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.
Despite using a raft of different technologies, COVID-19 vaccines all aim to do the same thing: introduce our immune system to antigens — specific parts of a disease-causing organism which the body uses to identify the invader — to shore up defences against the disease down the track.
By why inject into muscle? Why not in fat just under the skin, straight in a vein, or even up the nose, given that’s where we’re most likely to encounter the virus?
Nasal vaccines are approved in some countries — such as the US, where some people can get the flu vaccine as a mist sprayed up their nose.
Apart from knowing the safety and effectiveness of injected vaccines, we have large-scale manufacturing and logistics chains set up to make and distribute jabs, especially now COVID-19 vaccination programs are rolling out.
And 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.
Muscle contains and recruits immune cells called dendritic cells, which take up antigens quickly and stick them on their surface, like a flag.
Dendritic cells then migrate to and slip into lymph nodes, “which are like large meeting places for the immune system”, Dr Groom says.
There, they encounter T cells and B cells — white blood cells that help defend our body against specific pathogens.
A dendritic cell will present its flag to T and B cells until it finds those that recognise the antigen, then gives them the signal to multiply and, in the case of B cells, start manufacturing antibodies.
“In the case of COVID-19 vaccines, that amplification means they can block SARS-CoV-2 spike proteins so that the virus can no longer get into a cell,” Dr Groom says.
“But they also start to form that pool of long-lived memory, which is really what we want from an outcome of a vaccine.”
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.
As well as providing a ready pool of dendritic cells, muscle acts as a “deposit”, where the vaccine can linger a while and be used over a longer period.
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.
“There are other nonspecific immune cells that can mop up the vaccine and degrade it before it has the opportunity to get to the lymph node,” Dr Groom says.
“Then it doesn’t have the opportunity for this information to be shared with B and T cells.”
As well as being easier to do, injecting vaccines into muscle also has very few severe side effects, and overall invokes less inflammation than a vaccine in a vein.
Most vaccines comprise two parts: the virus-specific antigen portion and a substance that creates a stronger immune response called an adjuvant.
“Adjuvants can sometimes lead to overall inflammation, more so via an intravenous route than when it’s in the tissue,” Dr Groom says.
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.”
Adapted from other news agencies