June 8 (Reuters) - The U.S. Food and Drug Administration advisers on Thursday backed the use of Sanofi and partner AstraZeneca's experimental antibody to prevent respiratory syncytial virus (RSV) infections in infants.

The advisers voted unanimously in favor of using the antibody nirsevimab in newborns and infants to prevent infections in their first RSV season and in a 19-2 vote backed its use in children aged up to two years who are vulnerable to severe illness through their second RSV season.

Swedish Orphan Biovitrum's treatment Synagis is currently the only preventive therapy against RSV in infants.

Unlike Synagis, which is approved for high-risk infants and has to be given as monthly injections, Sanofi-AstraZeneca's nirsevimab is a long-acting therapy that is expected to prevent the RSV infection regardless of additional medical conditions in infants.

The FDA, which is not obligated to follow the recommendations of the advisory panel but usually does, is expected to make a decision on the therapy by the third quarter of this year. (Reporting by Mariam Sunny and Raghav Mahobe in Bengaluru; Editing by Maju Samuel)