UPDATE 3-Sanofi-AstraZeneca's preventive RSV therapy appears safe- FDA staff

(Adds RSV market size estimate in paragraph 5, and analyst estimate in paragraph 6)

June 6 (Reuters) - The U.S. Food and Drug Administration's staff reviewers said on Tuesday Sanofi and partner AstraZeneca's experimental therapy to prevent respiratory syncytial virus (RSV) infections in infants appeared safe and effective.

The RSV prevention antibody, nirsevimab, showed an overall favorable safety profile in clinical trials, the FDA staff said in documents published ahead of the health regulator's advisory committee meeting on Thursday to discuss the therapy's marketing application.

The companies are seeking the FDA's approval for use of the therapy in infants and children up to 24 months of age to protect them against lower respiratory tract disease that could cause pneumonia and bronchiolitis.

The FDA staff said there was an "imbalance in deaths", 12 deaths among infants who received nirsevimab compared to 4 among those on a placebo or an older antibody drug, but did not consider the deaths to be related to the therapy.

A potential approval in the U.S. would expand the companies' access to an RSV market that is estimated to cross $10 billion by 2030. Nirsevimab was approved by the European Commission in November last year.

Morningstar analyst Damien Conover estimated peak sales for the therapy would be close to $2 billion.

In a late-stage study, Sanofi-AstraZeneca's therapy showed a 74.5% reduction in incidence of lower respiratory tract infections that require medical care in infants.

While RSV generally causes a mild illness, infants are among those at the greatest risk of severe disease from the highly contagious virus.

One to two out of every 100 babies below six months of age with RSV infection may need to be hospitalized, according to government data.

Two studies showed the therapy was effective in preventing lower respiratory tract infections, but benefits and risks for babies older than eight months may need additional consideration given limited enrollment from that age group in the trials, the FDA staff said.

(Reporting by Raghav Mahobe and Mariam Sunny in Bengaluru; Editing by Krishna Chandra Eluri, Shounak Dasgupta and Shinjini Ganguli)