Why Does Everyone React Differently to the COVID-19 Vaccines? Here's What Experts Say

I'm a healthy person in my 30s, and when it was finally my turn to get the COVID-19 vaccine, I figured I'd do just fine. While that's mostly true, things didn't go as seamlessly as I thought they would.

I had some pretty severe soreness in my arm after receiving my first dose of the Moderna vaccine, but I just didn't sleep on that side on night one, and figured that was the worst of it. Honestly, I felt a little smug about the whole thing.

Things were different after the second shot. I got it at a local drugstore, and the pharmacist warned me that women my age tend to have more extreme side effects than others. I smiled, assumed he was just issuing a standard warning, and went home. I went to bed that night thrilled that I was on my way to being fully vaccinated, but aware that I could be feeling a little crummy the next day—and I definitely did.

black woman getting a vaccine at doctor's office Credit: Marko Geber / Getty Images

I woke up sometime around 3 a.m. feeling…off. I felt nauseous and I just couldn't get back to sleep, which is rare for me. I lay there for about an hour before I decided to go to the bathroom. As I made my way back to the bed, I felt like something wasn't quite right. The next thing I knew, I heard the sound of my head hitting the floor. I had passed out.

I called for my husband Chris from the floor, and he came rushing to me. He called the doctor, who told me to drink plenty of fluids and to go to the ER if things got worse. I stayed on the floor—it felt more…comfortable, I guess—but I felt too weak to even pick up my head to drink the water Chris had put on the ground next to me. I eventually crawled to the bathroom to throw up, before Chris helped me to bed again. After that, I called out of work and spent most of the day sleeping. But by that evening, I felt much, much better.

Related: Martha Just Received Her Second Dose of the Coronavirus Vaccine and Documented the Experience on Instagram

I was hesitant to share my story at first, with so many weighing in about their post-vaccine symptoms, I didn't want to add to the anxiety. But I came around to the idea precisely because there's so much out there right now about vaccine side effects—and lots of times we aren't given the full story. Sure, my vaccine reaction was a little more dramatic than I would have preferred, but there is an explanation for most of it.

See, as much as I hate to admit it, I'm a fainter—when I'm sick, the odds are pretty good I'm going to hit the floor. Just a few examples: I passed out when I developed heat exhaustion during cross country practice in college, I fainted when I was pregnant with twins, I blacked out several times after losing a lot of blood when I had a miscarriage, and I collapsed on my bathroom floor just before I was diagnosed with severe appendicitis in October. Each time I've completely freaked out my poor husband—and myself.

According to my doctor, passing out is probably just my body's response when I'm sick. She explained that my vagus nerve, the longest and most complex cranial nerve in the body, may simply get overly stimulated when I'm sick. "Then, your body just wants to be on the ground," she said. And it does.

My point is this: Everyone's experience with the COVID-19 vaccine is different, and there's often more to someone's story than you might realize. Still, it's understandable to have questions—I know I did. Here's what you need to know.

What are the more common reactions to the COVID-19 vaccines?

The Centers for Disease Control and Prevention (CDC) lists these as the most common side effects for any of the COVID-19 vaccines:

  • Pain in the arm where you got the shot
  • Redness in the arm where you got the shot
  • Swelling in the arm where you got the shot
  • Fatigue
  • Headache
  • Muscle pain
  • Chills
  • Fever
  • Nausea

The CDC also warns that side effects after your second shot, if you are given either the Pfizer or Moderna vaccine, may be more intense than the ones you had after your first shot.

Related: Everything You Need to Know About Your COVID-19 Vaccine Card

OK, but why do some people have more severe COVID-19 vaccine side effects than others?

Some people, like my husband, have no symptoms after getting the COVID-19 vaccine. Others, like me, can have a bad day afterward. But why? Doctors say there are a few possible reasons.

One is age. "Side effects are clearly related to age," William Schaffner, MD, an infectious disease specialist and professor at the Vanderbilt University School of Medicine, tells Health. "The older you are, the less likely you are to have side effects of the vaccine." The exact reason for this isn't clear, Dr. Schaffner says, but it could be that younger people's immune systems are more likely to react strongly and quickly to the vaccine.

Another is just the way your immune system works. "Every person's immune system has some idiosyncrasies that influence how they respond to a vaccine," infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security, tells Health. "Some people have no symptoms at all post vaccine; others have more."

There are also some people who seem to react in more extreme ways to medications, vaccines, and illness, Dr. Schaffner says, adding, "they usually know who they are." (Cough, me, cough.)

Still, other than allergy information, there's no data that clearly spells out why some people will have more extreme reactions to the COVID-19 vaccine than others. "It is not entirely well-established why some people have more or less significant side effects from the COVID-19 vaccines," Prathit Kulkarni, MD, assistant professor of medicine in infectious diseases at Baylor College of Medicine, tells Health.

How to prepare for your vaccine if you tend to have an extreme response to illness

Honestly, I should have known this would happen to me. It took me this long to realize that I'm a fainter, which is a little embarrassing.

If you're getting vaccinated soon, doctors say it's a good idea to be honest with yourself about how you tend to respond to illness and make plans accordingly. Dr. Schaffner says that could mean stocking up on Gatorade, making sure someone will be around to help you, if needed, and preemptively taking the day off of work, if you're able.

"You know best how you generally react," he says. Dr. Kulkarni agrees, and suggests at least planning to deal with some fatigue or muscle aches. If it doesn't happen, great. If it does, you're at least more prepared than I was.

"This is worth doing in order to get protection against COVID-19," Dr. Schaffner says. "COVID can put you in the intensive care unit…and that'll make you take a few days off."

As for me, I felt back to 100% the day after all the drama. And, while my experience wasn't exactly fun, I'm so happy to be vaccinated.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

This article originally appeared on Health.

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