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Why Do Vaccines Cause Side Effects?

It’s common for vaccines to cause side effects like a sore arm or fatigue, but these usually aren’t cause for concern.

Mild side effects are a sign a vaccine is working. And it’s not just COVID-19 vaccines that can cause side effects, other vaccines can cause them as well.

After a dose of a COVID-19 vaccine, it’s common to experience side effects like soreness in the arm where you received the shot, chills, aches, fatigue, headache, or fever. While these side effects are usually short lived—lasting between one and three days in most cases—they’re enough for some people to opt out of getting a COVID-19 vaccine.

While side effects are uncomfortable, they can actually be a good sign—they mean that the dose has triggered a reaction by the immune system and the immune system is preparing to respond if it encounters the virus.

In other words, side effects are a sign a vaccine is working. And it’s not just COVID-19 vaccines that can cause side effects, other vaccines can cause them as well.

Here, we look at two big questions about COVID-19 vaccine side effects—what is the risk of severe side effects and how do you treat vaccine side effects?

What is the risk of severe side effects?

For the vast majority of people, side effects from a COVID-19 vaccine are relatively mild and resolve within days. However, there have been cases of more severe side effects.

  • The Johnson & Johnson COVID-19 vaccine has been linked with Guillain-Barré syndrome, a rare autoimmune disorder. Cases occurred mostly in men ages 50 or older and are considered extremely rare. Most people who develop this condition recover within a few months.
  • The Johnson & Johnson COVID-19 vaccine has also been linked to severe blood clots. Again, occurrences are rare. Because of the risk of serious adverse effects, the CDC recommends other vaccines over the Johnson & Johnson vaccine.
  • There are also verified cases of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the outer lining of the heart) following vaccination. Most cases are associated with mRNA vaccines (Pfizer or Moderna) and occurred in people ages 30 years or younger.

While these side effects are extremely rare, concerns about them are legitimate.

If you have questions or concerns about your risk of serious side effects, your best source of information will be a healthcare provider.

After all, everyone’s health is different—a healthcare provider will be able to look at your medical history and overall health and help you make the best decision.

How to treat vaccine side effects

The person administering your vaccine can advise you on side effects to expect and how to treat them. Ask what over-the-counter medicines you can take to help ease things like pain, soreness, and fever.

In rare cases, side effects can be stronger. If the pain or redness where you were injected increases after 24 hours, or if side effects aren't going away after a few days, contact your healthcare provider.

Remember that the benefits of getting vaccinated far outweigh temporary side effects. Once you are fully vaccinated, you will be far better protected against contracting COVID-19 and having severe illness from COVID-19—the kind of severe illness that has led to hospitalizations, breathing through a ventilator, and death for many people.

Article sources open article sources

Memorial Sloan Kettering Cancer Center. "COVID-19 Vaccine Side Effects: Why They Happen and How to Treat Them."
Centers for Disease Control and Prevention. "Symptoms of COVID-19."
Cleveland Clinic healthessentials. "What to Know About Johnson & Johnson’s COVID-19 Vaccine and Guillain-Barre Syndrome."
Centers for Disease Control and Prevention. "Possible Side Effects After Getting a COVID-19 Vaccine."
Centers for Disease Control and Prevention. "Selected Adverse Events Reported after COVID-19 Vaccination."
University of Michigan Health. "Blood Clots and COVID-19 Vaccines: What You Need to Know."

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