对 shielded的主体是the cost
但shield在这里是保障的意思.
Shielded by third-party payers from the cost of our care,we demand everything that can possibly be done for us,even if it's useless.
由于我们受到第三付款人（指纳税人）的保护免交保健费用,所以我们要求可能为我们所做到的一切,即使这样做是徒劳的.