UPDATE 1-FDA staff flag reaction risk with Astra's RSV drug

* More hypersensitivity reaction with motavizumab vs Synagis

* Question over potential bias in clinical studies

(Adds details on drug, background)

LONDON, May 28 (Reuters) - AstraZeneca's (AZN.L) new infant lung drug motavizumab appears to carry a higher risk of causing severe allergic reactions than the company's older product Synagis, U.S. reviewers said on Friday.

In briefing documents prepared ahead of a June 2 advisory committee on motavizumab, Food and Drug Administration staff said they were concerned about a clear safety signal suggesting motavizumab had more significant hypersensitivity reactions.

AstraZeneca has developed motavizumab for preventing serious respiratory syncytial virus (RSV) in infants, but the drug has already suffered delays and some analysts have been sceptical about its commercial potential.

FDA staff said that while AstraZeneca contended motavizumab had comparable activity to Synagis, or palivizumab, clinical studies on the new product were potentially biased. (Reporting by Ben Hirschler, editing by Kate Kelland)