Night of the Comet (1984) on Netflix

Two sisters, high school seniors in the early 1980s, awaken one morning to blood red skies and the realization that the human race has been wiped out. It becomes apparant that anyone who was not surrounded by steel when the Earth recently passed through the tail of a comet has been reduced to a pile of red ash (the trace elements of human chemical makeup.) They encounter a rebellious Native American man and take over the air waves at a local radio station in an attempt to get help. Unfortunately, they attract the attention of a group of scientists who knew what the effects of the comet would be. They hid underground in a steel laboratory, but idiotically left the vents open. Now, as various survivors of the comet who were only partially surrounded by steel are decaying into flesh-hungry zombies, the scientists are attempting to come up with a vaccine made of the blood of people who were not infected by the comet. Thus the trio must fight their way past not only roving packs of the cannibalistic dead, but also outsmart a pack of scientists desperate not to become that way themselves.