Most heat pumps being installed these days are mini-split heat pumps. There are cold climate models available that (without using resistive elements) put out their full rated heating capacity at -5 F. I know because I have one. Even at -5 F it produces three kW of heat for every kW of electricity consumed. This doesn't violate Newton's 1st law of thermodynamics (conservation of energy) because the heat is not being created, it is being pumped from outside to the inside. And your 98.6 degree body temperature might make you think there is very little heat in air that is -5 F but, actually there is. You have to go all the way to absolute zero before there is no heat energy. Another way heat pumps efficiency can be increased in cold climates is to go to a ground source heat pumps (not all heat pumps are air to air). But even air to air heat pumps have made huge strides in cold weather efficiency by increasing the size of the heat exchangers, using highly efficient variable speed pumps and fans and tightly controlling the pressures inside the refrigerant lines to optimize cold weather performance. I'm really happy with the cold weather performance of my heat pump during those periods of time when the temperature at my ski cabin is between 15 and 30 degrees F. It's over three times more efficient that the electric baseboards it replaced (we don't have natural gas or propane) and this is more than apparent everytime I look at my electrical bill.