Documented Risks
All vaccines carry some risk of adverse events. Most are mild and
temporary, but serious events do occur at low rates. The following risks
are documented in peer-reviewed literature and regulatory agency data.
Common mild reactions (very frequent):
- • Injection site soreness, redness, or swelling
- • Fatigue, headache, or low-grade fever
- • These typically resolve within 1–3 days
Rare but serious adverse events (documented):
-
Anaphylaxis (severe allergic
reaction): ~1–2 per million doses across most vaccines
-
Febrile seizures: associated
with MMR and MMRV vaccines in young children; generally self-limiting
with no long-term effects
-
Myocarditis/pericarditis:
documented primarily in young males following mRNA COVID-19 vaccines;
most cases mild and resolve without treatment
-
Intussusception: historically
linked to the original RotaShield rotavirus vaccine (withdrawn 1999);
monitored in current rotavirus vaccines
-
Guillain-Barré Syndrome: small
elevated risk documented with some influenza vaccines; risk ~1–2
additional cases per million doses
-
Thrombosis with Thrombocytopenia Syndrome (TTS): rare risk documented with adenoviral vector COVID-19 vaccines
(J&J/Janssen, AstraZeneca); contributed to restricted use
Ongoing monitoring:
-
• Post-market surveillance systems (VAERS, Yellow Card,
EudraVigilance) continuously monitor for new signals
-
• ACIP benefit-risk frameworks reassess vaccines as new data emerges
Learn more about known adverse events →