In some hospitals, health centers and pharmacies in the United States, there are vials of Covid-19 vaccines that aren't making it into arms. Out of the more than 22 million doses of vaccine that have been distributed to hospitals and pharmacies so far in the United States, only about 6.7 million people have received their first dose, according to data from the CDC. There's no one reason for the slow rollout or doses going unused; experts say it was never going to be easy to begin a mass vaccination campaign during a pandemic.