Side Effects of the 2025 Moderna COVID Booster
The most common side effects from the Moderna COVID booster are injection site pain (experienced by nearly 100% of recipients), fatigue (68.5%), headache (63%), muscle pain (59.6%), joint pain (44.8%), and chills (43.4%), with most symptoms resolving within 1-3 days after vaccination. 1, 2, 3
Most Common Side Effects
The side effect profile for Moderna mRNA vaccines is well-established across multiple doses:
- Injection site pain: Nearly universal (91.6-100% of recipients) 1, 3
- Fatigue: 57-68.5% of recipients 1, 3
- Headache: 53-63% of recipients 1, 3
- Muscle pain (myalgia): 38.3-59.6% of recipients 1, 2
- Chills: 31.9-72% of recipients 1, 3
- Joint pain (arthralgia): 23.6-44.8% of recipients 1, 2
- Fever: 14.2-51% of recipients 1, 3
Duration of Side Effects
Side effects typically last 1-3 days after vaccination, with most symptoms resolving within 1-2 days. 2, 4
- The majority of side effects occur within the first day after receiving the booster (61.4% of cases) 3
- Median duration for common symptoms like fatigue, local pain, general pains, and headache is 1-2 days 4
- Most side effects are self-limiting and resolve without intervention 2
Severity Considerations
Severe adverse reactions occur in only 0.2-9.7% of participants, and these are more frequent after the second dose than the first dose. 1
- Only 11% of healthcare workers experienced moderate-to-severe side effects after their mRNA booster 4
- Severe reactions are generally less frequent in participants aged ≥65 years 1
- Most adverse events are mild to moderate (grade 1 or 2) 5
Key Predictors of More Severe Side Effects
Certain factors increase the likelihood of experiencing moderate-to-severe side effects:
- Previous moderate-to-severe side effects: The strongest predictor (OR 3.64), meaning if you had significant side effects with a previous dose, you're more likely to experience them again 4
- Female sex: Women have higher odds of moderate-to-severe side effects (OR 1.49) 4
- Younger age: Side effects tend to be more frequent in younger individuals compared to those ≥65 years 1
Important Clinical Caveats
While side effects are common, serious adverse events remain rare. The frequency of serious adverse events with Moderna is similar to placebo (1.0% for both groups) 1. However, providers should be aware of:
- Myocarditis risk: Highest after the second mRNA dose in young males aged 12-29 years (39-47 cases per 1 million), though most cases are mild and self-limiting 6, 5
- Chest pain evaluation: Any chest pain occurring early after mRNA vaccination should prompt evaluation with ECG, cardiac troponin, and echocardiogram 6, 5
Comparative Context
The Moderna vaccine has a slightly higher rate of side effects compared to Pfizer/BioNTech, but both are considered safe and effective. 7 The benefits of vaccination—including prevention of hospitalization, ICU admission, and death—substantially outweigh the temporary discomfort of side effects 6, 5.