There are hard study findings that correlates a decline of vaccination in the certain populations/geographic regions have indeed result in wide spread measles outbreak in many European countries. That is why measles vaccination is mandatory now. Also many EU countries have universal healthcare and highly regulate medication cost provided by pharmaceuticals and limit how much they can charge.

Many disease is been reduced or even eliminated due to vaccination such as small pox, cholera, and Hepatitis. Even still we heavily vaccinate for Hepatitis because if we stop for even a short period it can come back. Look at Yemin, they have no vaccination for cholera and over half a million people are infected. Another example is Malaria vaccination. It has been proven to work and cost pennies to produce.

As for your point about baby boomers and vaccines. One primary use of vaccines is to protect against disease infections during critical stages like childhood where immunity is weaker and exposure is higher. Its like protecting your self from bankruptcy early in your business. Once your business takes off and is reliable then bankruptcy insurance is no longer needed. So after early child hood the vaccination immunity wears off but its ok cause you no longer need it.

As for heard immunity, it is not a blank concept that can be applied to every vaccination and disease. It does not apply for less transmittable diseases such as lime disease. Herd immunity is critical facing highly transmittable diseases. Especially when in confined spaces like that in breeding farms and uncleanly conditions like war. Many US soldiers died during our many wars due to insufficient supply of vaccination and herd immunity was compromised.

But I get your point, why have mandate vaccination for zika in Montana when there is almost zero chance an out break happening.