Riding isn't free

The problem with vaccination is that it is very vulnerable to free riding. If 99% of people get vaccinated, it is safer for your kid not to be vaccinated. It's probably safer at 95% vaccination levels. But as levels fall to, say, 85%--particularly if they are clustered among children whose parents like exotic travel--it stops being safer. It starts being deadly.

Any one person's actions will never, by themselves, tip the balance. But in aggregate, their decisions are disastrous.

That's why we create, for these situations, social and legal norms that say "No, you can't opt out." You can't not pay your taxes, even though the rest of us won't notice the added fraction of a cent this heaps on our tax burden. You can't go to the bathroom in the reservoir, even though the dilution would make your . . . er . . . contribution negligible. You don't get to dodge the draft just because you would prefer that someone else get killed defending the country.

We allow people to opt out of some of these social compacts when they are a genuine matter of conscience--you don't have to kill if you think it is morally wrong (though you still have to risk being killed by serving as a medic or in some other non-combat arm). You don't have to vaccinate your children if you're a member of a tiny cult whose children rarely leave the farm. We tolerate this largely because such genuine exceptions are few in number, and because the people who harbor them generally pay a higher price than the rest of us.

But we do not let you opt out because in your opinion, this would make you better off. The very essence of free rider problems is that every individual is better off not complying with the general practice, but the rest of us are all worse off. The only way to secure general compliance is to establish the norm that this is something you have to do even if you'd really rather not. Once you start making general exceptions, the whole system breaks down. And that system has made us all--including yes, your children--vastly better off.