WASHINGTON (AP) — New research suggests a one-two punch could help battle polio in some of the world’s most remote and strife-torn regions: Giving a single vaccine shot to children who’ve already swallowed drops of an oral polio vaccine greatly boosted their immunity.

The World Health Organization officials said the combination strategy already is starting to be used in mass vaccination campaigns in some hard-hit areas and is being introduced for routine immunizations in developing countries, too.