Difficulty in getting appointments

[M_T]

Studies indicate that a single dose of Pfizer or Moderna provides more protection than Johnson & Johnson.

The Lancet

Actually, the 80% effectiveness of the mRNA first doses is only known for the 1 to 2 weeks from 2 weeks after the first dose until the 2nd dose is given.  The 67% figure for J&J is from 2 weeks after the shot until end of study.  Whether the first dose of the mRNA first doses continues to be effective or fades is not known.  (I would have thought it like lasts for at least 4 months.)