What Are the Side Effects of COVID-19 Vaccines?

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En español | As the coronavirus vaccine rollout continues across the country, health experts say one thing is critical for people to understand before they roll up their sleeves: The vaccines may cause side effects. 

Three vaccines — developed by Moderna, Pfizer-BioNTech and Johnson & Johnson — have been authorized by the U.S. Food and Drug Administration (FDA) to combat the coronavirus. However on April 13, federal health officials called for a pause in the use of Johnson & Johnson's coronavirus vaccine “out of an abundance of caution.” (More on this below.) Johnson & Johnson’s vaccine is a single shot; both the Moderna and Pfizer-BioNTech vaccines require two doses.

Not everyone will experience side effects, but most people who do will have flu-like symptoms that resolve in a few days, including:

• Injection site pain and swelling
• Fatigue
• Headache
• Chills
• Fever
• Muscle and joint pain
• Nausea
• Delayed swelling, redness or a rash at the injection site
• Swollen lymph nodes (typically manifests as a lump in your armpit or above your collarbone) 

The side effects are similar for the three vaccines and are an indication that the vaccines are helping to build protection against disease. Since you may feel under the weather, experts recommend not making any big plans for a few days after you get a dose of the vaccine. 

“Where a mistake could be made is in people being surprised or not being prepared for side effects,” says William Moss, M.D., executive director of the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health.

CDC data released Feb. 19 indicated that the side effects from the Moderna and Pfizer-BioNTech vaccines were as expected and not serious among the vast majority of the first 22 million people who received them. 

Side effects are a sign the vaccine is working

Side effects from vaccines are not uncommon. The seasonal flu shot, for example, can cause fever and fatigue, among other reactions. And the vaccine to prevent shingles can induce shivering, muscle pain and an upset stomach.

In some ways, these mild to moderate reactions are “a good thing,” Moss says, because they are “a sign that the immune system is responding to the vaccine.” 

The key, experts say, is to weigh the temporary discomfort against the long-term benefits: a potentially high level of protection from a disease that has uprooted everyday life for many of us and has killed more than 2.5 million people globally.

“We are willing to tolerate discomfort in other aspects of our life — many people exercise and have muscle aches afterward, and don’t say, ‘I’m never going to exercise again,’ ” Moss points out. “There are just many aspects of our lives where we need to be willing to make the trade-off of some degree of discomfort for a longer-term gain.”

However, if your side effects are worrying or don’t fade after a few days, contact your health care provider. The same goes if you experience redness or tenderness where you got the shot that gets worse after 24 hours.

Reports of adverse events

Adverse events — which the CDC defines as any serious health problem that happens after a shot — are rare, but have been reported.

Most notably, the CDC and FDA on April 13 recommended that U.S. vaccination sites pause their use of the Johnson & Johnson vaccine while the agencies review data involving six reported cases of a rare but serious type of blood clot, called cerebral venous sinus thrombosis, in individuals after receiving the vaccine. One case was fatal and one patient is in critical condition.

All six cases occurred in women between the ages of 18 and 48, six to 13 days after vaccination. People who recently received the Johnson & Johnson vaccine and develop symptoms of severe headache, abdominal pain, leg pain or shortness of breath within three weeks after vaccination should contact their health care provider, officials say.

Also, providers should be on the lookout for patients who present with these symptoms and be advised that “treatment of this specific type of blood clot is different from the treatment that might typically be administered,” says a joint statement from the CDC and FDA. “Usually, an anticoagulant drug called heparin is used to treat blood clots. In this setting, administration of heparin may be dangerous, and alternative treatments need to be given.”

The CDC is planning on an expeditious review of the Johnson & Johnson vaccine and will update guidance as soon as possible.

To date, no cases of cerebral venous sinus thrombosis have been linked to the Moderna and Pfizer-BioNTech vaccines. However, a small number of allergic reactions, including anaphylaxis, have been reported, according to early safety monitoring data from the CDC.

Because of this, the CDC recommends that anyone who has ever had a severe allergic reaction to any ingredient in a COVID-19 vaccine abstain from receiving it. (You can find the ingredients of authorized vaccines on the FDA's website.) If you’ve had an allergic reaction to a vaccine for another disease, ask your health care provider if you should get a COVID-19 vaccine.

When you go to get your COVID-19 vaccine, expect to be monitored after your shot. People with a history of anaphylaxis should be observed for 30 minutes after vaccination; other recipients should be observed for 15 minutes.

Still, severe allergic reactions are “exceedingly rare,” the CDC says, and should not discourage people from getting vaccinated.

Older adults could experience fewer side effects

While the coronavirus vaccines have been shown to be effective in older adults, people age 50 and older experience fewer side effects than younger recipients. 

Only about 25 percent of people ages 50 to 64 and 4 percent of those ages 65 to 74 who received the Moderna or Pfizer-BioNTech vaccine between Dec. 14 and Jan. 13 experienced side effects, according to CDC data. Meanwhile, 65 percent of those under 50 reported a reaction.

The clinical trial data from the Johnson & Johnson vaccine showed a similar effect. 

Researchers are still studying why this is the case, but they say it’s likely related to the declining immune response that comes with age. 

Studies also show that most people experience more severe side effects after the second dose of the Moderna or Pfizer-BioNTech vaccine.

How to treat side effects

Although side effects may affect your ability to do daily activities, most should go away on their own after a few days, the CDC says. Plan for plenty of time to rest in the days immediately after you get a dose of the vaccine.

If you have pain or discomfort, an over-the-counter pain reliever such as a nonsteroidal anti-inflammatory drug (Advil, Motrin) or acetaminophen (Tylenol) can help you feel better, doctors say.

“If your fever is making you uncomfortable, taking acetaminophen or a nonsteroidal anti-inflammatory should bring it right down,” says Dean Blumberg, M.D., an infectious disease specialist at UC Davis Health in Sacramento, California.

The CDC advises against the use of pain relievers before vaccination “for the purpose of preventing post-vaccination symptoms.” So wait until after you are experiencing side effects to take any medication.

If you have a delayed reaction at the injection site – typically described as a rash, itchiness or redness that appears five to 10 days after vaccination – it’s likely a mild allergic reaction, Blumberg says. He recommends treating it with an over-the-counter antihistamine like Benadryl or a topical steroid like hydrocortisone.

Another side effect that may last more than a few days is a swollen lymph node, which may feel like a lump under your armpit or over your collarbone. The swelling is not harmful, but it can last a few weeks, Blumberg says. Eventually, it should go away on its own.

Safety monitoring doesn't stop 

Just because the vaccines have expanded from trial participants to the public doesn’t mean monitoring for them will stop. Individuals who receive the vaccines will continue to be watched for long-term side effects and adverse events or disease. 

One way health officials are tracking side effects is with an app called V-safe that you download to your smartphone. When you get your first dose of the vaccine, your health care provider will give you information about how to get started. 

Infectious disease experts urge vaccine recipients to participate because it gives them important safety information about the vaccine. The daily survey takes only about 30 seconds, and the app protects your privacy by erasing your phone number after you take the survey. 

Editor's note: This story, originally published Dec. 2, has been updated to reflect new information.

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