Question: How do scientists know how to make a flu vaccine if viruses can be different every year?

Mark Travis answered on 13 Jun 2010:

Hi Mollie,
Good question! You are right that viruses can often change, as this is a good strategy to avoid getting attacked by the immune system or drugs that are used to try and get rid of the virus. How a vaccine works is that a small part of the virus is injected into us- a small part that on its own doesn’t harm us, but is enough to activate the immune system. This activation of the immune system means that the next time the virus enters the body, the immune system quickly gets activated and gets rid of the virus (so called ‘immune memory’).
When designing vaccines, what scientists will try and do is design them based on a very important part of the virus that is unlikely to change from year to year. You are right that virsuses do change from year to year, but these changes are usually subtle- so the vaccines are still effective.