ADVERSE EFFECTS OF MESSENGER RNA VACCINES: An Evidence Review from the Penn Medicine Center for Evidence-based Practice

Authors: Project director: Nikhil K. Mull, MD (CEP), Lead analyst: Matthew D. Mitchell, PhD (CEP), Clinical review: Patrick J. Brennan, MD. (CMO)


1. There are no specific guidelines for use of messenger RNA (mRNA) vaccines or contraindications to mRNA vaccines.
2. No large trials of any mRNA vaccine have been completed yet.
3. The only evidence on safety of mRNA vaccines comes from small phase I and phase II trials of SARS-CoV-2 vaccines, with follow-up typically less than two months.
4. Systemic adverse events such as fatigue, muscle aches, headache, and chills are common.
5. Severe systemic adverse events were reported by 5 to 10 percent of trial subjects.
6. Localized adverse events such as pain at the injection side are common.
7. Both systemic and local adverse events usually are resolved within one or two days.
8. The rate and severity of adverse events appears to be higher for the second dose of vaccine than for the first.
9. Higher vaccine doses appear to increase the rate and severity of adverse events.
10. Larger trials of SARS-CoV-2 vaccines are in progress, with results expected in mid-2021.
11. There is not sufficient evidence to support any conclusions on the comparative safety of different mRNA vaccines.
12. Direct evidence on the comparative safety of mRNA vaccines and other vaccines is lacking.

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