Is there a recent randomized controlled trial covering the period where recent strains were dominant which I could reference in order to get an idea?

That is not a vaccine. No one calls corticosteroids or antibiotics vaccines yet both meet the definition of lowering the probability someone will dies from an infection. So does taking care of yourself, getting exercise, eating a healthy diet, not sitting still all day, and a large swath of other things none of which are called vaccines.

My google medical degree defines a vaccine as:

"a substance used to stimulate the production of antibodies and provide immunity against one or several diseases, prepared from the causative agent of a disease, its products, or a synthetic substitute, treated to act as an antigen without inducing the disease."

Steriods and antibodies don't meet this definition. No vaccine has a 100% success rate, so reducing symptoms doesn't make it not a vaccine.