Extract from ABC News
It's normal to feel a bit lousy in the day or two after your COVID-19 jab — in fact, it can be helpful to think of it as a positive sign.
Experts say mild side effects like tiredness, fevers, muscle aches, and arm pain are "normal signs your body is building protection".
But if feeling rubbish offers some reassurance that the vaccine is working, what does it mean if you don't have symptoms after the jab?
Thankfully, it's nothing to worry about: an absence of symptoms is not a sign the vaccine isn't doing its job, according to research published this week in JAMA Internal Medicine.
It suggests people who don't experience any side effects from mRNA COVID-19 vaccines still produce a robust antibody response.
US researchers measured the antibody levels of almost 1,000 healthcare workers two weeks after their second dose of the Pfizer or Moderna vaccine, and asked them to report any side effects.
They found almost 100 per cent of vaccine recipients "mounted a strong antibody response to the spike protein … independent of vaccine-induced reactions".
The findings echo what clinical trials show: COVID-19 vaccines are highly effective irrespective of age, sex, or the presence of side effects.
So why do vaccines leave some of us feeling worse than others? And how exactly does our body mount an immune response?
How do vaccines mimic a virus?
In order to train our bodies to recognise pathogens (and fight them off down the track), vaccines introduce our immune system to part of a pathogen — known as an "antigen" — which triggers an immune response.
This antigen might be a weakened or inactivated virus, or it might be just one part of a pathogen — for example, the spike protein found on the surface of SARS-CoV-2 (used by the virus to latch onto and enter human cells).
Traditional vaccines, including some COVID-19 jabs, deliver antigens directly to the body.
But other COVID-19 vaccines, such as the Pfizer, Moderna and AstraZeneca jabs, use different technology.
Instead of delivering the antigen itself, the vaccines contain a genetic blueprint (or set of instructions) that tell the body to make the SARS-CoV-2 spike protein using the body's own cells.
To do this, the Pfizer and Moderna jabs contain a single strand of genetic material — that's the mRNA or messenger RNA — which is encapsulated in a protective fatty coating.
The AstraZeneca vaccine, on the other hand, contains double-stranded DNA, which is carried into the body via a weakened version of a common cold virus, engineered so it doesn't replicate.
"The DNA gets taken up by your cells, that DNA then encodes the mRNA, and then it turns into a protein … which is what your body is going to respond to," said Stuart Tangye, an immunologist from the Garvan Institute of Medical Research.
"The mRNA vaccine just skips that first step."
What happens in your body as soon as the jab hits your arm?
When the vaccine is first injected into your arm, the mRNA or DNA it contains is taken into your cells, which "read" the genetic material and begin to build antigens.
"Our muscle cells start making the spike protein, and it gets expressed on the surface of the muscle cell … because that's where we're getting injected," Professor Tangye said.
As this is happening, your body's first line of defence against invading pathogens — the innate immune system — is already kicking into gear, immunologist Larisa Labzin says.
"Our innate immune system has got these receptors that are really good at detecting bits of viruses and bacteria, or any sign of cellular damage," said Dr Labzin from the University of Queensland.
"So when we get that needle in our arm … that mRNA and DNA tricks the innate immune system into thinking it's seeing a virus."
Two key types of white blood cells arrive on the scene: macrophages and dendritic cells, which help to screen the blood, tissues and organs for suspicious signs.
"They survey the body … looking for junk and other stuff that shouldn't be there," Professor Tangye said.
As they circulate, macrophages eat up dead cells (and in the case of infection, destroy foreign cells), while dendritic cells start to collect samples of the antigen that's been introduced — to help inform the body's adaptive immune response (more on that later).
When the cells sense there is a problem, they call for backup: triggering the release of a group of proteins called cytokines, which help signal other immune cells to the injection site.
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