The COVID-19 vaccines have significantly reduced severe illness and hospitalizations. While recognized for their safety and effectiveness, like all medical interventions, they can cause side effects. Most reactions are mild and temporary, but a very small number of individuals may experience uncommon responses.
What Qualifies as a Rare Side Effect
A side effect is considered “rare” in the context of vaccine safety when it occurs in a very small proportion of vaccinated individuals. Common, mild reactions, such as pain at the injection site, fever, or muscle aches, are expected and indicate the body’s immune response to the vaccine. These differ from rare effects, which are observed in between 1 in 1,000 and 1 in 10,000 people, or even fewer than 1 in 10,000 for “very rare” events.
Specific Rare Side Effects Identified
Several specific rare side effects have been identified with COVID-19 vaccines through post-market surveillance.
Myocarditis, inflammation of the heart muscle, and pericarditis, inflammation of the lining around the heart, have been observed, particularly in adolescent and young adult males, most frequently within seven days after receiving a second dose of an mRNA COVID-19 vaccine. The incidence of myocarditis or pericarditis after a second dose of mRNA vaccine is approximately 12.6 cases per million doses among individuals aged 12 to 39 years.
Thrombosis with Thrombocytopenia Syndrome (TTS) involves rare blood clots accompanied by low platelet counts. This condition has been linked primarily to adenoviral vector vaccines, such as the AstraZeneca and Johnson & Johnson vaccines. The estimated incidence of TTS ranges from 3.2 to 16.1 cases per million doses for the AstraZeneca vaccine and 1.7 to 3.7 cases per million doses for the Johnson & Johnson vaccine. These severe blood clots often occur within three weeks following the first dose.
Guillain-Barré Syndrome (GBS), a rare neurological disorder where the body’s immune system damages nerves, has also been observed following some COVID-19 vaccinations. Data from the Vaccine Safety Datalink indicated an increased risk of GBS among adults 18 years and older after the Johnson & Johnson COVID-19 vaccination, with an unadjusted incidence rate of 32.4 per 100,000 person-years within 21 days after vaccination. However, no increased risk was found after Pfizer-BioNTech or Moderna COVID-19 vaccination.
Anaphylaxis, a severe allergic reaction, is another very rare side effect. It occurs quickly, usually within minutes of vaccination. Initial reports indicated an incidence of 11.1 cases per million doses for the Pfizer-BioNTech vaccine, though more recent data suggest rates similar to other vaccines, around 5 cases per million doses. Healthcare professionals administering vaccines are trained to recognize and treat anaphylaxis immediately.
When to Seek Medical Help
Recognizing potential symptoms of rare side effects and seeking prompt medical attention is important. For myocarditis or pericarditis, symptoms to watch for include acute chest pain, shortness of breath, or heart palpitations, particularly in adolescents and young adults. Younger children might show less specific signs like irritability, vomiting, fast breathing, or lethargy.
For Thrombosis with Thrombocytopenia Syndrome (TTS), symptoms typically appear within three weeks of vaccination. These can include severe or persistent headaches, blurred vision, seizures, persistent abdominal pain, leg swelling, or unusual bruising or bleeding. Any new or worsening neurological symptoms, such as weakness or tingling sensations, could signal Guillain-Barré Syndrome (GBS) and warrant medical evaluation. For anaphylaxis, immediate signs like difficulty breathing, swelling of the face or throat, a widespread rash, dizziness, or a rapid heartbeat require emergency care. It is always best to contact a healthcare provider promptly if any concerning symptoms arise after vaccination.
How Rare Side Effects Are Monitored
Robust systems are in place to monitor vaccine safety and identify rare side effects after vaccines are widely administered. Post-market surveillance includes both passive and active reporting mechanisms. The Vaccine Adverse Event Reporting System (VAERS) in the U.S. is a passive system where individuals, parents, or healthcare providers can report any adverse health events following vaccination. VAERS helps detect unusual patterns or “safety signals” that might indicate a potential issue.
Active surveillance systems complement passive reporting by systematically collecting and analyzing health data from large populations. The Vaccine Safety Datalink (VSD) project, for example, is a collaboration between the Centers for Disease Control and Prevention (CDC) and several large healthcare organizations. This system uses comprehensive medical records to study vaccine safety concerns, allowing for more rigorous assessment of potential links between vaccination and adverse events. The Clinical Immunization Safety Assessment (CISA) Project is another network of vaccine experts and health centers that conducts research and assists the CDC in vaccine safety evaluations. These interconnected systems are designed to detect even very rare events.