PARIS (Reuters) -French biotech firm Valneva said on Friday no conclusions should be drawn on the effectiveness of its COVID-19 vaccine by a British study, which found it was the only shot out of seven that offered no immunity boost when given to people previously immunized with Pfizer's vaccine.
Shares in Valneva fell by up to 24% on Friday following the publication of results of Britain's COV-Boost study, which looked at the effectiveness of alternative vaccines as boosters for people who previously received Pfizer or AstraZeneca shots.
All seven vaccines given in the COV-Boost study increased immunity when given as booster shots to people who had previously received two doses of AstraZeneca's vaccine.
The other six - but not Valneva's VLA2001 - were also found to increase immunity when given to people who had been vaccinated with Pfizer.
Valneva said participants in the study had been given their booster doses after a shorter interval than usual, and that vaccines made from inactivated viruses, such as its candidate, typically require a longer interval to be effective.
"The setting in this study leads us to believe that COV-Boost does not allow any conclusions to be reached regarding the use of VLA2001 as a booster in a real-life setting," it said in an e-mailed statement in response to a Reuters query.
"Valneva believes it is likely that the short interval between the second shot and booster shot could have adversely impacted the results for VLA2001, given that a longer interval is generally required for inactivated vaccines."
The European Union's drug regulator said on Thursday it had started a rolling review of Valneva's vaccine - which could speed up approval of the shot - weeks after the EU signed a supply deal with the company.
Unlike shots by Pfizer-BioNTech, AstraZeneca, Johnson & Johnson and Novavax, the Valneva shot exposes the immune system to the entire coronavirus, not just the spike protein.
Experts hope that could potentially make the vaccine less susceptible to losing its effectiveness against variants that have mutations on the spike protein.
Valneva has said it is hopeful its vaccine candidate would protect people against variants of the virus, adding it would test it specifically against Omicron.
(Reporting by Silvia AloisiEditing by Tassilo Hummel and Peter Graff)