Background: The mRNA COVID vaccines are only licensed for intramuscular injection but it is unclear whether successful intramuscular administration is required for immunogenicity.
Methods: In this observational study, eligible adults receiving their first Comirnaty/BNT162b2 dose had their skin to deltoid muscle distance (SDMD) measured by ultrasound. The relationship between SDMD and height, weight, body mass index, and arm circumference was assessed.