I came across this recently:
https://pubmed.ncbi.nlm.nih.gov/33849373/
I thought it would have been near 100%. It wouldn't have occurred to me to NOT go back and get the second shot. It seems doubtful to me that the two-dose Covid-19 shot regimen will attain the same completion rate as Shingrix.
My Shingrix interval was 3 months between doses. My Pfizer Covid-19 was 27 days (second delayed almost a week by record bad weather).
I had just assumed that anyone who decided to go get a specific vaccine would follow through with a second dose if it was part of the regimen. Unless they became deceased, bad accident, or a true serious reaction to the first dose, like anaphylactic shock. But then, I'm the type of person who if they were prescribed an antibiotic for an infection, completes the treatment as prescribed. Not one to stop taking them because "I felt better, so I stopped". So now I need to add "Always completes prescribed drug regimen" to my list for application for sainthood.