Expert Explainer: Steven Kerfoot, PhD, on how vaccines work in our bodies

vials of COVID-19 vaccine in the foreground with a synringe in the background

What exactly happens inside your body after you receive a COVID-19 vaccination? Why do some people experience side effects and others don’t? Will our immunity to COVID-19 eventually wane?

As vaccine eligibility expands in Canada, Steven Kerfoot, PhD, Associate Professor at Western University’s Schulich School of Medicine & Dentistry, provides a clear picture of how our immune systems respond to the COVID-19 vaccine.

What is the difference between mRNA (Pfizer and Moderna) and non-replicating viral vector vaccines (AstraZeneca and Johnson & Johnson)?

Kerfoot: There are a number of different ways to make a vaccine, but the goal is the same: to train your immune system to recognize one specific piece of the virus that causes COVID-19 so that if you are infected with the virus, your immune system will already recognize it, know how to deal with it and can fight it off.

In this case, that piece of the virus are the spike proteins, which are the protrusions on the outside of coronaviruses that allows them to get entry into your cells.

The viral vector vaccines present the spike protein to your immune system by attaching it to another type of virus that doesn't make you sick.

The mRNA (messenger ribonucleic acid) vaccines work by supplying the instructions to your own cells which start to make the spike protein.

What is happening in our bodies during the first two days after getting the vaccine?

Kerfoot: When your body’s immune cells detect the spike protein, the cells start getting the signal that there may be a virus in the area. The immune cells sound the alarm bells and respond by flooding the area with antiviral compounds.

Depending on how strong your immune response is, you may start feeling side-effects throughout your body or get a fever which is part of this general, non-specific anti-virus response. If you feel tired or achy, a lot of that is your immune system declaring a body-wide shut down.

How you’re feeling in these first few days is an indication that you've tricked your immune system to believe that there is a real infection that needs to be dealt with. However, the specific immune response that will lead to long-term immunity won’t be ready for another ten days.

Are stronger side effects a sign of stronger immunity?

Kerfoot: There hasn't been a lot of work to directly understand how the strength of those early side effects compare to the strength of your long-term protective immune response. One recent study showed that those who had the most severe fever or chills after getting the mRNA vaccines had a small, but measurable increase in one part of the immune response, what they call the antibody response. That said, when you looked at other parts of the immune response like the number of memory cells, known as T-cells and B-cells, and how effective they were, there was no difference at all.

Research has shown that vaccines are highly effective in the large majority of people. It's clear that for the majority of people, the response that's important - the T-cell, B-cell, and antibody response - isn't necessarily related to how bad you feel one day post-injection.

Why do people have more severe side effects than others?

Kerfoot: In general, younger people tend to respond more strongly to immune signals compared to older people, but, that's not a 100 per cent rule.

There are certainly examples of younger people who didn't feel any negative effects at all, and older adults who did, so it's going to be very individual, and there isn't necessarily a way to predict it.

When do we get long-term immunity and how is that different than our initial response?

Kerfoot: To be protected from COVID-19, we need our T-cells and B-cells, which are the immune cells responsible for identifying specific targets to attack, to learn to identify the spike protein from SARS-CoV-2. Viruses are tricky and they hide themselves well, so we need the tools to find those that specific virus wherever it’s hiding.

It’s not until 10 days to two weeks after immunization that you have enough T-cells and B-cells and antibodies (made by B cells) that are specific to the SARS-CoV-2 spike protein to provide long-term protection. Training your immune system to recognize the virus takes time.

Will we need a booster at some point?

Kerfoot: Whether or not we will need a booster shot further down the line is still an open question. There are two scenarios where a booster may become important. The first is if the immune response that we all developed to the vaccination wanes and disappears over time. It’s true that immune memory isn't always forever, and a booster shot in that scenario is designed to remind your immune system to increase that memory response to keep it at a protective level.

There is evidence that the immune responses we're developing to the current vaccines aren't going to go away anytime soon, but we won't know that definitively for another two or three years down the line.

The second scenario looks at the potential emergence of versions of the SARS-CoV-2 virus that have changed their spike protein sufficiently so that our immune system can't recognize it well enough anymore to be protective.

At this point, it looks like the vaccines available to us can handle virtually all of the variants that are currently out there. However, if we do struggle to get this pandemic in check globally, there is potential for a version of the virus to appear that can bypass our current immune response.

I should note however, our immune systems deal with viruses that mutate a little bit all the time and our B-cells can change the antibodies that they make and tweak them so they match the new variants. So, while it may get to a scenario where it's not as protective immediately, our immune system is pretty good at catching up quickly.

Only time will tell if a booster becomes necessary, but at this point I wouldn't say it's a certainty.