The most common side effect of the Moderna COVID-19 vaccine is pain at the injection site, reported by roughly 90% of adults aged 18 to 64 after their second dose. Beyond that soreness, fatigue, headache, and muscle aches are the next most frequent reactions, each affecting more than 60% of adults in clinical data. These side effects are temporary, typically resolving within a day or two, and reflect your immune system responding to the vaccine.
Local Reactions at the Injection Site
Pain where the needle went in is by far the most reported local reaction across every age group. In adults under 65, about 90% experienced injection site pain after the second dose. For adults 65 and older, the rate was slightly lower at around 83%. Children aged 6 to 11 had the highest rate at nearly 95%.
Some people also develop swelling, redness, or tenderness in the lymph nodes near the injection site (often in the armpit). Redness and swelling each affected roughly 9 to 19% of recipients depending on age, while lymph node tenderness was reported by 8 to 21%. A small number of people experienced delayed skin reactions, particularly local redness appearing after the second dose, but these mostly cleared up within about 10 days.
Systemic Side Effects in Adults
After the second Moderna dose, adults aged 18 to 64 reported systemic reactions at these rates:
- Fatigue: 67.6%
- Headache: 62.8%
- Muscle aches: 61.3%
- Chills: 48.3%
- Joint pain: 45.2%
- Nausea or vomiting: 21.3%
- Fever: 17.4%
Older adults (65 and up) generally experienced these same side effects at lower rates. This pattern holds across most vaccines: the stronger immune response typical of younger adults tends to produce more noticeable reactions.
Side Effects in Children and Teens
Children’s side effects vary by age in ways that reflect how kids express discomfort. In infants and toddlers (6 months to 2 years), the most common reactions were irritability or crying (64%), sleepiness (35%), and loss of appetite (32%). Injection site pain was reported less frequently in this group, partly because very young children can’t easily describe localized pain.
In children aged 2 to 5, fatigue (48%) and fever (16%) were the leading systemic reactions. Headache and muscle pain each affected about 16% of this group. By ages 6 to 11, the pattern starts looking more like adults: fatigue hit 64.5%, headache 54.3%, and muscle pain 28.2%.
Teenagers aged 12 to 17 had the most adult-like reaction profile, with headache (70.2%) and fatigue (67.8%) topping the list. Nearly 47% reported muscle pain, and 43% experienced chills. Fever was less common in this group at about 12%.
The Second Dose Typically Hits Harder
If you felt fine after your first Moderna shot but rough after your second, that’s a well-documented pattern. Both Moderna and Pfizer mRNA vaccines produce a higher frequency of fever and other systemic reactions after the second dose compared to the first. This happens because your immune system has already been primed by the first dose. When it encounters the vaccine’s instructions a second time, it mounts a faster, stronger response, which translates into more noticeable symptoms.
The numbers listed throughout this article reflect second-dose reactions for that reason. First-dose reactions follow the same pattern but are generally milder and less frequent.
Why the Vaccine Causes These Reactions
The side effects aren’t random. When the vaccine is injected, your innate immune system kicks into action and releases a flood of inflammatory signaling molecules. These molecules are the same ones your body produces when fighting an infection, and they’re directly responsible for the familiar constellation of symptoms: muscle aches, chills, fatigue, fever, headache, and soreness at the injection site.
The lipid nanoparticles that carry the mRNA (essentially tiny fat bubbles that protect the vaccine’s genetic instructions) also trigger some of this inflammatory response on their own. This is part of why even the local reaction at the injection site can be so pronounced. The inflammation is a feature, not a bug. It helps recruit immune cells to the area so they can learn to recognize the spike protein and build lasting protection.
How Long Side Effects Last
Most reactions appear within a day of vaccination and resolve within one to three days. Injection site pain often starts within hours and is usually the first symptom to show up and the last to fully fade. Systemic symptoms like fatigue, headache, and chills tend to peak on the day after vaccination and clear up quickly. Swollen lymph nodes occasionally take a bit longer but generally resolve within a day or two.
You can manage discomfort with a cool, damp cloth on the injection site and over-the-counter pain relievers. Moving your arm throughout the day helps reduce stiffness. Staying hydrated and resting are the most practical things you can do while your immune system does its work. If you develop a fever, lightweight clothing and fluids will help more than bundling up.
Serious side effects from the Moderna vaccine are rare. The common reactions described here, while sometimes unpleasant, are short-lived signs that your body is building immune protection.