There were 238 preterm births among 3,496 vaccinated moms (6.8%) compared with 86 among 1,739 pregnant women (4.9%) in the placebo group. There were also 13 neonatal deaths in the vaccine arm, compared with three in the placebo group, according to a recent report in the BMJ.

The FDA's Vaccine and Related Biological Products Advisory Committee, or VRBPAC, will meet Thursday and vote on whether to recommend approval for Pfizer's maternal RSV vaccine.

In terms of efficacy, FDA reviewers said the vaccine was most effective for the youngest infants, and its protection waned over time.

For the first three months after birth, the vaccine was 82% effective at preventing severe RSV disease and 57% effective at keeping babies from needing to see the doctor because of an RSV infection. By six months after birth, the vaccine was 69% effective at preventing severe RSV disease and 51% effective at preventing a doctor's visit for RSV-related breathing problems.

After six months, the vaccine was about as effective as the placebo at preventing RSV that became severe enough to send babies to the doctor's office.

For the most part, the vaccine appeared to be safe, with reactions and serious adverse events balanced between the vaccine and placebo groups.

There were 17 deaths in the main study: five in the group that got the vaccine and 12 in the group whose moms got a placebo.

The FDA agreed with Pfizer that four of the five deaths in the immunized group were probably unrelated to the vaccine. The agency said it couldn't exclude the possibility that one death, in a baby born very prematurely 10 days after its mother was vaccinated, was related to the vaccine.

One infant in the placebo group died of an RSV infection.

Pfizer's RSV vaccine is made with a key piece of the virus: its F proteins, the part of the virus that docks onto and infects cells. The vaccine uses F proteins from the two major subgroups of the virus, A and B, making it a bivalent vaccine.

The vaccine tested in pregnant women is the same dose and structure as the one the company is testing for adults 60 and older.

The FDA has reviewed Pfizer's vaccine for seniors and is due to make a decision on whether to approve it by the end of the month.

If it is approved, it will become the second RSV targeted at older adults. The agency recently approved an RSV vaccine for seniors made by GSK.