COVID-19 vaccine side effects are normal. What to expect when you get your vaccine.

March 30, 2021
Side effects for COVID-19 vaccines are normal. What to expect. Woman receives her vaccine at a vaccination clinic in Denver.
A woman receives her COVID-19 vaccine at Shorter Community AME Church in Denver. Photo by Cyrus McCrimmon for UCHealth.

Vaccines for COVID-19 are extremely safe and effective. And, people who get them are able to embrace new freedoms like spending time with family and friends.

In the coming months, fully vaccinated people may be able to travel more freely, dine in restaurants, work out in gyms and attend barbecues and parties again.

It’s normal for many people who receive COVID-19 vaccines to experience side effects. We consulted with medical expert, Dr. Thomas Campbell, to answer your questions about COVID-19 vaccine side effects. He provides answers to commonly asked questions so you’ll know what to expect when you get your vaccine.

Campbell has overseen the clinical trials for the Moderna COVID-19 vaccine at UCHealth University of Colorado Hospital and enthusiastically recommends both Moderna and the Pfizer BioNTech vaccine, which is a similar type of vaccine.

“The safety of these vaccines is excellent,” said Campbell, who is a Professor of Medicine in the Division of Infectious Diseases for the University of Colorado School of Medicine on the Anschutz Medical Campus.

The efficacy or effectiveness of the vaccines in preventing illness along with hospitalizations is also far better than medical experts expected when vaccines were first being developed last year. Scientists hoped vaccines would be at least 50% effective in preventing illness, which is similar to annual flu vaccines. In fact, the Moderna and Pfizer vaccines outperformed expectations. They prevented severe cases of COVID-19 about 95% of the time in clinical trials and are performing nearly as well — with efficacy of about 90% in the real world now as millions of people get their vaccines.

Here are some answers to top questions about the side effects for COVID-19 vaccines.

What is the most common COVID-19 vaccine side effect?

A sore arm is the most common side effect, Campbell said.

“The main thing we see is injection site reactions that occur shortly after getting one or both injections, usually within 24 to 48 hours after getting the vaccine, and sometimes as long as seven days later. This can be pain or swelling,” Campbell said.

What are the other common side effects of the COVID-19 vaccine?

Other common side effects include flu-like symptoms such as fever, chills, muscle aches, headaches and exhaustion. A small percentage of people also report some digestive symptoms such as nausea or diarrhea.

Are the side effects different after the first and second doses of the vaccines?

Some people don’t have any side effects. Side effects tend to be more frequent and more severe after one receives a second dose.

Do people who have previously had COVID-19 have stronger side effects?

Yes, that can be true. Many people who had previous infections have stronger side effects after the first dose (with the two-dose vaccines, Moderna and Pfizer). On the other hand, people who have not previously been exposed to the virus more typically have stronger side effects after their second vaccine dose.

While people who knew they already had COVID-19 were not allowed in the clinical trials, all volunteers had baseline tests for antibodies in their blood. Some volunteers already had antibodies to COVID-19, meaning they had previously been exposed and probably had asymptomatic infections.

When these people received vaccines, they tended to have stronger reactions, such as fever, to the first dose.

“Their bodies had seen the spike protein before and had developed some immune response. When their immune system sees it again in the vaccine, the memory kicks in and drives up the immune response, just like it does after the second dose for people who haven’t been infected previously,” Campbell said.

What’s the best timing for people who have had COVID-19 to get their vaccines?

If you’ve had COVID-19, the best time to get your vaccine is about 90 days after you recover, Campbell said. However, you can get the vaccine sooner as long as you have recovered from in your infection and are no longer on isolation.

I’m concerned about side effects. Should I still get a vaccine for COVID-19?

Yes, get your vaccine, Campbell and other medical experts say.

“Everyone should get their vaccine as soon as it is offered to them. Getting the vaccine is the best way to protect yourself against severe and potentially life-threatening illnesses from COVID-19,” Campbell said.

“If you don’t get the vaccine, you will remain susceptible to COVID-19, and COVID-19 is not going to go away. It’s going to be here for the rest of human history. The only way to protect yourself is to get a vaccine. Don’t rely on natural infection. You can have bad consequences both in the short term and the long term,” Campbell said.

He advises people to be aware of side effects, but not to worry too much about them.

“These side effects can make people feel sick in some cases. But, they don’t last that long. There have not been any long-term side effects that can be attributed to these vaccines, whereas some people with mild cases of COVID-19 can get very debilitating long-term symptoms. Those are symptoms that these vaccines prevent,” Campbell said.

What percentage of people experience side effects from the COVID-19 vaccines?

About 70% of people who participated in the Moderna and Pfizer clinical trials experienced at least one side effect, Campbell said. None were serious.

Were there any severe side effects during the clinical trials for COVID-19 vaccines?

No. The clinical trials did not detect any serious side effects, Campbell said.

Are the side effects a good sign? What is happening in my body when I have a reaction to the vaccine?

Yes. Side effects mean that the vaccine is beginning to work. At the same time, the absence of side effects doesn’t mean that the vaccine is not working. Each person can respond a little differently.

“Your body is responding to the foreign protein, the COVID-19 spike protein that’s in the vaccines,” Campbell said. “Symptoms like fever, chills and muscle aches are signs that your body is mounting a response against the foreign protein. That’s what it’s intended to do.”

Do people have side effects long after they’ve gotten their COVID-19 vaccines?

No. In almost all cases, the side effects emerge and resolve quickly, within about 48 hours after a person has received a vaccine dose.

Should I take over-the-counter pain medication before or after my COVID-19 vaccination?

It’s best not to take pain medication in advance of your appointment so you can allow the vaccine to start working. If you have a fever or muscle pain afterward, you can take acetaminophen or ibuprofen after your vaccine. If you have any kidney or liver problems, please consult with your doctor about which medication is safe for you. Many people won’t need any medication after receiving their vaccines.

I’ve heard that some people had allergic reactions. Is that true?

There were no allergic reactions during the clinical trials for the Moderna and Pfizer vaccines. Once millions of people started getting vaccines, a very rare allergic reaction occurred among some people who had previously experienced anaphylactic shock to bee stings or certain foods like peanuts and shellfish.

“Since these vaccines have been rolled out and given to tens of millions of people, we are seeing some allergic reactions pop up. These appear to be very rare,” Campbell said. “All three vaccines have excellent safety records.”

What should I do if I’ve had anaphylactic shock in the past? Should I get my COVID-19 vaccine?

Yes, go ahead and get your vaccine. If you have a history of experiencing anaphylactic shock, you should discuss vaccines with your doctor and alert vaccine clinic managers. Bring your epinephrine pen with you when you get your vaccine.

But, Campbell said it’s still wise to get your vaccine.

“Not everyone (with a history of previous allergic responses) who gets the vaccine has the reaction. If a person has an allergic reaction that requires injections with an EpiPen, it can be serious. But, getting COVID-19 can be serious too. You have to balance the risk of the vaccine with the risk of getting COVID-19,” Campbell said.

“These anaphylactic reactions are very rare. We only started seeing them after millions of people started getting injected. They did not show up at all in the trials when 30,000 to 40,000 people were getting vaccinated,” he said.

Here’s what experts at the Centers for Disease Control and Prevention recommend regarding vaccines and allergies: “People (should) get vaccinated even if they have a history of severe allergic reactions not related to vaccines or injectable medications — such as food, pet, venom, environmental, or latex allergies. People with a history of allergies to oral medications or a family history of severe allergic reactions may also get vaccinated.”

Do younger people have stronger side effects than older people?

Yes, in some cases younger people experience stronger side effects when they get their COVID-19 vaccines.

“It’s not a huge difference, but there is a slight difference,” Campbell said.

Younger clinical trial participants — those ages 18 to 64 — reported side effects like pain at the injection site or headaches about 8% to 10% more frequently than clinical trial participants who were 65 and older, Campbell said.

Interestingly, this variation by age also held true for people who received the placebo instead of the actual vaccine.

Campbell said there are two theories that explain different side effects to COVID-19 vaccines by age (or perceived side effects to the placebo).

Older people generally have weaker immune systems, so when an older person receives a vaccine, their response might not be as vigorous. Another theory is that younger people haven’t experienced as much illness or pain in their lives. So, it’s possible that they notice pain at the injection site or chills and muscle aches more than an older person would.

“It could be that younger people think more (about possible side effects) and tune in more to them. It could be that pain thresholds change with age. Any of these are possible,” Campbell said.

People can have variable responses to the vaccines and they shouldn’t worry if they’re on either end of the spectrum and experience almost no side effects or a fever and body aches.

Of course, anyone who has a severe allergic reaction should seek immediate help. That’s why all vaccine clinics require recipients to wait at the clinic for 15 minutes after receiving their vaccine in case they need medical attention.  People who have had serious allergic reactions in the past should be observed for 30 minutes after vaccination.

Do men and women or people of various races and ethnicities respond differently to COVID-19 vaccines?

Vaccine efficacy is high across all groups of people regardless of sex, race and ethnicity.

How did pregnant women respond after getting vaccines? Do we know if pregnant women have different side effects if they get the COVID-19 vaccine?

Women who were known to be pregnant were not allowed to participate in the clinical trials for COVID-19 vaccines. But, some women who were already enrolled in the clinical trials ended up getting pregnant during the studies.

“There were no adverse outcomes,” Campbell said. “And there was no evidence that vaccines affected fertility.”

(New research unrelated to the clinical trials now appears to show that vaccines may be safe for pregnant women and that the antibodies to COVID-19 may cross from the woman to her unborn baby. One mom in western Colorado had received the vaccine while she was pregnant. After her daughter was born, she tested positive for antibodies to COVID-19. Researchers are working to learn more about vaccines for pregnant women, infants and children as fast as they can. Right now, UCHealth medical experts do not recommend testing your baby for COVID-19 antibodies if you got the COVID-19 vaccine during your pregnancy.)

Read more about COVID-19 and pregnancy.

Do vaccines provide even better protection from a future infection than a previous case of COVID-19?


“The vaccines do a better job of providing better protection,” Campbell said.

That’s because the SARS-CoV-2 virus that causes COVID-19 is very good at camouflaging itself until it finds the opportunity to infect cells, Campbell said.

“The virus has developed ways to keep key parts of itself protected from the immune system so the immune system doesn’t see it. It’s kind of stealth in that way,” Campbell said.

“Imagine that your hand is the spike protein. If you clench your hand into a fist, you can’t see what’s inside the fist. If you open it up, then you can see inside. The spike protein keeps itself closed up and hides the receptors until it’s ready to infect the cell. There’s not much time for the immune system to see the spike protein when it’s in the open state,” Campbell said.

The vaccine works by opening up the part of the spike protein that normally remains hidden.

“What the vaccines do is present the spike protein so it’s frozen in that open state. It’s called ‘pre-fusion complex.’ All three of these vaccines do that. That’s a key difference between what the vaccine does and natural immunity,” Campbell said.

So, people who have had COVID-19 have some natural immunity, whereas people who have gotten vaccines have higher levels of antibodies that can more easily find and incapacitate the virus.

“With vaccines, we see very high levels of neutralizing antibodies. With a natural infection, there’s a wide range. Some people make a lot and some make a little,” Campbell said.

Will vaccines have to be altered because of COVID-19 variants? Is it likely that we’ll need new COVID-19 vaccines in the future?

Medical experts don’t know yet if we’ll need annual COVID-19 shots as we do for the flu. But, Campbell thinks it’s likely that researchers will continue to adapt COVID-19 vaccines.

“We will have to modify the vaccines to cover variant viruses. Whether we’ll have to have boosters, we don’t know yet,” he said.

About the author

Katie Kerwin McCrimmon is a proud Coloradan. She attended Colorado College thanks to a merit scholarship from the Boettcher Foundation and worked as a park ranger in Rocky Mountain National Park during summers in college.

Katie is a dedicated storyteller who loves getting to know UCHealth patients and providers and sharing their inspiring stories.

Katie spent years working as an award-winning journalist at the Rocky Mountain News and at an online health policy news site before joining UCHealth in 2017.

Katie and her husband, Cyrus — a Pulitzer Prize-winning photographer — have three adult children and love spending time in the Colorado mountains and traveling around the world.