Why is it possible to only be vaccinated once for measles but the flu vaccine is required yearly?
Oh, well for all intents are purposes, there is only one strain of the measles. One vaccinted you are vaccinated forever. (Depending on the vaccine, tetatnus is every 10 years.) But, there are thousands of strains of the flu. Once vaccinated you do stay vaccinated, but each year there are many, many brand-new flus. If you catch the flu you are immune to that strain forever too, which is why the old ones don't keep coming back. The flu vaccine is made to protect against the 3 or 4 that doctors predict will be the worse each season. It doesn't protect against them all and it can't protect against the new mutations that we will see next year. Hope this clears things up for you!