Every year, health authorities come up with a flu vaccine that they think will be effective against the variation or variations of the flu that they believe will be dominant in that particular year. Most people simple assume that a “flu shot” will provide them with complete protection against “the flu”, but that is actually not true at all. In some years the match between the flu vaccine and the variations of the flu that runs around is pretty good, and some years that is definitely not the case. The following comes from the official CDC website…
During years when the flu vaccine is not well matched to circulating influenza viruses, it is possible that little or no benefit from flu vaccination may be observed. During years when there is a good match between the flu vaccine and circulating viruses, it is possible to measure substantial benefits from flu vaccination in terms of preventing flu illness and complications. However, even during years when the flu vaccine match is good, the benefits of flu vaccination will vary, depending on various factors like the characteristics of the person vaccinated, what influenza viruses are circulating that season and even, potential, which type of flu vaccine was used.
Influenza mutates every year. COVID mutats too, and scientists assure us that it continue doing so. Does that mean that the current (new) vaccines being completely useless against new mutant strains of the virus?
At this point, Pfizer and Moderna both seem convinced that their vaccines will be effective against the dominant strain of COVID that sweeps across the globe.
It indicates the legal terms to protect themselves when they talk about dominant.