Why do COVID-19 vaccines have to be injected?

We recently received a question from a Cosmos reader: “Why can’t you drink a COVID vaccine?” So why do vaccines have to be injected?

We said:

There are some vaccines that can be taken orally, but they are different to the COVID-19 vaccines we use in Australia.

“One of [the oral vaccines] is the polio vaccine, that children use to get on a sugar drop in a spoon,” says Professor Nikolai Petrovsky, a coronavirus vaccine developer at Flinders University.

“That worked because polio was actually a live virus that infects the cells in the gut.

“But vaccines for COVID are based on inactivated viruses’ synthetic proteins. They just don’t stimulate an immune response if you were to swallow them.”

There are some oral COVID vaccines in development that use an adenovirus, which works a bit like the polio vaccine. But they haven’t been fully tested or approved yet.

Here are all the ingredients in vaccines approved for use in Australia.

Why injections?

The best place for a COVID vaccine to be introduced is in the muscle – known as an intramuscular injection. This is because muscle contains lots of immune cells that can quickly recognise the antigen – in this case, a viral vector or mRNA fragments of SARS-CoV-2.

These cells also carry their new information off to the lymph nodes, which are a key part of the immune system. 

But why in our arm?

There is a whole cluster of lymph nodes under our armpits, so the immune cells in the muscle don’t need to travel far to meet the lymph node. Even better, these lymph nodes are close to our lungs, where coronavirus usually attacks.

But in fact the injection can go into almost any muscle. Arms and thighs are simply the safest.

“These are places where there’s muscle but no dangerous blood vessels or nerves that could be damaged,” explains Petrovsky.

“So that’s why we typically give them there. Technically they could be given in other places, but they would either be more uncomfortable or not as safe.”