The diagnosis of a young boy and girl paralysed with polio has sparked an emergency mass vaccination campaign in Nigeria. Having celebrated its first full year free of the disease in 2015, the country looked set to be polio free by 2017, along with the rest of Africa.

Health officials are now busy planning their next steps to alleviate the effects of what Michel Zaffran, director of polio eradication for the World Health Organization (WHO), said in a conference call was a “reminder to all of us to reinforce the fact that we cannot be complacent”. A first round of vaccinations for children in affected areas will begin as early as next week.

Clearly any resurgence of polio is bad news, but this particular outbreak is especially troubling. Here’s why:

Nigeria was nearly polio free

The country was on the cusp of eradicating polio completely, which would have made the entire continent polio free. In order to be certified by the WHO as polio free, countries cannot register a case of wild polio for three years in the presence of high quality surveillance. These diagnoses have set the clock back three years.

“These two cases have been detected after two years of thinking that Nigeria was free of polio, so this is a true disappointment,” Zaffran said. “In 2012, the country was representing half of the cases of polio in the world, and a huge amount of progress had been made to the point where we thought we had interrupted transmission of the wild polio virus.”

The two children with polio live in an area threatened by Boko Haram

A boy aged around 15 months and a two-year-old girl were diagnosed with polio in early summer of this year, and both live in the heavily-forested region of Borno – a state in north-eastern Nigeria that is regularly raided by militant group Boko Haram.

This means that there are serious logistical issues with accessing some areas in order to vaccinate local children. To get around this, Zaffran explained that the teams will keep an eye on areas they are unable to access, and if there is an opening a “hit-and-run strategy” will be used.

“This is when we sent the vaccination teams in a very quick manner to access the children,” he explained. “It is very tricky in some areas but in the past we have been able to use it.”

The teams will also do their best to spread the message about the vaccinations as much as they can, in the hope that people in areas that are not easily accessible because of security reasons will hear about the campaign and reach out to them – but it won’t be simple.

Medical teams also need to access the countries of the Lake Chad basin

Children across the border in Chad also need to be vaccinated, as well as in Cameroon, Niger and the Central African Republic. “We want to absolutely avoid that the virus circulates and is transmitted internationally, thus causing an outbreak in all of this area and potentially affecting other countries,” said Zaffran. However, like in Borno, access to immunization is not easy in these areas and the security situation is not optimal.

Four vaccinations are needed to properly protect someone from polio

Most of the children in the affected areas will not have kept a written record of how many doses of the vaccination they have received in the past. It relies on a family member to remember how many times the child was involved in a vaccination campaign or whether they were taken to the health centre – but it will not always be clear.

This is an endemic virus – and was not brought in from elsewhere

Having investigated the polio caught by the children, WHO officials say it is clear that they have type one of the virus – type two was last seen in India in 1999 and was declared eradicated last September, and type three has not been seen since 2011.

Type one is endemic in Pakistan and Afghanistan – and now it looks like it’s back in Nigeria. However, testing has shown that it is a Nigeria-specific virus that has circulated out of Nigeria in the past. It hasn’t been transported from Pakistan or Afghanistan, meaning that Nigeria was never totally polio free.

Despite this disappointing news, Zaffran remains optimistic. “Although this is a setback and a disappointment, we still believe that polio eradication is absolutely feasible,” he said.

Get our Health Newsletter. Sign up to receive the latest health and science news, plus answers to wellness questions and expert tips. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Write to Kate Samuelson at kate.samuelson@time.com.