The standard length needle might not reach the muscle of some people, such as those with big arms.

We wanted to assess if the immune response to the COVID vaccine was lower in the small number of people who have a skin to muscle distance, measured by ultrasound, that was longer than the needle used for their vaccination.

The antibody response was the same in people who were vaccinated with needles that were probably too short to reach the muscle and those vaccinated with appropriate length needles.

This is reassuring and means that injection into the muscle may not be needed for the vaccine to produce a good immune response.