For the first time in Pakistan's history all the previous month's environmental samples for polio have tested negative—a sign of progress in the campaign to eradicate the virus, an official said Monday.

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.

Safdar said the negative samples represent a "significant achievement" but that complete eradication would require more "consistent efforts." He said new immunization campaigns have been planned in vulnerable areas starting in July, after the end of the Islamic fasting month of Ramadan.

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.

Pakistan and Afghanistan are among the only countries in the world where polio is endemic. Eleven polio cases have been reported so far this year in Pakistan, compared to 54 last year. The highly infectious virus mainly affects small children, and can cause paralysis.

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