Pharmacists involved in the covid vaccination drive say it’s common to have half a dose left in a Pfizer vial after five or even six doses have been administered — and to have half a dose left after 10 doses have been drawn out of a Moderna vial. Combining two half-doses could increase vaccinations by thousands at a time when 2 million or so doses are being administered every day in the country. So, they want to use a single hypodermic needle to withdraw leftover vaccine from two vials from which all full doses already have been removed.