July 8 (Reuters) - Pfizer's ( PFE ) respiratory

syncytial virus (RSV) vaccine for pregnant women was not tied to

a higher risk of pre-term or early births, according to a study

published on Monday that analyzed real-world use of the shots.

The vaccine Abrysvo was approved by the U.S. Food and Drug

Administration last year for women in their second or third

trimesters of pregnancy to prevent infections tied to RSV in

babies in the first six months after birth.

However, some experts had raised concerns over a higher

number of pre-term births among participants taking the Pfizer ( PFE )

shot compared to those who received a placebo in the company's

clinical trial.

Researchers from Weill Cornell Medical College in New York

conducted the retrospective study in women who were 24 to 36

weeks into pregnancy, and found no significant statistical

difference between the vaccinated women's pre-term birth rate of

5.9% compared with unvaccinated women's 6.7%.

Health regulators however recommended a narrower gestational

window, said Annette Regan, from the University of San

Francisco's School of Nursing and Health Professions, in an

editorial accompanying the study.

"Despite the need for further research and surveillance,

results from this study should offer initial reassurance to

regulators, policymakers, health care professionals, and

pregnant patients," Regan wrote in the article published in

JAMA.

In most of the continental U.S., the Centers for Disease

Control and Prevention (CDC) and the Advisory Committee on

Immunization Practices (ACIP) recommend the vaccine for women

who are 32 to 36 weeks pregnant between September and January.

The study analyzed electronic health records of women across

two New York City hospitals who gave birth between September

last year to late January.

RSV leads to the hospitalization of about 58,000 to 80,000

children younger than 5 years annually.

Pfizer's ( PFE ) vaccine is the only maternal RSV shot approved in

the US. The vaccine, along with rivals by GSK and

Moderna ( MRNA ), is also approved for older adults.