You might feel like crap for a day or two after getting vaccinated — but that doesn't mean you should avoid it.
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Just a few short days after Pfizer's COVID-19 vaccine received emergency use authorization from the Food and Drug Administration, some people are already getting vaccinated. On December 14, 2020, the first doses of Pfizer's vaccine were given to health workers and nursing home staffers. In the coming weeks and months, the vaccine will continue to be rolled out to the general population, with essential workers and older adults being among the first to receive doses after high-risk health-care professionals. (See: When Will a COVID-19 Vaccine Be Available — and Who Will Get It First?)

It's an exciting time, but if you've been seeing reports about the COVID-19 vaccine's "intense" side effects, you probably have some questions about what to expect when it's your turn to get the shot. Here's what you need to know about COVID-19 vaccine side effects.

First, a recap on how the COVID-19 vaccine works.

The COVID-19 vaccines from Pfizer and Moderna — the latter of which is expected to receive emergency authorization in a matter of days — use a new type of vaccine called messenger RNA (mRNA). Instead of putting an inactive virus in your body (as done with the flu shot), mRNA vaccines work by encoding a part of the spike protein that's found on the surface of SARS-CoV-2 (the virus that causes COVID-19). Those pieces of the encoded protein then trigger an immune response in your body, leading you to develop antibodies that can protect you from the virus should you become infected, Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Centers for Health Security, previously told Shape. (More here: How Effective Is the COVID-19 Vaccine?)

Think of the encoded protein pieces as a genetic "fingerprint" for the SARS-CoV-2 virus, says Thad Mick, Pharm.D., vice president of pharmaceutical programs and diagnostic services at ZOOM+Care. "The goal of the COVID-19 vaccines is to introduce that viral fingerprint that warns your body early on so that the immune system recognizes it doesn't belong there and builds up an immune response to it before the virus has a chance to overtake your natural defenses," he explains.

In the process of building up that immune response, it's normal to experience some side effects along the way, adds Mick.

What kind of COVID-19 vaccine side effects should I expect?

As of now, we only have preliminary research on the safety data side effects of Pfizer's and Moderna's COVID-19 vaccines. Overall, though, Pfizer's vaccine is said to have a "favorable safety profile," while Moderna's similarly shows "no serious safety concerns." Both companies say they're continuing to collect safety (and efficacy) data to confirm these findings.

That said, as with any vaccination, you could experience some side effects from a COVID-19 vaccine. The Centers for Disease Control and Prevention lists these potential COVID-19 vaccine side effects on its website:

  • Pain and swelling at the injection site
  • Fever
  • Chills
  • Fatigue
  • Headache

Other COVID-19 vaccine side effects may include muscle aches and joint pain, adds Mick. "From what we know, most side effects will likely appear in the first day or two after receiving the vaccine, but may potentially present later," he explains. (It's worth noting that flu shot side effects are relatively similar.)

If these side effects sound a lot like symptoms of COVID-19, that's because they basically are. "The vaccine stimulates the immune system to fight the virus," explains Richard Pan, M.D., a pediatrician and California state senator. "Most of the side effects are symptoms of that response such as fever, fatigue, headaches, and muscle aches."

However, that doesn't mean the COVID-19 vaccine can give you COVID-19, notes Dr. Pan. "It is important to remember that the mRNA [from the vaccine] does not permanently affect any of your cells," he explains. Rather, that mRNA is just a temporary blueprint of the spike protein located on the surface of the virus. "This blueprint is very fragile, which is why the vaccine needs to be kept so cold before it is used," says Dr. Pan. Your body eventually eliminates that blueprint after you have been vaccinated, but the antibodies you develop in response will remain, he explains. (The CDC notes that more data is needed to confirm just how long antibodies built from COVID-19 vaccines will last.)

"It is impossible to catch COVID-19 from the vaccine, just like having a blueprint for building a steering wheel doesn't give you the plans to build an entire car," adds Dr. Pan.

How common are COVID-19 vaccine side effects?

The FDA is still evaluating data on exactly how common the above COVID-19 side effects might be in the general population. For now, though, the information released by Pfizer and Moderna on their large-scale clinical trials suggests that a small number of people will experience "significant but temporary symptoms" after receiving a COVID-19 vaccine, says Dr. Pan.

More specifically, in Moderna's trial of its COVID-19 vaccine, 2.7 percent of people experienced injection site pain after the first dose. Following the second dose (which is given four weeks after the first shot), 9.7 percent of people experienced fatigue, 8.9 percent reported muscle aches, 5.2 percent had joint pain, 4.5 percent reported a headache, 4.1 percent experienced general pain, and 2 percent said the second shot left them with redness at the injection site.

So far, Pfizer's COVID-19 vaccine side effects appear to be similar to Moderna's. In Pfizer's large-scale trial of its vaccine, 3.8 percent of people reported fatigue and 2 percent experienced a headache, both after the second dose (which is given three weeks after the first injection). Less than 1 percent of people in the clinical trial reported a fever (defined in the research as a body temperature above 100°F) after either the first or second dose. A small number (0.3 percent, to be exact) of vaccine recipients also reported swollen lymph nodes, "which generally resolved within 10 days" of vaccination, according to the research.

While these side effects are temporary and don't appear to be that common, they can be "significant" enough that some people "may need to miss a day of work" after getting vaccinated, notes Dr. Pan.

You might have also heard concerns about allergic reactions to Pfizer's COVID-19 vaccine. Shortly after the vaccine rolled out in the UK, two health-care workers — who both routinely carry an EpiPen and have a history of allergic reactions — experienced anaphylaxis (a potentially life-threatening allergic reaction characterized by impaired breathing and a drop in blood pressure) following their first dose, according to the New York Times. Both health-care workers have recovered, but in the meantime, health officials in the UK have issued an allergy warning for Pfizer's COVID-19 vaccine: "Any person with a history of anaphylaxis to a vaccine, medicine, or food should not receive the Pfizer/BioNTech vaccine. A second dose should not be given to anyone who has experienced anaphylaxis following administration of the first dose of this vaccine." (Related: What Happens When You Go Into Anaphylactic Shock?)

In the U.S., a fact sheet from the FDA on Pfizer's COVID-19 vaccine similarly states that "individuals with known history of a severe allergic reaction (e.g. anaphylaxis) to any component of the Pfizer-BioNTech Covid-19 Vaccine" should not be vaccinated at this time. (You can find the full list of ingredients in the Pfizer vaccine in the same fact sheet from the FDA.)

Why You Should Get a COVID-19 Vaccine, Regardless of Side Effects

Truth is, you might feel like crap for a day or two after you receive a COVID-19 vaccine. But all in all, COVID-19 vaccines are "much safer" than the virus itself, which has already killed approximately 300,000 people in the U.S., says Dr. Pan.

COVID-19 vaccines will not only help you avoid serious COVID-19 complications, but they'll also help protect people who can't be vaccinated yet (including those with severe allergic reactions, pregnant people, and those who are younger than 16 years old), adds Dr. Pan. (Wearing your mask, social distancing, and washing your hands will also continue to be important in protecting people from COVID-19.)

"While many are concerned about the COVID-19 vaccine, there are many benefits to getting vaccinated," explains Mick. "These vaccines are being thoroughly evaluated and will only hit the market if any risks of the vaccine are outweighed by the benefits."

The information in this story is accurate as of press time. As updates about coronavirus COVID-19 continue to evolve, it's possible that some information and recommendations in this story have changed since initial publication. We encourage you to check in regularly with resources such as the CDC, the WHO, and your local public health department for the most up-to-date data and recommendations.