Are side effects a sign something has gone wrong?

Last reviewed: 2026-06-08

Quick answer

Usually not. Soreness, redness or swelling at the injection site, low fever, tiredness or headache are common reactions that mean the immune system is building protection. They typically appear within a day or two and settle within a few days. Severe allergic reactions are very rare, usually happen shortly after the jab, and vaccination sites are equipped to handle them. See a doctor for any persistent or severe symptoms.

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 see a doctor

Seek medical care promptly for a persistent high fever, severe swelling at the injection site, difficulty breathing, swelling of the face or throat, or any severe or lasting symptom that worries you.

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.

Answer six short questions to see which vaccines are worth discussing with your GP, and why.

Based on your age, health, and travel plans, using HK Department of Health and CDC guidance. A ranked list of vaccines, with the reason each one applies to you.

Rule-based

The recommendations follow HK Department of Health and US CDC guidance, and you can see how each one was reached.

Private

Your answers stay on your own device. We never receive or store them.

Explained

Every recommendation comes with the reason it applies to you.

Start the recommender →