August 22, 2021

Here’s What to Know About the COVID-19 Vaccine Side Effects

What to expect and why they happen

A healthcare provider uses a syringe to withdraw a COVID-19 vaccine from a small bottle.

Every day, more and more people receive one of the COVID-19 vaccines. As vaccination becomes more widely available, you may wonder about the potential side effects.


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It’s important to note that vaccine side effects aren’t new or unusual. For example, it’s normal for some people to experience mild side effects like headaches, muscle aches and soreness after receiving their annual flu shot.

We talked to critical care specialist Rachel Scheraga, MD, about potential side effects, what causes them and why the vaccines are safe.

The vaccines

Currently, three vaccines have been given the green light for distribution:

  1. Pfizer/BioNTech: Given in two doses, 21 days apart. It’s approved for those 12 and older.
  2. Moderna: Given in two doses, 28 days apart. It’s approved for those 18 and older.
  3. Johnson & Johnson: Given in one single dose. It’s approved for those 18 and older.

So far, only the Pfizer/BioNTech has been fully approved by the Food and Drug Administration (FDA), which allows them to be sold to hospitals and healthcare providers. The other two are approved for emergency use authorization (EUA) by the FDA because of the current state of the pandemic. Also, keep in mind that data about side effects is still being collected and studied.

What are the known side effects of the vaccines?

As with many vaccines, there are some side effects, says Dr. Scheraga, but those side effects are relatively mild. The side effects are mainly:

The data shows that side effects are more commonly felt after the second dose of Pfizer or Moderna’s vaccine rather than after the first dose. With Johnson & Johnson’s vaccine, commonly reported side effects were arm soreness, headache, muscle aches and nausea. Most of the side effects occurred one or two days after vaccination.

Uncommon side effects

As studies continue, some less common side effects have been identified.

Blood clotting

In April, a small number of recipients of the Johnson & Johnson vaccine developed a rare blood clotting disorder.

These cases occurred among women between the ages of 18 and 59, and the symptoms set in around six to 15 days after vaccination. Upon further review, these reactions were determined to be very rare, and the FDA recommended resuming the use of this vaccine.

Guillain-Barré Syndrome

The Johnson & Johnson vaccine has also been associated with cases of Guillain-Barré Syndrome, a rare autoimmune disorder that attacks the body’s nerves. The likelihood of being diagnosed with GBS after receiving the Johnson & Johnson COVID-19 vaccine is extremely small — about 0.0008%.

Heart conditions

According to the CDC, there have been reported cases of myocarditis and pericarditis in adolescents and young adults. These cases predominantly occurred among teens 16 or older and adolescent males after receiving the second dose of an mRNA COVID-19 vaccine.

However, the CDC maintains its stance that these reports are rare and the benefits of COVID-19 vaccination outweigh the known and potential risks.


Delayed periods

Though some people have reported changes in their periods shortly after vaccination, there currently is no concrete evidence that COVID-19 vaccines can have a direct effect on women’s menstrual cycles. Instead, these changes may be associated with the body’s response to stress.

What causes these side effects?

According to Dr. Scheraga, the side effects have to do with how the COVID-19 vaccines work. Both the Pfizer/BioNTech and Moderna vaccines use messenger RNA (or mRNA). The strategy has been in development for years, but this is the first time it’s been used in a distributed vaccine.

In this case, the harmless “spike” protein of the virus in the mRNA vaccine triggers the secretion of antibodies and recruitment of memory immune cells in the bloodstream. This reaction allows protection if one gets infected with the virus at a later time.

With the Johnson & Johnson vaccine, a piece of DNA from the COVID-19 spike protein is injected into an adenovirus (the kind of virus that typically causes colds). This modified adenovirus can carry the DNA segment but can’t replicate it inside your body and cause illness. The DNA causes cells to make harmless versions of the spike protein, and your body creates an immune response to it.

“This activation of your immune system is what leads to some of these non-specific symptoms we’re seeing,” says Dr. Scheraga.

Do the vaccine side effects mimic COVID-19 symptoms?

Dr. Scheraga notes that there are some overlapping features between the side effects and the symptoms of COVID-19. Specifically, fever and muscle aches are some of the general side effects that mirror COVID-19 symptoms.

It’s important to keep an eye on the side effects you experience because it’s possible that these symptoms could be a result of COVID-19.

“It’s possible that if you contract the virus a few days before you receive your first dose, those symptoms could be indicative of COVID-19 infection instead of vaccine side effects,” Dr. Scheraga says.

But, she adds, there are a few ways to tell if it’s just the side effects or something more serious. Most times, the side effects are mild and usually fade within 24 hours. You shouldn’t feel prolonged effects from the vaccine itself.

Everyone’s reaction will be on a case-by-case basis. If you find those symptoms worsening or lingering longer than a day or two, you should contact your healthcare provider. They’ll be able to direct you on the best next steps, including, potentially, a COVID-19 test.

No, you can’t get COVID-19 from the vaccine

The COVID-19 vaccines will not give you COVID-19. You’re just getting a sequence of the protein spike of the virus. You’re not getting either an active or inactive part of coronavirus.

“You’re just getting what is needed from the virus so your immune cells can develop a memory of it. That way if you do contract COVID-19 after full vaccination, your immune system will be able to fight it off quickly so you don’t get sick,” says Dr. Scheraga.


Are there any pre-existing medical conditions that could interfere with or intensify the side effects?

One question has been whether or not pregnant patients would experience any additional adverse reactions to the vaccines. So far, that’s unknown because the initial clinical trials didn’t include pregnant recipients. However, research is ongoing.

Recent guidance developed by the Centers for Disease Control and Prevention (CDC) advises that if a pregnant patient is part of a group recommended to receive the vaccine, that patient would have the ultimate choice, but should consult with their healthcare provider first.

As with other vaccines, like a flu shot, the healthcare provider administering the vaccine will check with each recipient if they’ve experienced allergies with previous vaccines. Specifically, those reactions are generally caused by additives in the vaccine.

“Once you receive a dose, you’ll be monitored for 15 minutes, which is typically enough time to see any indication of a severe allergic reaction,” Dr. Scheraga points out.

As of now, the CDC says that patients with a history of such reactions can receive the vaccine but they should have those incidents fully assessed by a healthcare provider to determine a fuller context around those incidents.

The CDC says, “These persons may still receive vaccination, but they should be counseled about the unknown risks of developing a severe allergic reaction and balance these risks against the benefits of vaccination.”

In those instances, the CDC calls for a post-vaccination observation period of 30 minutes instead of 15 minutes.

Are the vaccines safe?

Yes, all three COVID-19 vaccines are safe and effective.

“There’s a lot of concern because of the speed of development and whether or not mRNA vaccines are a brand new platform,” says Dr. Scheraga. “But it’s not brand new technology. This platform has been under development for this kind of real-time distribution for a while and was worked on prior to the pandemic.”

The vaccines are safe, effective and potentially life-changing.

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