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What short-term side effects are most common after the Moderna COVID-19 vaccine and how do they compare to Pfizer?
Executive summary
Clinical trial fact sheets and multiple news analyses show the most common short‑term reactions to both Moderna and Pfizer mRNA COVID‑19 vaccines are injection‑site pain, fatigue, headache, muscle aches, chills and fever; Moderna trial documents report higher percentages for several symptoms (for example, Moderna lists fatigue 64.7% and muscle pain 61.5% in one fact sheet summary) while Pfizer’s trial summaries report lower point estimates for some symptoms (e.g., fatigue reported as 3.8% in some summaries cited) [1] [2]. Large surveillance and reporting studies also found most reactions are mild, peak the day after vaccination, and resolve within a day or two [3] [4].
1. What people most commonly experience after an mRNA shot — the short list
Both vaccines produce a predictable set of short‑term, reactogenic symptoms: pain, redness or swelling at the injection site plus systemic symptoms such as fatigue, headache, muscle pain, chills and fever. The CDC and multiple news outlets summarized these as the most frequent reactions after either Pfizer or Moderna doses [5] [6] [7].
2. How Moderna and Pfizer compare in reported frequencies
Multiple reports and fact‑sheets published in 2021 indicate Moderna recipients tended to report higher rates of several symptoms than Pfizer recipients in trials and surveillance. One compilation of Moderna trial data lists headache (64.7%), muscle pain (61.5%), joint pain (46.4%), chills (45.4%), nausea/vomiting (23%) and fever (15.5%) [1]. Other outlets quoting manufacturers’ trial breakdowns note Moderna reported fatigue at 9.7% versus Pfizer’s 3.8% and headache 4.5% versus Pfizer’s 2% in the specific figures cited [2] [8]. Independent surveillance (V‑safe) and later studies also found more self‑reported side effects after Moderna than Pfizer in real‑world data [3] [9].
3. What the big studies say about severity and duration
A CDC‑linked peer‑reviewed analysis summarized in reporting found roughly 92% of self‑reported side‑effects from Pfizer and Moderna were mild and faded within a day or two; less than 1% of people reporting side effects sought medical care, and no causal link to deaths was found in that dataset [4]. JAMA Insights–referenced reporting on V‑safe data found reactions peak on day one and decline through day seven after vaccination for both vaccines [3].
4. Why Moderna might show more side effects — plausible explanations
Reporting attributes some of the Moderna–Pfizer differences to reactogenicity linked to dose and formulation: Moderna’s authorized primary dose was 100 micrograms vs. Pfizer’s 30 micrograms, and higher antigen or adjuvant effect can increase short‑term inflammation and symptoms. Media summaries and product comparisons flagged the higher Moderna dose as a likely contributor to more frequent reactogenic reports [10]. Independent symptom‑report aggregations also found more mentions per report for Moderna in the samples they analyzed [9] [11].
5. Limits of the available comparisons and data caveats
Comparisons mix clinical‑trial summaries, manufacturer self‑reported percentages, passive surveillance (VAERS) and active surveillance (V‑safe) and third‑party user reports; each has biases. Manufacturer trial figures quoted in some articles focus on specific trial subgroups or definitions and may not be directly comparable across studies [2] [8]. Aggregated symptom‑report services and small convenience samples can overstate differences because of reporting bias [11] [9]. Available sources do not present a single harmonized head‑to‑head table that reconciles every analytic method.
6. How boosters and dose number change the picture
Booster‑dose data show similar patterns: both mRNA boosters produce local and systemic reactions, and Moderna booster recipients in some reports had higher rates of fatigue, muscle pain and headache than Pfizer booster recipients; overall side effects remained mostly mild to moderate and self‑limited [12] [7].
7. Practical takeaways for people getting vaccinated
Expect local soreness and possible systemic symptoms (fatigue, headache, muscle aches, chills, fever) that are usually strongest the day after vaccination and resolve within a couple of days [3] [4]. Moderna has been reported to produce these symptoms more often than Pfizer in multiple data sources, but most reactions are short‑lived and non‑serious [9] [1]. If you have concerns about severe allergic reactions, ask your provider — current reporting emphasizes that severe events are rare and that monitoring after vaccination is routine [5] [4].
Sources cited above draw from manufacturer trial summaries, news analyses of CDC/FDA documents, and surveillance studies reported in mainstream outlets; differences in percentages reflect divergent datasets and methodologies rather than a contradiction in the basic finding that both vaccines are reactogenic but generally safe [2] [1] [3] [4].