What Are the COVID-19 Vaccine Side Effects? Here’s What Doctors Know So Far

Side effects are usually mild—and a sign that the coronavirus vaccine is doing its job.

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Once a distant dream, the coronavirus vaccine is now a reality. More than 10% of American adults have been fully vaccinated against COVID-19, and that number is expected to rise quickly as rollout kicks into high gear.

President Joe Biden said during a March 2 press conference that the U.S. will have enough COVID-19 vaccine doses for every adult in America by the end of May. “That’s progress,” he said, per CNN. “We must remain vigilant, act fast and aggressively, and look out for one another. That’s how we’re going to get ahead of this virus, get our economy going again, and get back to our loved ones.”

There are three COVID-19 vaccines that have received FDA authorization based on clinical testing: the Pfizer-BioNTech, Moderna, and Johnson & Johnson vaccines.

Vaccine distribution differs among states, so your spot in line may look a bit different compared to another person across the country. But depending on where you live, you may be able to receive your dose soon.

So, it’s totally understandable to have questions about how the vaccines work, what kind of potential side effects they could have, and why it’s important to get one when you become eligible, so that you feel fully informed by the time your turn comes to get vaccinated. Here’s everything you should know.

First, what’s in the COVID-19 vaccines?

It depends on the vaccine. For example, Pfizer’s and Moderna’s respective mRNA vaccines contain similar ingredients that are just packaged a bit differently, says infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security.

Vaccines contain ingredients like preservatives (to prevent contamination), adjuvants like aluminum salts (to help boost the body’s response to the vaccine), and stabilizers like sugar or gelatin (to keep the vaccine effective after it’s manufactured), according to the Centers for Disease Control and Prevention (CDC).

“A vaccine has got to have materials in it to make sure that it is stable and can really function,” says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. “No vaccine is just purely the antigen,” he adds, which is any substance that causes your immune system to produce antibodies to it.

How do the COVID-19 vaccines work?

The Pfizer and Moderna vaccines use messenger RNA (mRNA), a newer technology that encodes a part of the spike protein gene in SARS-CoV-2, the novel coronavirus. This is the part of the virus that latches onto human cells. The mRNA vaccines do not inject live or inactive virus into your body, but rather use pieces of genetic material from SARS-CoV-2.

Essentially, your cells are given instructions to develop a piece of the spike protein (the antigen), according to the CDC. This triggers an immune response and you develop antibodies unique to SARS-CoV-2. Your body eliminates the protein and the mRNA, but those antibodies stick around to help protect you from future COVID-19 infection. (It’s also important to note that mRNA does not alter your DNA, per the CDC.)

The Johnson & Johnson vaccine is a viral vector vaccine, which means it uses a different, inactivated virus to deliver instructions in the form of a gene to your cells, the CDC explains. For this vaccine, a modified adenovirus (a common cold virus that cannot reproduce in the body or cause illness) delivers a gene that also instructs your cells to make a piece of the coronavirus spike protein. This triggers production of those infection-fighting antibodies, so your immune system remembers how to respond to the spike protein should you get infected in the future.

What are the potential side effects of the COVID-19 vaccine?

It’s important to remember that you will be briefed on side effects before you are given your first dose of the vaccine, Dr. Schaffner says. Since healthcare workers were offered the vaccine first, your doctor can offer information to you based on personal experience, as well as newly released data.

What experts have learned so far is promising. Data from both animal and human trials show that the vaccines have a “favorable safety profile” Dr. Adalja says. Plus, it’s important to remember that these are potential side effects—meaning you may experience none at all.

Each vaccine is slightly different but, in general, experts say it is possible to have the following side effects with any of the COVID-19 vaccines:

  • sore arm at the injection site
  • fever or chills
  • fatigue (tiredness)
  • headache
  • joint pain
  • muscle aches

    “This is similar to what you’d expect with the flu vaccine,” says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. “[The side effects] usually only last a day or so. They’re not serious or concerning.” For example, the flu shot can also cause arm soreness, swelling at the injection site, a low-grade fever, and other flu-like symptoms.

    Vaccine side effects “basically show that the immune system is being primed,” says Richard Watkins, M.D., an infectious disease and professor of internal medicine at the Northeast Ohio Medical University. Remember, your body is learning to mount a response to SARS-CoV-2, so that can lead to short-lived symptoms.

    It’s also important to note that the second dose may cause more side effects than the first dose if you get the Pfizer or Moderna vaccine, says Dr. Schaffner.

    How common are the potential side effects from the COVID-19 vaccine?

    Moderna shared in mid-November that the following side effects were the most common among patients in its trial:

    • fatigue (9.7%)
    • muscle aches (8.9%)
    • joint pain (5.2%)
    • headache (4.5%)
    • pain (4.1%)
    • injection site pain (2.7%)
    • redness at the injection site (2%)

      Pfizer shared that the following side effects happened in some patients:

      • fatigue (3.8%)
      • headache (2%)

        The FDA says that patients experienced these side effects with the Johnson & Johnson vaccine, but did not specify how often they occurred:

        • pain at the injection site
        • redness at the injection site
        • swelling at the injection site
        • headache
        • fatigue
        • muscle aches
        • nausea
        • fever

          In addition to redness and swelling around the injection site shortly after vaccination, a small number of people are also experiencing a delayed arm rash with the Pfizer and Moderna vaccines. This can occur seven to 10 days after a person receives their dose (most often the second one). While it can be annoying due to redness, swelling, or itching, experts say it resolves on its own and goes away within a few days after it appears.

          What about long-term side effects?

          Even though the COVID-19 vaccines have undergone thorough testing, the results of which were closely evaluated by both the FDA and CDC, it will take some time for medical experts to gather data on potential long-term side effects. Dr. Russo notes that vaccine makers, as well as the FDA, will continue to gather detailed data as the vaccines continue to be released to the general public.

          How to tell the difference between vaccine side effects and actual coronavirus symptoms

          Just like the flu shot can’t give you the flu, the COVID-19 vaccine will not give you COVID-19. However, there is still a chance that you can become infected with COVID-19 right after (or before) you’ve been vaccinated. The risk is greatly reduced afterward, but only after your body has the time to build strong immunity to the virus, which can take a few weeks.

          If you do get infected, “the symptoms may be indistinguishable with the exception of shortness of breath, which should not occur post-vaccine,” Dr. Adalja says. “Loss of taste and smell is also not something you experience after the vaccine.”

          Any side effects that follow your immunization will usually only last a day or two, while true COVID-19 symptoms will persist for longer. If you have mild symptoms that last two to three days post-vaccination (for any authorized vaccine) or occur in between vaccine doses (for Pfizer or Moderna), call your doctor for guidance.

          Should I worry about allergic reactions?

          If you have a history of a severe allergic reaction to any of the ingredients in a particular vaccine or have a history of a severe allergic reaction to any vaccine or injectable therapy, ask your doctor if you should get the COVID-19 vaccine.

          While severe allergic reactions to the vaccine can happen, they are rare. A January report from the CDC found that the risk of developing a severe allergic reaction after the vaccine is 0.001%, based on data available so far. Usually, allergic reactions happen within 10 to 15 minutes of receiving your dose, based on data we have from the mRNA vaccines, Dr. Adalja says. That’s why the CDC recommends sticking around after your vaccination for 15 minutes if you have no history of allergies and 30 minutes if you do.

          It’s possible to develop symptoms later, like a red rash around the injection site. If you’re bothered or concerned in any way by a reaction, Dr. Watkins recommends calling your doctor. If you develop symptoms of a severe allergic reaction, like facial swelling or difficulty breathing, seek medical attention immediately.

          Why is it important to get the COVID-19 vaccine?

          Getting the vaccine has several benefits, Dr. Adalja says. The big one? We can safely establish community immunity, so you, your loved ones, and those who are most vulnerable can be protected from the virus if a threshold of immunization is reached. Getting vaccinated, above all, offers protection against severe complications of the virus if a person becomes sick, preventing the risk of hospitalization or death.

          It’s a tall order, as experts estimate that roughly 70% of people in the U.S. (200 million) need to be vaccinated to reach this level of protection for COVID-19 specifically. But with the approval of a third vaccine and an ongoing rollout, the U.S. is finally starting to see a dip in confirmed coronavirus infections. Until then, keep following recommended COVID-19 prevention guidelines, like wearing a mask, social distancing, and washing your hands often.

          This article is accurate as of press time. However, as the COVID-19 pandemic rapidly evolves and the scientific community’s understanding of the novel coronavirus develops, some of the information may have changed since it was last updated. While we aim to keep all of our stories up to date, please visit online resources provided by the CDC, WHO, and your local public health department to stay informed on the latest news. Always talk to your doctor for professional medical advice.

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