Officials foresee a grinding effort to convert people one shot at a time that could last for months, and that risks being complicated by growing Republican resistance to what was once a largely bipartisan bid to get the country immunized. “We are under no illusions that each person in this stage will take longer to reach," said a senior administration official. “The first 180 million were much easier than the next 5 million.”... Though a range of prominent health experts contend the Food and Drug Administration could prime demand by fully approving the vaccines, there’s skepticism within the administration that millions are holding out until the agency gives its final endorsement... Another potential conduit for accelerated vaccinations — primary care doctors — has not expanded as quickly as some had hoped, with some physicians hesitant to turn their practices into vaccine distribution points when the shot is already widely available elsewhere. Others have embraced the effort, only to be forced to throw away much of the multidose vials after vaccinating only a couple patients in a day.