Vaccine side effects: How the body responds and why being ill after a jab ‘is good’

For a fraction of people, getting these first COVID-19 vaccines could be unpleasant—more than the usual unpleasantness of getting a shot. They might make you feel sick for a day or two, even though they contain no whole viruses to actually infect you. Both the Pfizer/BioNTech and Moderna vaccines are quite “reactogenic”—meaning they stimulate a strong immune response that can cause temporary but uncomfortable sore arms, fevers, chills, and headaches.

Immunologists are well aware that the immune system uses a complex set of sensors to understand not only whether or not something is foreign, but also what kind of threat, if any, a microbe might pose to the system.

It can tell the difference between viruses and parasites, like tapeworms, and activate specialised arms of your immune system to deal with those specific threats accordingly.

It can even monitor the level of tissue damage caused by an invader and ramp up the immune systems response to match.

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How it works

Vaccines work by introducing a safe version of a pathogen to a patient’s immune system.

When vaccinated, a person’s immune system needs to sense danger before responding.

The requirement for danger means that one’s immune system is programmed not to respond unless a clear threat is identified.

Vaccines work by tricking the body into thinking it has been infected with symptoms occurring being an indication it has successfully done so.

The fever, fatigue, and other signs associated with a COVID-19 infection are typically caused by the immune responses, not the virus itself.

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Side effects and symptoms caused by the vaccine include redness and swelling at the injection site, stiffness and soreness in the muscle, tenderness and swelling of the local lymph nodes and, if the vaccine is potent enough, even fever.

Experts claim these side effects are due to the balance of vaccine design which is maximising protection and benefits while minimising the uncomfortable, however necessary side effects.

Two of the most discussed serious side effects, anaphylaxis (a severe allergic reaction) and Guillain-Barre Syndrome (nerve damage due to inflammation), occur at a frequency of less than one in 500,000 doses.

When a person gets the COVID-19 vaccine, the body cells encode a version of the coronavirus’s spike protein.

The virus uses spike protein as a key to get into the cells, but unattached from the rest of the virus, the spike protein can’t infect anything.

“However, the bodies innate immune system recognises the vaccine materials and the resulting spike protein as foreign,” said Matthew Woodruff, instructor with the Lowance Center for Human Immunology at Emory University.

He added: “This signal sets off a reaction that can feel a lot like getting sick.

“More immune cells become inflamed and sore, activating even more immune cells that might cause whole-body symptoms such as fever and fatigue.”

Early data has suggested that the mRNA vaccines in development against COVID-19 are highly effective, with as much as a 90 percent effectiveness it’s been claimed.

That means they are capable of stimulating robust immune responses, complete with sufficient danger signalling, in greater than nine out of 10 patients.

Woodruff added: “Initial data has stated that more than 2 percent of the Moderna vaccine recipients experienced temporary side effects which included fatigue and headaches.

“The percentage of people who experience any side effects will be higher.

“Experts state that these are signs that the vaccine is doing what it was designed to do which is training the immune system to respond appropriately.”

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