The largest national vaccination program since the polio vaccine of the 1950s rolled out on Monday, with the first of 2.9 million doses of the Pfizer/BioNTech vaccine approved last week on the way to all 50 states. That’s about half the amount states were expecting of an initial 6.4 million doses from Pfizer, but the FDA directed that half be held back for the second of the two shots required for the vaccine to reach maximum effectiveness. “They don’t want us to hold anything back for the second shot,” Florida Emergency management director Jared Moskowitz said during an interview last week with the Florida Channel.