Why Does Everyone React Differently to the COVID-19 Vaccines?

Your post-vaccine experience is hard to predict—but side effects tend to be mild and temporary.

COVID-19 vaccines have been shown to be safe and effective, according to the Centers for Disease Control and Prevention (CDC). That means side effects tend to be mild and temporary, much like those you may get from other routine vaccinations, per the CDC.

As of July 2022, four COVID-19 vaccines had been allowed for use in the US—Moderna, Pfizer, Novavax, and Johnson & Johnson. The types of side effects you might get from them are similar, and they only last a few days, per the CDC.

It can be hard to predict how you'll react to a vaccine. Some people feel mild or no symptoms. Others can have trouble going about their day—either way, the vaccine can give you significant protection against COVID-19. And serious side effects are rare, per the CDC.

Everyone's experience with the COVID-19 vaccine is different, and there's often more to someone's experience than you might realize. It's understandable to have questions. Here's what you need to know.

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What Are the More Common Reactions to the COVID-19 Vaccines?

The Centers for Disease Control and Prevention (CDC) listed the following as the most common side effects of any of the COVID-19 vaccines in people 18 years or older:

  • 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 warned 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.

Why Do Some People Have More Severe COVID-19 Vaccine Side Effects Than Others?

Some people have no symptoms after getting the COVID-19 vaccine. Others can have a bad day afterward. But why? Healthcare providers 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, told 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 said, 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, told 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 said, and added, "they usually know who they are."

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, told Health.

How to Prepare for Your Vaccine if You Tend to Have a Severe Response to Illness

If you're getting vaccinated soon, healthcare providers 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 said 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," Dr. Schaffner said. Dr. Kulkarni agreed, and suggested to at least plan 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 said. "COVID can put you in the intensive care unit…and that'll make you take a few days off."

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 CDC, WHO, and their local public health department as resources.

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