Aug. 19, 2015 | CNN

Kano State, Nigeria (CNN) Ahmadu Kanduwa's home is just two kilometers away from the local clinic in Nigeria's northern Sumaila district -- two kilometers from the vaccine that could have prevented his son, Isa, from contradicting polio more than a year ago. It's something Kanduwa thinks about often.

"I have this thought, if he had received say five or six doses, he would have been immune from this ailment," Kanduwa said.

He says Isa received two of the oral polio vaccinations. Painfully close to the four doses recommended for complete immunity.

In a district that now has an immunization rate of around 85%, officials hope Isa's will be Nigeria's last new case of polio. More than a year of being polio-free highlights how close the country is to a major milestone. But Isa's case also shows just how difficult polio can be to fully eradicate.

Standing outside of the clinic, Kulchumi Hammanyero from the World Health Organization smiles when she sees the line of mothers with babies in their laps and immunization cards in their hands, waiting patiently for the vaccine. But her smile is matched by a heavy dose of caution.

"When we see there is no more wild polio virus (WPV) and all indicators are showing us that we have covered the necessary ground, then we can say, ok, we have reached a certain point. But we are not out of the woods, not out of the woods at all," Hammanyero said.

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