The first Americans inoculated against COVID-19 began rolling up their sleeves for their second and final dose Monday, while Britain introduced another vaccine on the same day it imposed a new nationwide lockdown against the rapidly surging virus. … Over the weekend, U.S. government officials reported that vaccinations had accelerated significantly. As of Monday, the CDC said nearly 4.6 million shots had been dispensed in the U.S., after a slow and uneven start to the campaign, marked by confusion, logistical hurdles and a patchwork of approaches by state and local authorities.