Pfizer Inc said on Wednesday it had started a late-stage U.S. trial of an influenza vaccine involving 25,000 patients, among the first such studies for a messenger RNA flu shot.

The company said that the first participants had been dosed with the vaccine, which is based on the same technology used in its widely-used COVID-19 shot developed in partnership with Germanay's BioNTech SE.

Influenza causes 12,000 to 52,000 deaths in the United States every year, but the strains used in vaccines have to be changed annually ahead of the flu season as circulating viruses keep evolving.

Messenger RNA technology allows changing the vaccine strains relatively faster, and Pfizer expects this flexibility and its rapid manufacturing to potentially allow better strain matches in future years.

Currently, vaccines against flu such as CSL Seqirus' Fluad Quadrivalent or Flucelvax Quadrivalent are inactivated vaccines, where viruses are grown in a culture and then killed using certain chemicals, a process that requires months.

Early-stage data from Moderna Inc's flu vaccine last year disappointed investors after it showed the company's mRNA-based flu vaccine was no better than the already approved shots in the market. Moderna, though, also started a late-stage trial of its flu vaccine in June this year.