The needle on the syringe is also available in different sizes, as they come in different lengths and gauges. The length of the needle determines how deeply it penetrates.

Syringe and needle sizes and lengths. The International Scientific Advisory Board for the Third Injection Technique Workshop states that there is no medical reason to use a needle longer than 8 mm.

But different bodies need different length needles, and needle length can affect how well a vaccine ends up working in your body. If you are about to go get this year’s COVID vaccine, or really ...