A measles vaccine can protect you for a lifetime, tetanus gives you about 10 years of cover, while flu vaccines need updating every year.

When it comes to Covid19, we're very confident the jabs are safe and effective, but we're still not sure exactly how long their protection will last.

So why do some vaccines protect you much longer than others? vaccines train your immune system to recognize a specific threat, such as a virus and produce the right weapons to fight it off, including BugBuster proteins called antibodies.

The first thing to think about is how quickly the virus changes, the vaccine against measles provides lifelong protection partly because the virus is quite stable and doesn't really mutate.

So once your body is familiar with it, it will be easy to recognize, whether you meet it again in one year's time or 50.

On the other hand flu viruses evolve very quickly, a vaccine will train your immune system to recognize one strain.

But it may not be very good at protecting you by the next winter when the virus could look very different.

Experts think the virus that causes Covid falls somewhere in between, it has mutated fast enough to escape some of the protection provided by the first round of vaccines which were based on an earlier covert strain.

But the immune system can still recognize it enough to block a lot of infections and to protect most people from getting very ill if they do become infected with a different covert variant.

Mutation is only one piece of the puzzle though, some infections just seem to be more memorable to the immune system than others, and exactly why that is is not fully understood.

Part of it might be to do with the different things your immune system produces to fight off invaders, while virus fighting antibodies may fade relatively quickly, they can leave behind slower and longer lasting T-cells.

They don't stop an infection getting into your cells, but they can kill off your infected cells once the virus has already broken in, stopping you getting really sick, this may well be what's happening with Covid.

A final thing to think about is how often you are exposed to the infection after being vaccinated.

For example you might only rarely encounter tetanus, meaning the vaccine could be your body's one chance to learn what it looks like and how to fight it off.

After a few years this memory of the infection will fade as it hasn't been given any refreshes, meaning extra shots may be needed particularly if you step on a rusty nail or get dirt in a wound.

You might come across other infections, like previously common childhood infections mumps or rubella, lots of times after being vaccinated, refreshing your immune system's memory and keeping it in fighting shape.

Experts say covert will probably remain a common infection that continues to circulate, so the hope is with vaccines making the virus less risky.

You can be exposed to it again and again, topping up your antibodies and strengthening your defenses for the next time you encounter it.