ISLAMABAD — A Pakistani health official says that for the first time in the country’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 in Pakistan.

Dr. Rana Safdar, the head of the National Emergency Operation Center, says Monday that a total of 40 samples were collected from 14 cities in mid-April under the supervision of World Health Organization. The negative samples, he says, show that the virus could eventually be eradicated through continued immunization campaigns.

Safdar says new campaigns have been planned in vulnerable areas starting in July after the end of the Islamic fasting month of Ramadan.

Pakistan and Afghanistan are among the only countries left in the world where polio is endemic.