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How do side effects of Moderna vaccine compare to AstraZeneca?
Executive Summary
Moderna and AstraZeneca vaccines both cause common short-term side effects such as injection-site pain, fatigue, headache, muscle pain, chills, fever, and nausea, but the available analyses show inconsistent, incomplete comparisons and some studies report higher reported rates with Moderna while others note more adverse reaction reports for AstraZeneca. The evidence base contains gaps in direct head-to-head comparisons and varied reporting that prevent a definitive ranking without caveats [1] [2].
1. What the existing summaries claim — similar short-term effects, rare serious events noted
Across the analyses, the most consistent claim is that mRNA vaccines like Moderna produce local and systemic reactogenicity similar to Pfizer, including injection-site pain, redness or swelling, tiredness, headache, muscle pain, chills, fever, and nausea; rare but serious events such as anaphylaxis and myocarditis/pericarditis are also mentioned for mRNA platforms [1] [3]. The summaries state AstraZeneca was discussed in some sources with common post-vaccination reactions but that direct, detailed side-effect lists specific to AstraZeneca versus Moderna were often absent, making the comparisons partial. This cluster of findings establishes a baseline: both vaccines provoke expected immune-response symptoms and carry small risks of rare serious adverse events, but the exact comparative frequency and severity remain variably reported [1] [4] [3].
2. Where direct comparisons appear — mixed signals on which vaccine has higher rates
Some compiled analyses and observational studies referenced indicate higher reported incidence of systemic and local side effects with Moderna, with figures such as systemic reactions in roughly 34–42% after Moderna doses versus about 13.3% for AstraZeneca in one pilot study, and local reactions likewise higher for Moderna in that dataset [2]. Conversely, other review-level summaries report that AstraZeneca had the largest number of adverse reaction reports overall in some surveillance datasets, which could reflect reporting practices, distribution volumes, or population differences rather than true per-dose risk differences [5] [6]. These mixed signals indicate that perceived differences depend heavily on study design, denominator choice, and reporting context [2] [6].
3. Why the evidence is inconsistent — limitations and reporting biases
The analyses emphasize major limitations: many sources do not directly compare Moderna and AstraZeneca, datasets vary by country, age group and surveillance system, and some studies are pilot or observational with small or non-representative cohorts. One review notes that AstraZeneca had more adverse reaction reports without breaking down exposure denominators, while other studies present dose-specific percentages for Moderna that cannot be directly equated to AstraZeneca figures in different populations [5] [6] [2]. Differences in reporting systems, vaccine rollout timing, and demographic mixes likely drive apparent inconsistencies, and the absence of head-to-head randomized comparisons in the provided analyses prevents unambiguous attribution of higher risk to one product over the other [5] [2].
4. Specific study evidence — what a hospital pilot and reviews reported
A pilot study from a Bangladesh hospital reported substantially higher reported rates of systemic and local side effects for Moderna—systemic reactions 34.3% and 42% for the first and second Moderna doses respectively, versus about 13.3% after AstraZeneca doses—while local side effects were also higher for Moderna in that dataset [2]. Broader scoping reviews of observational studies conclude that COVID-19 vaccines are safe overall with predominantly mild to moderate side effects but stop short of clear Moderna–AstraZeneca ranking due to heterogeneity across studies [5]. These specific data points highlight that single-cohort studies can show notable differences but cannot be generalized without corroborating, multi-jurisdictional evidence [2] [5].
5. How journalists and public health summaries frame the trade-offs
Public-facing summaries in news and health outlets compiled here state that Moderna’s reactogenicity broadly resembles Pfizer’s mRNA profile, and that non-mRNA vaccines like AstraZeneca may exhibit different reactogenicity patterns; some reports cite studies suggesting other platforms (e.g., Novavax in other work) could have fewer side effects than mRNA vaccines, but AstraZeneca-specific comparisons remain spotty in the provided materials [1] [7] [8]. These communications often emphasize vaccine safety and the rarity of serious events while acknowledging variable short-term reactogenicity, reflecting an agenda to maintain vaccine confidence while informing about expected transient symptoms [1] [7].
6. Practical takeaway — what this means for decisions and surveillance needs
Based on the assembled analyses, the practical conclusion is that both Moderna and AstraZeneca cause common, usually short-lived side effects, with some datasets showing higher reported reactogenicity for Moderna and other datasets reporting more total adverse reports for AstraZeneca; neither dataset in this collection definitively proves one is categorically safer across all populations [2] [6]. The evidence calls for more standardized, head‑to‑head surveillance and stratified analyses by age and prior infection status to guide nuanced individual choice and policy, and readers should interpret single-study differences in the context of study design, population, and reporting systems [5] [2].