Balanced precariously in a narrow canoe, two Nigerian nurses traverse a mile-wide Atlantic estuary to run the daily gauntlet of militant gangs, kidnappers, and unforgiving waters, on a mission to immunise children.

Only last month nine similar health workers delivering doses of polio vaccine in the north of the country were shot dead by Islamist gunman, possessed by the myth that western inoculation programmes are a ploy to sterilise Muslim men.

But while the risks are high, the consequences of doing nothing are worse.

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Each year 861,000 under-fives die here in Africa's third richest nation - more than the entire population of Leeds’ urban sprawl.

It's one of only three nations in the world, along with Pakistan and war-ravaged Afghanistan, where polio is endemic.

And it's why the vaccinators - determined to cut the deaths of children from preventable diseases and wipe out polio altogether by 2018 - are prepared to endure all the country throws at them in a battle to succeed.

“Last week I was held at gun point by masked men – bandits from the deltas,” said Universe Enebong, during her two-hour journey along the tidal river.

“They held us there for 20 minutes.

“They knew we were only helping people. But you worry in case one time it is someone who doesn't care.”

Neither of the two community health workers can swim. And neither has been given a life jacket.

But it is the only way of reaching the village of Ikot Archigbong, a fishing community hours from civilisation which lies on the luscious banks of Cross River.

“The water terrifies me,” says Angelica Ujaga.

“More so than the militiamen. But it is the only way to reach the children so it is what we have to do.”

Once a week the nurses board the boat to Ikot. Sometimes there is money to pay for a motor boat which will get them there in less than ten minutes. Sometimes there is only enough for a boatman to paddle them. And often, they just have to paddle themselves.

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“That’s when we are most frightened,” admits the 44-year-old nurse.

Earlier we had seen the knife-edge balance between life and death that rests on the vaccination efforts.

In a small shack amid a maze of buildings of a slum neighbourhood of the Cross River State capital Calabar, Esther Maurice collapsed in grief as she prepared to place the body of her 16-month old son Valentine into a tiny white coffin.

Only 24-hours before health workers proudly told us that that toddler was recovering from a severe illness thanks to his vaccinations. He was meant to be one of the few 'good' news stories this area could offer.

But as we tried to contact the family the next day there was no answer.

Valentine's small body was overcome, and around midday he slipped into unconsciousness.

“We prayed for him, and so did everyone else, but he didn’t wake up,” whispers his mother, Esther.

How could a child have fallen to pneumonia - a disease easily treated in the West, and one which he had supposedly been vaccinated against?

A local health worker explains: “We can’t be sure he was vaccinated.

“Unfortunately it’s likely that this woman is illiterate. She turned up to a clinic when the child was young and is now certain he was vaccinated against everything. But we can’t be certain."

These atrocious infant mortality rates have made Nigeria a focal point for aid agencies, and the International Vaccine Access Center (IVAC) at the Johns Hopkins Bloomberg School of Public Health is spearheading the nation’s vaccination programme.

Results are starting to be seen. In the last decade coverage of the diphtheria, tetanus, and pertussis vaccine has increased from 29 per cent to 69 per cent, and a vaccination against meningitis A has just been introduced.

IVAC is funded by The Global Alliance for Vaccines and Immunisation, who have spent nearly £100million on vaccination programmes in Nigeria since 2001. A further £177 million has been pledged to be spent before 2016.

Some problems, however, fall outside of the remit of aid agencies.

Onojo Otowo is the man in charge of the nation’s central cold storage warehouse in the capital, Abuja, where millions of life-saving vials are delivered ready to be distributed.

A ‘priority’ power line supplying the warehouse constantly fails, leaving a two hour scramble to keep vaccines the cool, or face losing them altogether.

He tells us of one drastic outage that saw him keeping millions of doses on life-support in row upon row of refrigerated lorries. The makeshift solution worked, and the vials were saved.

And it’s thanks to the efforts of individuals like Onojo, the paddling river nurses Universe and Angelica, and a sustained international relief effort, that results are starting to be seen.

It's a fight that must continue.

A recent study by Johns Hopkins University predicted that a 90 per cent coverage rate for vaccines would save more than 600,000 lives and £11.2 billion over the next ten years. Economics aside, IVAC’s Nigeria expert Chizoba Wonodi is clear of her mission.

“With almost a million children dying in Nigeria each year, we must ensure that every child has access to life-saving vaccines. We are all working hard to make that happen.”