All vaccines (and medicines) have side effects. COVID-19 vaccines have mild side effects, which differ slightly among the vaccines that are available. Most COVID-19 vaccines cause mild fever or pain or redness at the injection site. Other side effects include high fever, fatigue, headache, muscle pain, rash at the injection site, chills, and mild diarrhoea. Most reactions to vaccines are mild and go away within a few days on their own. Allergic reactions (medically known as ‘hypersensitivity reactions’) are not uncommon after any kind of vaccine. Allergic reactions can be mild (such as a rash or itchiness around the injection site), or very uncommonly, severe. A severe allergic reaction known as anaphylaxis leads to low blood pressure, collapse, difficulty breathing with/without skin rash. This reaction needs emergency treatment including fluid, oxygen and adrenaline.

More serious or long-lasting side effects to vaccines have been reported but extremely rare. The most well publicised adverse reaction is a condition known as vaccine-induced thrombosis and thrombocytopenia (VITT). Symptoms appear 10-14 days after vaccination and may include symptoms of a stroke, or bleeding, and depend on which blood vessels and organs are affected. The exact mechanism of this is not clearly understood, but is likely to occur because of antibody responses to parts of the vaccine.

When adverse events are investigated, they are classified as:

minor local reactions – these include swelling, redness or rash at the injection site
minor systemic reactions – these include mild headache, body aches and pains, fainting or fever less then 38°C
severe local reactions – these include longer duration of symptoms at the injection site, but also includes swollen glands or abscess at the injection
severe systemic reactions – any condition that results in hospitalisation, severe allergic reactions, high and prolonged fevers or collapse