Add to my previous discussion two recent articles on the flu vaccine:

Should I Get the High-Dose Flu Vaccine?

How to Turbocharge Flu Protection

Also, re post #50359:
"... the high-dose (= double-dose) versions are only available as trivalent (= containing hemagglutinin of 3 strains of the influenza viruses) vs the standard flu vaccines, which are now quadrivalent (4 strains, this season 2 influenza A and 2 influenza B strains)."
Correction:
The high-dose version contains 4 times the antigen of the regular-dose version: 60 µg vs 15 µg of hemagglutinin of each strain.