ELI5: What are vaccines made of?

Vaccines are made of tiny pieces or weakened forms of germs, plus other ingredients that help them work safely.

Think of a vaccine like a practice lesson for your body’s defense system. It shows your body a tiny “picture” of a germ so your body can learn how to fight it later without getting very sick.

• The main part is usually an antigen, which is the part that teaches your immune system what the germ looks like.
• Some vaccines also have adjuvants, which are helpers that make your body’s response stronger, like turning up the volume on a lesson.
• Stabilisers help the vaccine stay working properly, kind of like keeping milk cold so it stays good.
• Preservatives help stop bad germs from growing in the vaccine bottle.
• Diluents are liquids, like sterile water, that mix with some vaccines so they can be given as a shot.
• There can also be tiny leftover bits from making the vaccine, called residues, but only in very small amounts.

So, in everyday life, a vaccine is like a training tool for your body: it gives your immune system a safe way to practice before the real germ shows up.