What group is especially vulnerable to the ravages of covid-19 even if fully vaccinated and boosted? Seniors. And who will have an especially tough time getting free at-home covid tests under the Biden administration’s plan? Yes, seniors. As of Jan. 15, private insurers will cover the cost of eight at-home rapid covid tests each month for their members — for as long as the public health emergency lasts. Finding the tests will be hard enough, but Medicare beneficiaries face an even bigger hurdle: The administration’s new rule doesn’t apply to them. It turns out that the laws governing tradition…