Can a vaccine give you the disease it's meant to prevent?

Last reviewed: 2026-06-08

Quick answer

The vast majority of vaccines cannot cause the disease, because they use an inactivated pathogen or just a part of it, such as a protein, which can't replicate. A few live-attenuated vaccines (such as MMR or the nasal flu vaccine) contain a heavily weakened live virus that can occasionally cause mild, short-lived symptoms, but not the actual disease. A low fever or tiredness after a vaccine is usually the immune system working as expected.

Whether a vaccine can cause illness depends on what's in it.

Inactivated and subunit vaccines

Most vaccines use a killed pathogen or just a part of one. Pneumococcal vaccines, hepatitis B vaccine, the inactivated flu shot and mRNA vaccines contain no live virus or bacteria that can replicate. They can't multiply in the body, so they can't cause the disease.

Live-attenuated vaccines

A few vaccines contain a live pathogen that has been heavily weakened in the lab, such as the MMR vaccine, the chickenpox vaccine and the nasal flu vaccine. The weakened virus is enough to train the immune system but generally not enough to cause illness. A small number of people may get a mild rash or low fever one to two weeks after MMR; that's an immune response, not actual measles.

Common reactions after a vaccine

Soreness, redness or swelling at the injection site, low fever, headache or tiredness are common in the first day or two and usually settle within a few days. These are signs the body is building protection, not signs you've caught the disease.

Who needs extra care

People with a seriously weakened immune system may not be suitable for live-attenuated vaccines and should check with a doctor first. For them, doctors usually choose an inactivated or subunit vaccine.

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 →