Most people who get a COVID-19 vaccine experience mild, short-lived side effects that clear up within a few days. The most common reactions are soreness at the injection site, fatigue, headache, muscle aches, and low-grade fever. These are signs your immune system is responding to the vaccine, not signs that something has gone wrong. Some people feel nothing at all.
Common Side Effects and When They Appear
Side effects typically show up within a day or two of vaccination. The most frequently reported reactions include pain, redness, or swelling where the shot was given, along with fatigue, headache, muscle pain, chills, joint pain, and fever. These tend to be slightly more noticeable after the second dose of an mRNA vaccine compared to the first.
For most people, symptoms resolve within one to three days. You don’t need to take pain relievers before your shot to try to prevent side effects. The CDC specifically recommends against that approach. If you’re uncomfortable afterward, over-the-counter options like ibuprofen or acetaminophen can help, though it’s worth checking with your doctor first, especially for children.
Why Vaccines Cause These Reactions
The short answer is inflammation, and it’s intentional. mRNA vaccines activate several immune pathways that trigger your body to produce signaling molecules called cytokines, along with a surge of inflammatory immune cells. This is the same basic process your body uses to fight an actual infection, which is why vaccine side effects can feel like a mild version of being sick. The fatigue, achiness, and fever are byproducts of your immune system ramping up, not damage from the vaccine itself.
People who’ve already had a COVID-19 infection sometimes notice stronger side effects after vaccination because their immune system already recognizes parts of the virus and mounts a faster, more intense response.
Side Effects in Young Children
Children aged 6 months to 5 years generally experience similar types of side effects as older age groups, though the rates vary by age and vaccine brand. Local reactions like injection-site pain occurred in roughly 19% to 47% of young children depending on the dose and product. Fever was reported in about 11% to 31% of children, again depending on dose number and brand.
Younger children (6 months to 2 years) tended to have more systemic symptoms like fussiness and fever compared to 3- to 5-year-olds, while older toddlers reported more injection-site soreness. High fevers were uncommon. Very severe fever occurred in fewer than 0.5% of children across all age groups and doses. For parents managing a fussy child post-vaccination, a non-aspirin pain reliever and comfort measures like cool cloths and extra fluids are standard approaches.
Myocarditis and Pericarditis
The most closely watched rare side effect has been myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the tissue surrounding the heart). This risk is real but uncommon, and it follows a clear demographic pattern. It occurs most often in younger males after the second dose of an mRNA vaccine, typically within the first week.
CDC data from the initial vaccine rollout showed rates of about 12.6 cases per million second doses among people aged 12 to 39. The highest rates were in males aged 12 to 17 (roughly 67 cases per million second doses) and males aged 18 to 24 (about 56 per million second doses). By comparison, females in the same age groups had rates of 9 and 5.5 per million second doses, respectively. For adults over 50, rates dropped below 2.5 per million regardless of sex.
Importantly, ongoing safety monitoring has found no statistical signal for myocarditis with the updated 2024-2025 vaccines. Reporting rates are now similar to background rates of fewer than 2 cases per million doses. Most cases that did occur during the initial rollout were mild, with patients responding well to rest and anti-inflammatory treatment and recovering within days to weeks.
Symptoms to be aware of include chest pain, shortness of breath, and a feeling of a rapid or pounding heartbeat, especially in the week following vaccination.
Guillain-Barré Syndrome
Guillain-Barré syndrome, a rare neurological condition where the immune system attacks nerve cells and causes weakness or paralysis, was linked to the Johnson & Johnson (Janssen) COVID-19 vaccine. CDC studies found evidence of an increased risk in adults 18 and older after that specific vaccine. No similar signal was found for the Pfizer or Moderna mRNA vaccines. The J&J vaccine is no longer available in the United States, so this risk is largely historical at this point.
Serious Allergic Reactions
Anaphylaxis, a severe allergic reaction, can occur with any vaccine, including COVID-19 shots. It’s extremely rare, estimated at roughly 2 to 5 cases per million doses. This is why vaccination sites ask you to wait 15 minutes afterward for observation. Symptoms include difficulty breathing, swelling of the face or throat, rapid heartbeat, dizziness, and a widespread rash. These reactions happen quickly, almost always within 30 minutes, and are treatable on the spot.
If you’ve had an allergic reaction to a previous dose or to a specific ingredient in the vaccine, that’s something to discuss before getting another shot.
What the Latest Safety Data Shows
The CDC’s Vaccine Safety Datalink, which tracks millions of vaccinated individuals in near-real time, has not detected increased risks of myocarditis or other serious adverse events with the 2022-2023, 2023-2024, or 2024-2025 COVID-19 vaccine formulations. The safety profile of the updated vaccines has been consistent and reassuring across multiple surveillance seasons. The FDA continues to list myocarditis and pericarditis as a warning for mRNA and Novavax vaccines based on earlier post-marketing data, but the observed rates with current formulations are in line with what would be expected in the general population without vaccination.