In all the research I have done I have never found an insurer that covers just trip cancellation. Really the real cost is the trip cancellation that is what is claimed the most, they just "throw" in the medical coverage in reality. You can buy JUST the medical coverage and it is so cheap that even if you could buy just cancellation the cost would probably be the same.
I checked out a bunch of insurers and overall liked CSA the best. They have a plan you can buy at the time of final payment and cover pre-existing conditions where most plans made you buy within in certain number of day or weeks of deposit.
They also have a cheaper plan that does not cover pre-existing conditions.
This is the coverage graywolf had when she had her heart attack and she said they were wonderful. So I would check that company out, nice to know they handled a serious claim so well. It is "secondary" coverage you claim on your own medical first.
Travelex is one of the few that is "primary" coverage claim straight to them for any medical problems.
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