With the Trump administration aiming to deliver 300 million doses of vaccine against the coronavirus as early as January, state officials and health experts say they remain in the dark about key details and, therefore, are inadequately prepared for what is expected to be the largest single vaccination campaign ever undertaken. Getting shots into the arms of millions of Americans is a massive undertaking, they say, requiring extraordinary coordination, planning and communication. But with only six months to the government’s target date for approving a vaccine, the administration has shared limited and often confusing information about its plans for distribution, making it difficult for overwhelmed state and local officials, including those who run immunization programs, to prepare.