A total of 40 samples were collected from 14 cities in mid-April under the supervision of the World Health Organization and analyzed at the National Health Institute, said Dr. Rana Safdar, the head of the National Emergency Operation Center. WHO officials could not immediately be reached for comment.
Last year 22 percent of environmental samples tested positive, and 9 percent tested positive in the first quarter of 2016.
Past immunization campaigns have been met with resistance by Islamic extremists, who have spread conspiracy theories that the vaccines are part of a plot to sterilize children or gather intelligence for Western nations. Islamic militants have attacked vaccination centers and health workers taking part in the campaigns.
But Safdar said outreach programs supported by religious scholars have helped to combat the propaganda, bringing the refusal rate down from 1.5 percent to 0.05 percent over the last two years.