Lymph nodes or lymph glands are small lumps of tissue that contain white blood cells that can help fight infections by attacking and destroying germs that are carried through the lymph fluid.
There are hundreds of lymph nodes throughout the body, including in the neck, armpit, chest, abdomen and groin.
A recent report from the Therapeutic Goods Administration (TGA) found that swollen lymph nodes were the most common adverse event reported following a booster shot.
Of the 2.5 million booster doses administered in Australia to January 2, the TGA received approximately 600 reports of suspected adverse events.
"[Swollen lymph doses are a normal and known side effect of vaccines and occurs when the immune system is stimulated," the TGA said, adding it was observed during clinical trials for the Covid-19 vaccines.
For Pfizer, it has been found to occur more frequently after a third dose, with about 5 per cent of people reporting experiencing swollen lymph nodes.
This is compared to the less than one per cent of people who reported this side effect after the first or second doses in the clinical trials.
For Moderna, this side effect occurred in up to 10 per cent of people.
Swelling may be noticed near the injection site within a few days of the vaccination. For example, in an armpit where the lymph nodes are located.
"This normally resolves without treatment after a week or so. People should seek medical attention if swelling persists for more than a few weeks to rule out other causes," the TGA said.
What causes the swelling?
The Covid-19 vaccines contain spike proteins that start to build when injected into your body.
These proteins are then carrying into the lymph nodes, activating some of the white blood cells.
Immunology researcher at the Walter and Eliza Hall Institute (WEHI) Joanna Groom told the ABC some of the white blood cells that are stimulators are known "effector" cells.
These cells pump out antibodies that neutralise the virus and therefore multiplying, possibly leading to inflammation.
"The delay between Covid vaccinations has been much shorter than what we normally experience with, say, the annual flu vaccine, so potentially that means we still have those effector cells from the last vaccine," Dr Groom said.
"It's likely that in some people, there's hyper-proliferation of immune cells and ongoing activation, and so that's causing some additional inflammation."
Fourth booster shot being considered for Australians
It appears that a fourth Covid-19 booster shot could be on the cards for Australians, with the country's most senior health official dropping a big hint about additional vaccine doses on Saturday.
Chief Medical Officer Paul Kelly said the Australian Technical Advisory Group on Immunisation would "absolutely" be deliberating on the possibility of a fourth shot.
"(ATAGI) is continuing to meet weekly … They will absolutely deliberate on fourth or subsequent doses," he told reporters in Canberra.
Earlier this month, Israel became the first country to begin rolling out a fourth dose of the vaccine.
Denmark and Chile have also begun rolling out a second booster to people with risk of serious illness.
A fourth dose is already recommended for immunocompromised people in Australia.
Professor Kelly said he had spoken to his Israeli counterparts who assured him at this stage a fourth dose was only for the immunocompromised, as well as healthcare workers.
"They are keeping (the fourth dose) very tight to people with a higher risk of severity," he said.
"So older people – I believe it's people over 60 – people with chronic diseases, particularly those that lead to immune-compromised and health care workers.
"They are still evaluating that programme and they have promised in the coming weeks to share that evaluation with us."
