What Do Your COVID-19 Vaccine Side Effects Tell You?

What Do Your COVID-19 Vaccine Side Effects Tell You?

If someone gets a headache or feels a bit under the weather after receiving a COVID-19 vaccine, it's become common to hear them say something like "Oh, it just means my immune system is really working hard." On the flip side, when people don't notice any side effects, they sometimes worry the shot isn't doing its job or their immune system isn't reacting at all.

Is there any link between what you can notice after a vaccine and what's happening on the cellular level inside your body? Robert Finberg is a physician who specializes in infectious diseases and immunology at the Medical School at the University of Massachusetts. He explains how this perception doesn't match the reality of how vaccines work.

What Does Your Body Do When You Get a Vaccine?

Your immune system responds to the foreign molecules that make up any vaccine via two different systems.

The initial response is due to what's called the innate immune response. This system is activated as soon as your cells notice you've been exposed to any foreign material, from a splinter to a virus. Its goal is to eliminate the invader. White blood cells called neutrophils and macrophages travel to the intruder and work to destroy it.

This first line of defense is relatively short-lived, lasting hours or days.

The second line of defense takes days to weeks to get up and running. This is the long-lasting adaptive immune response. It relies on your immune system's T and B cells that learn to recognize particular invaders, such as a protein from the coronavirus. If the invader is encountered again, months or even years in the future, it's these immune cells that will recognize the old enemy and start generating the antibodies that will take it down.

In the case of the SARS-CoV-2 vaccines, it takes approximately two weeks to develop the adaptive response that brings long-lasting protection against the virus.

When you get the vaccine shot, what you're noticing in the first day or two is part of the innate immune response: your body's inflammatory reaction, aimed at quickly clearing the foreign molecules that breached your body's perimeter.

What Side Effects Are People Reporting?

covid chart

The most common side effect people reported after the the Moderna or Pfizer vaccine was fatigue.

It varies from person to person, but how dramatic the initial response is does not necessarily relate to the long-term response. In the case of the two mRNA COVID-19 vaccines, well over 90 percent of people immunized developed the protective adaptive immune response while fewer than 50 percent developed any side effects, and most were mild.

The bottom line is you can't gauge how well the vaccine is working within your body based on what you can detect from the outside. Different people do mount stronger or weaker immune responses to a vaccine, but post-shot side effects won't tell you which you are. It's the second, adaptive immune response that helps your body gain vaccine immunity, not the inflammatory response that triggers those early aches and pains.