mRNA vaccine technology instructs the body’s cells to produce a harmless piece of a virus, triggering an immune reaction that prepares the body to fight off future infections. Like all medical interventions, mRNA vaccines carry potential risks. Understanding these risks is important for informed health decisions.
Common and Expected Reactions
Following mRNA vaccination, individuals frequently experience mild, temporary reactions. These are a normal part of the immune system’s response. Common local reactions occur at the injection site, including pain, redness, or swelling of the arm. These effects typically resolve within a day or two.
Systemic reactions can also occur. Individuals often report fatigue, headaches, muscle aches, and chills, sometimes accompanied by a low-grade fever. Nausea may also occur. These reactions are usually mild to moderate and generally subside within one to three days after vaccination.
Rare Serious Adverse Events
While most vaccine reactions are mild and temporary, some rare serious adverse events have been identified. Myocarditis, an inflammation of the heart muscle, and pericarditis, an inflammation of the sac surrounding the heart, are examples. These conditions have been observed predominantly in adolescents and young adult males, usually within days after receiving an mRNA COVID-19 vaccine, particularly after the second dose.
The incidence of myocarditis and pericarditis after mRNA vaccination is low, with rates generally reported as a few cases per million vaccine doses. Studies indicate rates of approximately 2.3 to 10.6 cases per million doses for myocarditis, with higher rates in males aged 12-29 years. Symptoms can include chest pain, shortness of breath, and a fluttering or pounding heart. Most cases have been mild, responding well to conservative management and resolving quickly.
Severe allergic reactions, known as anaphylaxis, are another rare but serious adverse event. Anaphylaxis is a rapid, severe allergic response that can occur immediately after vaccination. The incidence of anaphylaxis following mRNA COVID-19 vaccines has been estimated at 4.7 to 5 cases per million doses. Vaccinators are trained to recognize and treat anaphylaxis with epinephrine, and individuals are monitored for 15-30 minutes after vaccination to ensure prompt response.
Factors Influencing Risk
Certain individual characteristics can influence the risk profile for mRNA vaccines. For most individuals, the benefits of vaccination continue to outweigh potential risks. Age plays a role, particularly regarding the rare risk of myocarditis and pericarditis. Younger individuals, especially adolescent and young adult males, have shown a slightly higher risk of developing these heart-related inflammations.
Biological sex also influences some risks, with males having a higher observed incidence of myocarditis following mRNA vaccination. Pre-existing medical conditions are carefully considered when assessing vaccine suitability. Individuals with a history of severe allergic reactions to any vaccine component are advised to consult with a healthcare provider, and vaccination may occur under close medical supervision.
For specific populations, such as pregnant individuals or those who are immunocompromised, the risk-benefit assessment is important. Studies show that mRNA vaccines are safe for pregnant individuals, and the benefits of protection against severe disease generally outweigh any theoretical risks. Immunocompromised individuals may have a reduced immune response to the vaccine but typically do not experience a higher rate of adverse events.
Ongoing Safety Monitoring and Data
Robust systems globally monitor vaccine safety after authorization. In the United States, the Vaccine Adverse Event Reporting System (VAERS) collects reports of adverse events following vaccination from healthcare providers, manufacturers, and the public. These reports are then analyzed by health authorities.
Other systems, such as the Vaccine Safety Datalink (VSD) and the Clinical Immunization Safety Assessment (CISA) Project, actively monitor data to identify potential new risks and investigate signals from VAERS. In the United Kingdom, the Yellow Card Scheme serves a comparable purpose. Regulatory bodies like the Centers for Disease Control and Prevention (CDC), the Food and Drug Administration (FDA) in the US, and the European Medicines Agency (EMA) in Europe rigorously analyze this data.
This continuous monitoring identifies new risks, updates recommendations, and maintains transparency. It also compares vaccination risks to the known risks of contracting the disease, which can include severe illness, hospitalization, or long-term complications.
References
CDC. Myocarditis and Pericarditis After mRNA COVID-19 Vaccination. https://www.cdc.gov/vaccines/covid-19/clinical-considerations/myocarditis.html. Accessed July 24, 2025.
CDC. Anaphylaxis after COVID-19 Vaccination. https://www.cdc.gov/vaccines/covid-19/clinical-considerations/managing-anaphylaxis.html. Accessed July 24, 2025.
McNeil, M.M., et al. (2021). Anaphylaxis After mRNA COVID-19 Vaccine. JAMA. 325(11):1108-1109.