If a person and his friend were lost in the desert one had double the water he needed to stay alive and one had no water left. However, the friend with ample water does not want to give any away and his friend does not make it.
What would you say in such a case ?
At any moment they don't know how long they'll need to wait to be saved - they don't know how much water each person needs.
Are you obligated to reach into a pool to pull out a drowning child? would you do it even if you're not obligated?
How about if it's a heavy adult in the deep end? how about if you're a small woman?
How about if you see someone drowning in a lake? an ocean? What if you're not that good/ strong a swimmer?
We have excess vaccines. We are a richer country than most. We have the capacity to help others at a cost to us. Is the cost too much? Are we obligated? or do we do this because we want to?
We're not killing others by not helping. Sometimes it's the right and noble thing to help others even when it's a risk to ourselves. Sometime the risk is so small, we'd be wrong not to help. But we are not doing any killing.