The US Food and Drug Administration has delayed a decision on whether to approve Merck & Co's new vaccine for shingles, Zostavax, according to the company.

Merck said that the agency has moved back the deadline for its decision on Zostavax by three months to May 25 in order to review additional information that the company has submitted to support its application.

Last December 2005, an FDA advisory committee said that while Merck's data supported approving the Zostavax vaccine for patients aged over 60, the data were not compelling enough to back approval in those aged 50 to 59. However, only about 150 of the 38,000 patients in Merck's Zostavax study were in the 50-59 year age range, so a restricted label may not be too damaging for the vaccine’s commercial prospects.

Merck originally filed its application for approval on April 25, 2005. Generally, the FDA makes a decision within 10 months of the original filing date.

Zostavax is one of three Merck vaccines being considered for approval in the USA this year. Earlier this month, the FDA approved the rotavirus vaccine Rotateq and granted an expedited review of Gardasil, a vaccine against human papillomavirus, which causes cervical cancer.