Dhaka, Oct. 5 -- For a vaccination campaign meant to find children who slipped through the cracks, Bangladesh is starting with a number that is itself difficult to pin down.

The government says 3.4 million children are being brought under its latest measles-rubella catch-up drive, including 2.4 million who were missed during earlier campaigns.

Yet two days before the new campaign began, the Expanded Programme on Immunization said it had received information on only around 1.2 million children who had missed measles vaccination.

How, then, did the list of 3.4 million come together?

The question goes beyond simple math. With measles still spreading and children outside the campaign's age range also falling ill, health experts are questi...