Why vaccines are not injected in our veins but in our upper arm muscle

Around the world, more than 17 million sleeves have rolled up for the first COVID-19 vaccinations, and Australia will start its program next month.

Like most adult jabs, this slew of vaccines — including those developed by Oxford University and AstraZeneca and Pfizer and BioNTech — are injected into the deltoid: the thick, fleshy muscle of your upper arm.

There are a few good reasons to favour muscle injections over other routes.

The strength of muscle

First up: unlike the layer of fat just under our skin, muscle has an excellent blood supply to help disperse the vaccine, says Joanna Groom, an immunology researcher at the Walter and Eliza Hall Institute.

Longer — and safer — training sessions

Muscle is a bit of a Goldilocks tissue for doling out vaccines to our immune cells: not too slow, but not too fast either.

This allows for an extended immune system training session, Dr Groom says, which “is thought to result in maximal activation of the immune system”.

A vaccine injected directly into the bloodstream, on the other hand, is vulnerable to destruction.

Dr Groom says while common side effects of intramuscular vaccination may be a literal pain for a few days, they’re a sign that your immune system is doing what it should.

“There are actually some side effects from vaccinations that you want, like you want your arm to get a little bit red, or have your lymph nodes swell a bit — and that’s actually your immune system working.”