COVID-19 Resources

Everything You Want To Know About COVID-19 Vaccine Side Effects

Feb. 24, 2021

The vaccine is very safe — and side effects are a sign that it’s working.

As more and more Americans get the COVID-19 vaccine, more of us have heard firsthand stories about the side effects that can come along with it.

While some have reported side effects, it’s important to know that the COVID-19 vaccine is very safe. We know this because it was developed and tested through clinical trials in the exact same way as other vaccines. If there had been any severe or long-term side effects of the vaccine seen in the clinical trials, it would not have been released to the public.

Fortunately, most side effects of the vaccines are mild or moderate, and last one to two days. COVID-19, on the other hand, can cause mild to severe symptoms that last days to weeks, and can be fatal.

While side effects can be uncomfortable, they are actually a sign that the vaccine is doing its job by causing your body to build protection against the coronavirus.

Why does the vaccine cause side effects?

When your body identifies a virus, it creates defense molecules called antibodies that attack and destroy these germs. Your body keeps these antibodies, which remember how to protect you if they meet the virus again.

This is called immunity, and it’s why you often don’t get sick from the same illness twice.

Vaccines work by introducing material to your body that looks like a virus but won’t make you sick. This material tricks your body into thinking you have the live and working virus. In response, your body creates antibodies that are specifically designed to attack this virus. If your body ever identifies the real thing, it has the defenses needed to protect you.

This can help us understand why some may experience side effects after getting a vaccine. The vaccine causes the body to have this immune response as a way to build its defenses against the virus — and this immune response may make some feel sick. While many people report no side effects from the vaccine, you may feel some symptoms, which means that the vaccine is working.

Common Side Effects of the COVID-19 Vaccine

The VAERS (Vaccine Adverse Event Reporting System) is a system that tracks side effects of vaccines given throughout the country. Based on reports to VAERS, common side effects of the COVID-19 vaccines can include:

  • Injection site pain (in your arm).
  • Tiredness.
  • Headache.
  • Muscle pain.
  • Fever.
  • Chills.
  • Joint pain.
  • Injection site swelling.
  • Injection site redness.
  • Nausea.
  • Swollen lymph nodes.

Are side effects more common after the second dose?

For COVID-19 vaccines that require two doses, the second dose of the vaccine may cause more of the common side effects listed above than the first dose. This has been reported by healthcare workers who were the first to receive the vaccines in the U.S., and also in the clinical trials.

This happens because your body quickly recognizes the vaccine and can build a stronger immune response with the second dose. Again, it means that the vaccine is working and the body is building defenses against the virus. Without the second dose, the body may not be protected enough to prevent an infection from the virus.

Can the vaccine cause an allergic reaction?

All vaccines and medications carry a risk of causing allergic reactions. The COVID-19 vaccines contain various ingredients (such as lipids) that are needed to keep the vaccine stable so that the body builds an immune response against the virus. Some of these ingredients may lead to allergic reactions, but it is important to note that these ingredients are fairly common and found in many other vaccines and medications.

Ingredients of the COVID-19 vaccines that could cause an allergic reaction:

  • Pfizer-BioNTech vaccine:
    • Lipid ALC-0315.
    • Lipid ALC-0159.
  • Moderna vaccine:
    • PEG2000-DMG (polyethylene glycol), which is similar to polysorbate.

Symptoms of Severe Allergic Reaction

There is a very small chance (0.001%) that the vaccine could cause an allergic reaction. The symptoms of an allergic reaction would usually occur within 15 minutes after getting a dose of the vaccine, and include:

  • Difficulty breathing.
  • Swelling of your face and throat.
  • A fast heartbeat.
  • A bad rash all over your body.
  • Dizziness and weakness.

Though treatable, it is recommended not to receive the vaccine if you have a history of allergic reactions to any ingredients in the vaccine. Additionally, the Centers for Disease Control and Prevention (CDC) recommend that those who have experienced a severe allergic reaction to any other vaccine, medication or food, talk to their doctor before getting the vaccine. If you and your doctor decide to proceed with vaccination, then your monitoring period will be extended as a precaution (see below).

Safeguards (If you don’t know if you have an allergy.)

  • All people who receive a vaccine are monitored on-site by medical personnel for at least 15 minutes. People who have a history of allergic reactions are monitored for at least 30 minutes after receiving the vaccine.
  • Vaccination providers have the appropriate medications and supplies to treat allergic reactions, if they occur. This includes epinephrine (the medication in an EpiPen), antihistamines (like Benadryl), stethoscopes, blood pressure cuffs and pulse monitors.
  • If you were to experience a severe allergic reaction after receiving the COVID-19 vaccine, providers would treat the reaction and enact rapid response care for emergency medical services (911), and you would be monitored in a medical facility for several hours.

In conclusion, though serious, allergic reactions are very rare and treatable in the unlikely event that they would occur.

When do side effects occur?

It depends on the type of side effect.

Common side effects can happen anywhere from within an hour to three days after the vaccine is given. They typically resolve within one day.

If an allergic reaction occurs, it will likely happen within the first 15 minutes of receiving the vaccine. Therefore, an observation period of about 15 minutes under medical supervision is recommended after receiving the vaccine. Most of the time, a side effect like this can be avoided by communicating any prior history of allergic reactions that you may have to the healthcare provider who is administering the vaccine.

Other Side Effects

You may have heard about other side effects, such as fainting and Bell’s palsy. However, these reactions to the vaccines are not direct effects of the vaccines. Each of these are addressed below.

Side Effect Myth-Busting


A nurse in Tennessee, Tiffany Dover, fainted on camera shortly after receiving her COVID-19 vaccine. This event was reported in news outlets and on social media. While this prompted many to speculate that fainting is an immediate side effect of the COVID-19 vaccine, there is no evidence that this is the case.

Some people are prone to dizziness or fainting (known as vasovagal syncope) that is associated with receiving any kind of injection, or even when a blood sample is taken. Vaccination providers are trained to understand that fainting can occur with any kind of injection. The VAERS database, which releases this information publicly, indicates that fainting or syncope usually occurs with other symptoms of anxiety, or in people who have previously had anxiety from a needle-based injection. The nurse mentioned above reported after the event that she previously experienced fainting from other types of pain, and did not regret receiving the COVID-19 vaccine.

What does this mean for you?

If you have history of fainting because of anxiety and pain from needle injections, you should tell the healthcare professional who administers your vaccine about this beforehand. While fainting itself is not usually dangerous, it is important to avoid any potential injuries from falling.

Verdict: There is no evidence to suggest fainting is an immediate side effect of the COVID-19 vaccine, but rather a response by people who have a history of fainting because of fear or anxiety from needles.

Bell’s Palsy

What is Bell’s palsy?

Bell's palsy (link to UC Health Bell’s palsy) is an unexplained episode of full or partial facial muscle weakness or paralysis on one side of the face. Many times, symptoms – like sagging eyebrows or drooping mouth – can worsen over about two days. The symptoms occur suddenly and typically resolve over the course of a few weeks or months.


Only seven people out of the entire combined Moderna and Pfizer-BioNTech clinical trial populations, amounting to 74,400 total participants, experienced any symptoms of Bell’s palsy. This is only 0.0094% of participants, which is lower than the percentage of Americans in the general population who experience symptoms of Bell’s palsy, 0.01%.

What does this mean for you?

The reports of Bell’s palsy in the clinical trials very likely suggest a coincidence, and that Bell’s palsy is not caused by the vaccine itself.

Verdict: It’s likely a coincidence.

Could there be any unknown long-term side effects?

With the vaccines being new, unknown side effects are possible. However, the vaccines have been administered to a very large number of people already, both in clinical trials and to the public. While unknowns can cause worry, we do not have any evidence of side effects outside of those discussed in this article in the large number of people that have been immunized. No long-term side effects have been found to occur in any patients in the clinical trials to date, and those patients will continue to be closely monitored. It is unlikely that there are any negative long-term health effects, given the research and development that has been put into both mRNA-based and adenovirus-based vaccines over many years.

One positive long-term effect that we can be reasonably certain of is that the COVID-19 vaccines are very effective in preventing COVID-19 disease. The most important thing we can do to fight COVID-19 is to vaccinate as many people as we can. As more people receive the vaccine, the virus will not be able to spread, cause infection and mutate into new variants that may be more contagious or are harder to treat.

The COVID-19 vaccines are the best chance that we have to fight the pandemic and to return to normal daily life.

Where To Find More Information

In times of high emotion, it is important to stay informed while maintaining vigilance for false information or claims. If you would like to stay informed about side effects and symptoms of the vaccine, we recommend you gather information from reliable sources, such as the CDC, state and local health departments, and healthcare systems.