U.S. officials on Wednesday defended a tactic used by the CIA to attempt to verify the whereabouts of Osama bin Laden — the covert creation of a vaccine program in Abbottabad, the town in Pakistan where he was later killed in a U.S. raid. The vaccine drive was conducted shortly before the raid in early May on bin Laden’s compound, officials said, and was overseen by a Pakistani doctor who traveled to Abbottabad.