What side effects are common after Pfizer-BioNTech vs Moderna mRNA COVID-19 vaccines?
Executive summary
Both Pfizer‑BioNTech (Comirnaty) and Moderna (Spikevax and related Moderna mRNA products) use the same mRNA technology and in clinical trials showed similar high efficacy initially, and both commonly cause short‑term local and systemic reactions such as sore arm, fever, chills and fatigue [1] [2]. Regulators continue to track rare but serious events: the FDA expanded myocarditis/pericarditis warnings for males aged 16–25 after recent mRNA formulations, applying the caution to both Pfizer and Moderna [3].
1. What side effects are commonly reported — the short list
Clinical trials and public information repeatedly describe the most common reactions as local pain at the injection site and systemic symptoms like fever, chills, fatigue and muscle aches within hours to a day after vaccination; sources note a sore arm immediately after vaccination and systemic effects commonly developing within 8–12 hours [2] [1]. Health summaries comparing vaccines emphasize that both mRNA products produce similar immediate side‑effect profiles tied to the immune response [1].
2. How similar are Pfizer and Moderna in their side‑effect profiles?
Yale Medicine and other comparators state Moderna and Pfizer‑BioNTech “use the same mRNA technology” and had comparable efficacy when authorized, and the implication in multiple summaries is that side‑effect types are broadly similar across the two mRNA platforms [1]. FactCheck.org also treats the updated Comirnaty and Spikevax formulations as two of the updated mRNA vaccines in current use, further supporting a practical similarity in expected adverse reactions [4].
3. Differences that have been highlighted (age, dose, and label changes)
Regulators and reporting have pointed to differences in risk magnitude for specific rare harms: the FDA told both companies to expand warnings after reviewing seasonal/formulation data showing the highest estimated incidence of myocarditis/pericarditis in males 16–25, and the agency explicitly applied that expanded warning to both Pfizer and Moderna vaccines [3]. Some product fact sheets and clinical trial dosing differ (for example, Pfizer historically used 30 µg, some Moderna formulations used higher µg doses), which can influence reactogenicity, but available reporting here emphasizes regulatory warnings rather than precise comparative rates [5] [1].
4. What regulators say about myocarditis/pericarditis
The FDA publicly ordered expanded labeling on mRNA vaccines to warn about possible heart‑inflammation side effects, noting the highest estimated incidence in males 16–25 after the 2023–2024 formula and applying the label change to both Comirnaty and Spikevax [3]. The CDC has previously described post‑vaccine myocarditis/pericarditis as rare and told advisers that acute myocarditis “tends to resolve quickly” in many cases, but the FDA’s move signals continued surveillance and a narrower age band for heightened attention [3].
5. What the clinical literature and reviews add
Systematic and observational studies have generally affirmed favourable safety profiles for both mRNA vaccines in preventing COVID‑19, while some cross‑sectional and local studies compare adverse effects across vaccine brands; one peer‑reviewed cross‑sectional study cited limited research directly comparing adverse‑effect severity across Pfizer, Moderna and other vaccines, indicating more data were needed at that time [6]. Major science outlets also report ongoing research into broader immunologic effects of mRNA vaccination, reflecting active study rather than settled controversy [7].
6. Where reporting diverges and what’s missing
News analysis pieces highlight corporate and political developments around Moderna and vaccine contracts, which may color public perceptions of safety and priorities but do not alter the medical data about common short‑term side effects [8]. Detailed, head‑to‑head numerical rates for each specific side effect (for example exact percent rates of fever or local pain by brand and dose across age groups) are not fully enumerated in the provided sources; those granular comparative statistics are “not found in current reporting” among the supplied items (available sources do not mention exact side‑by‑side percentages).
7. Practical takeaway for readers deciding between vaccines
Both mRNA vaccines typically cause transient local and systemic reactions as the immune system responds (sore arm, fever, chills, fatigue), and regulators now require both manufacturers to warn about a small risk of myocarditis/pericarditis in young males aged 16–25 [2] [3]. Public health guidance continues to treat the vaccines as safe and effective overall, while emphasizing surveillance and updated formulations—so individual vaccine choice should consider age, approved age ranges and the most recent product fact sheets and clinical advice [1] [4].