Possible side effects Like all vaccines, Comirnaty can cause side effects, although not everybody gets them. Very common side effects: may affect more than 1 in 10 people injection site: pain, swelling tiredness headache muscle pain chills joint pain diarrhoea fever Some of these side effects were slightly more frequent in adolescents 12 to 15 years than in adults. Common side effects: may affect up to 1 in 10 people injection site redness nausea vomiting Uncommon side effects: may affect up to 1 in 100 people enlarged lymph nodes feeling unwell arm pain insomnia injection site itching allergic reactions such as rash or itching Rare side effects: may affect up to 1 in 1,000 people temporary one sided facial drooping allergic reactions such as hives or swelling of the face Not known (cannot be estimated from the available data) severe allergic reaction inflammation of the heart muscle (myocarditis) or inflammation of the lining outside the heart (pericarditis) which can result in breathlessness, palpitations or chest pain extensive swelling of the vaccinated limb swelling of the face (swelling of the face may occur in patients who have had facial cosmetic injections)