What COVID-19 Vaccine Side Effects Might I Expect?

IIf you haven’t received your COVID-19 vaccination yet, your number will likely be revealed soon. What can you expect when you get your shot? For many it is not a day in the park, but others feel nothing. It is impossible for experts to predict who will be comfortable and who will not. In the vast majority of cases, any side effects you experience will be gone within a few days and there is nothing to worry about.

But it is important that the medical and scientific communities speak out about the temporary side effects of these vaccines – and that the public know that there is a very small percentage of side effects.

I’m an immunologist who studies the fundamentals of the immune response to vaccinations, so part of that responsibility falls on me.

When you get these vaccines, a lot of people will likely feel crappy for a few days. That is a far better prospect than long-term illness or death. In case you’re wondering why someone feels bad in the first place, I’ll explain.

Immunology’s “dirty little secret”

In 1989, the immunologist Charles Janeway published an article summarizing the state of immunology. Up until that point, immunologists had suggested that immune responses are triggered when the immune system encounters something foreign – bacteria, viruses, and parasites – that it calls “not itself.”

Janeway suspected that there was more to the story and, as is well known, interpreted what he called “the immunologist’s dirty little secret”: Your immune system doesn’t just react to all strange things. It reacts to strange things that it perceives as dangerous.

Today, 30 years later, immunologists know that your immune system uses a complex set of sensors to not only detect whether something is foreign or not, but also what kind of threat a microbe might pose. It can differentiate between viruses – like SARS-CoV-2 – and parasites like tapeworms and activate specific arms of your immune system to deal with these specific threats accordingly. It can even monitor the level of tissue damage caused by an intruder and boost your immune response accordingly.

Recognizing the type of threat posed by a microbe and the intensity of that threat enables your immune system to choose the right responses, use them precisely, and avoid the very real risk of immune overreaction.

Vaccine adjuvants bring the danger we need

Vaccines work by introducing a safe version of a pathogen into a patient’s immune system. Your immune system remembers past encounters and reacts more efficiently when it sees the same pathogen again. However, it only creates memory if the vaccine contains enough danger signals to trigger a solid immune response.

As a result, your immune system’s need to recognize danger before it reacts is extremely important and problematic at the same time. Requiring a hazard means that your immune system is programmed not to respond unless a clear threat is identified. It also means that when I develop a vaccine, I have to convince your immune system that the vaccine itself is a serious threat.

Scientists can do this in a number of ways. One is to inject a weakened – immunologists have weakened – or even killed version of a pathogen. This approach has the advantage that a pathogen is presented almost identically to the “real” pathogen, triggers many of the same danger signals and often leads to strong long-term immunity, as is the case with the polio vaccination. It can also be risky – if you haven’t weakened the pathogen enough and you roll out the vaccine too quickly, there is a chance that large numbers of vaccine recipients could inadvertently become infected.

A safer approach is to use individual components of the pathogen that are harmless on their own but are able to train your immune system to recognize the real pathogen. However, these parts of the pathogen often do not contain the danger signals necessary to stimulate a strong memory reaction. Therefore, they need to be supplemented with synthetic hazard signals that immunologists refer to as “adjuvants”.

Adjuvants are safe but designed to ignite

To make vaccines more effective, entire laboratories have devoted themselves to testing and developing new adjuvants. All of them are designed with the same basic purpose in mind – to activate the immune system in such a way that the effectiveness and longevity of the response are maximized.

To do this, we use the same sensors that your immune system uses to detect damage from an active infection. This means that while they stimulate an effective immune response, they do so through transient inflammatory effects.

At the cellular level, the vaccine causes inflammation at the injection site. Blood vessels in the area become somewhat “leaky” to recruit immune cells into the muscle tissue, causing the area to turn red and swollen. All of this triggers a full-blown immune response in a lymph node somewhere nearby that will play out over the course of weeks.

In terms of symptoms, this can lead to redness and swelling at the injection site, stiffness and sore muscles, tenderness and swelling of the local lymph nodes, and, if the vaccine is strong enough, even a fever (and the generally crappy feeling that comes with it).

This is the balance of vaccine design – maximizing protection and benefit while minimizing the unpleasant but necessary side effects. That’s not to say that there aren’t any serious side effects – they do – but they are extremely rare. Two of the most talked about serious side effects, anaphalaxy (a severe allergic reaction) and Guillain-Barré syndrome (nerve damage due to inflammation), occur with a frequency of less than 1 in 500,000 doses.

Vaccination against SARS-CoV-2

Early data suggests that Moderna and Pfizer’s mRNA vaccines against SARS-CoV-2 are highly effective – over 90%. Johnson & Johnson’s vaccine is also highly effective, although it was not developed using mRNA technology. All three are able to stimulate robust immune responses with sufficient hazard signaling to prevent severe COVID-19 in more than 9 out of 10 patients. That is a large number under any circumstances and suggests that these vaccines are effective.

In an early release of the Phase 3 study data, more than 2% of Moderna vaccine recipients experienced what they classed as severe transient side effects such as fatigue and headache. However, minor side effects are common – especially after the second dose. These are signs that the vaccine is doing what it was designed to do – training your immune system to respond to something it might otherwise ignore so that you can be protected later. It doesn’t mean the vaccine gave you COVID-19.

It depends: By vaccinating, you protect yourself, your loved ones, and your community from a highly communicable and fatal disease. It can cost you sick for a few days.


  • You may experience redness and pain in the arm that received the injection, tiredness, muscle aches, chills, and nausea, but these symptoms will not last long.

  • After vaccination, you will be monitored for 15 to 30 minutes for more serious side effects, which are rare.

  • Side effects do not mean that you have contracted COVID-19. Vaccines work by training your immune system to safely recognize and remember a pathogen.

  • If you have flu-like symptoms after the injection, talk to your doctor about over-the-counter pain relievers, but don’t take pain relievers before you have the injection.

Matthew Woodruff, lecturer, Lowance Center for Human Immunology, Emory University

This article was republished by The Conversation under a Creative Commons license. Read the original article.