system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what is the best analogy explaining vaccination to laypeople?

[edit formatting] I've been trying (for about 2 years now) to come up with a good analogy for the vaccine debate... it's really quite difficult. I'd love to hear others' thoughts.

There are a few important components, and we'll refer to the analogy of vaccination as the "action" being considered for analogy, including, above all – something that is relatable to most people.

* the action is voluntary

* the action carries a significant benefit to the individual BUT ONLY when encountering a "challenge" and NOT AT ALL if there is no "challenge"

* the action carries a small risk to the individual (unpredictable) at the time of the action

* the action carries a benefit to others BUT ONLY when after encountering a "challenge" (and one could argue also ONLY if the "others" did not take the action, although not always... this is where complex herd immunity statistics come into play)

* the benefit to "others" increases (up to a point) as more and more people take the action

* non-action carries a risk to others BUT ONLY when encountering a "challenge" (although the "others" should typically be able to protect themselves by taking the action)

* some people cannot take the protective action due to mitigating circumstances

* _____ (am I missing anything?)

So... the best example I can think of is seatbelts (although it's not perfect). 

* you choose to wear a seatbelt (although it is typically required by law)

* it may save your life (significant benefit) BUT ONLY if you're in an accident, otherwise it's not helpful (and can be a little annoying/discomforting – like a needle stick)

* tiny risk of seat belt injuries is possible but small [and the risk is only applicable in the case of a "challenge" like an accident, whereas a vaccination has the risk in the "action" not the "challenge"]

* in a serious car accident "challenge" there is a real risk of people bouncing around the car and injuring other passengers as a projectile if they didn't take the action (even though wearing a seatbelt doesn't protect you from others' bodies bouncing around).
* there is a limited benefit to "others" no matter how many people wear their seatbelt
* not wearing your seatbelt does not cause you any significant risk if you are not in an accident
* basically everyone can wear some type of seatbelt

Obviously it's not a perfect 1:1 match-up. I'd be really curious to hear if anyone has suggestions. THANKS!
gpt
Think of your immune system as a bunch of soldiers in a fort.  If enemy soldiers get past the patrols and attack the fort, your soldiers might fight them off (you get well) or they might not (you get very ill and possibly die).  Going to the hospital to be treated for the infection (calling in reinforcements) can often help, but sometimes cannot.

How did the enemy soldiers get past your soldiers' patrols?  Their uniforms are unfamiliar, your soldiers have standing orders to attack only enemies they recognize.  If they attacked soldiers they should not, they might be killing soldiers that are on your side (this would be an autoimmune disorder).

A vaccine, then, is an intelligence dispatch to your immune system.  'Be on the lookout for these new enemy soldiers.  You'll recognize them as they are wearing the following uniforms...kill them on sight.'

And now, your immune system won't wait until the fort is attacked, the soldiers will attack the enemy soldiers immediately.

Continuing the analogy: some immunizations last a lifetime (such as for Polio) because the enemy soldiers never change their uniforms.  Other enemy soldiers, such as Influenza (the "Flu") swap parts of their uniforms around constantly, so your immune system fort needs intelligence dispatches every year.

Edit: expanding the analogy to address your question about risk.

So, there is a very small risk associated with being vaccinated.  Once in a while, someone's immune system fort will receive an intelligence dispatch and the soldiers will panic and attack friendly soldiers.  But as the experts will tell you, the risk is very small, and the risk of enemy soldiers overwhelming the fort is much greater.