Feeling unwell after a vaccine is often a sign the body is working as it should, not a warning.
Common, mild reactions
The most common reactions are pain, redness or swelling at the injection site, plus low fever, muscle aches, headache or tiredness. These reflect the immune system recognising the vaccine and building protection. They usually appear within one to two days and settle within a few days. Fluids, rest and, if needed, fever or pain relief as advised by a health professional usually help.
Severe reactions are rare
A severe immediate allergic reaction (anaphylaxis) is very rare and usually happens within minutes to about 15 minutes of the jab. That's why people are typically asked to wait at the site for a short observation period. Vaccination sites keep the medication and staff needed to respond.
When to seek emergency medical care
Seek emergency medical care immediately for difficulty breathing or swelling of the face or throat. Seek medical advice promptly for persistent high fever, severe injection-site swelling, or other severe, lasting or worrying symptoms.
Related question: What should I do about an allergic reaction after a vaccine?
Monitoring in Hong Kong
Hong Kong's Department of Health runs a pharmacovigilance system that continuously receives and reviews reports of adverse events after vaccination to monitor vaccine safety.