(Reuters) -Novavax ( NVAX ) shares surged more than 17% before the bell on Monday, following the long-delayed approval of its COVID-19 vaccine from the U.S. health regulator, albeit with new conditions.

The U.S. Food and Drug Administration approved the vaccine, Nuvaxovid, but limited its use to older adults and people over 12 years with conditions that put them at risk due to the illness.

The conditions on the label are medically unusual but consistent with the chatter to date, said BTIG analyst Thomas Shrader.

Analysts at Leerink Partners echoed the view. The approval is in line with expectations and "no more restrictive than we expected" from the Centers for Disease Control and Prevention's vaccine advisory committee, ACIP, they said.

The vaccine's future was called into question after the FDA missed its April 1 deadline to approve the shot - a more traditional protein-based vaccine unlike its messenger RNA-based rivals.

U.S. Health and Human Services Secretary Robert F. Kennedy Jr., a longtime vaccine skeptic, also expressed concerns about the vaccine's efficacy in a CBS interview.

Novavax ( NVAX ) missed out on the pandemic vaccine windfall - enjoyed by rivals Moderna ( MRNA ) and Pfizer ( PFE ), which make messenger RNA-based vaccines - due to manufacturing issues and regulatory hurdles.

The FDA's full approval of Nuvaxovid triggered a $175 million milestone payment to Novavax ( NVAX ) from Sanofi, related to their licensing deal in 2024.

Novavax ( NVAX ) expects to be ready for the commercial delivery of the shot in the U.S. this fall in partnership with Sanofi, pending strain recommendation.

Novavax ( NVAX ), along with its competitors, will need to seek additional approval to alter the strain of the virus its vaccine targets for the upcoming COVID-19 immunization season. This is set to be discussed at a meeting of FDA vaccine advisers later this week.

Through last close, Novavax ( NVAX ) shares have fallen 16.3% so far this year.