More than 150 developing countries are switching to a new polio vaccine in the first worldwide vaccine change ever attempted.

According to the World Health Organisation, every country using the oral polio vaccine will between April 17 and May 1 switch from the trivalent vaccine (tOPV) to the bivalent oral polio vaccine (bOPV).

“This is an essential part of the work that needs to be done to secure a polio-free world by phasing out oral polio vaccines to prevent future outbreaks of vaccine-derived polioviruses,” said WHO.

The tOPV protects children against the three types of wild polio viruses — Type 1, Type 2 and Type 3 — while (bOPV) targets types one and three. Types one and three are the two remaining strains of the wild polio virus. Wild polio viruses are those that occur naturally.

Type 2 polio transmission has been stopped since 1999, hence immunising against it is no longer necessary.

In rare cases, it also poses a risk that the weakened Type 2 virus in the vaccine can seep into circulation and cause “vaccine-derived” polio infections.

Seen as the largest globally synchronised project in the history of vaccines, switching from the vaccine that was successfully used to fight polio for more than 30 years is a huge logistical exercise. Thousands of people will monitor the exercise in the next fortnight.

“It is a massive undertaking and a major step towards eradication,” said WHO’s director of polio eradication, Michel Zaffran.

“We are entering into uncharted territory. This has never been done before. But there is no going back now,” he said.

Poliomyelitis (polio) is a highly infectious viral disease that mainly affects young children. The virus is transmitted from person-to-person mainly through faecal matter entering the mouth.

Polio cases are currently just a fraction of the one per cent of cases known in 1988, when 350,000 cases were recorded in 125 countries around the world.

In 2015, there were just 74 cases of the paralysing disease and there have been 12 so far this year, all recorded in Afghanistan and Pakistan.

On the other hand, Africa has been free of polio for more than a year.

READ: UN sets 2018 target to eradicate polio

According to Mr Zaffran, the switch needs to be co-ordinated to prevent outbreaks in places where the old vaccine is no longer being used.

The Global Polio Eradication Initiative estimates that it will cost about $5.5 billion to eliminate polio.

However, Mr Zaffran argued that even more money will need to be spent to keep the disease from returning.

“Taking our foot off the pedal now could mean polio will within a few years spread straight back into large parts of the world and create 100,000 or 200,000 cases,” Zaffran said. “The job has not been done and will not be done until we have fully eradicated the virus.”

The world has already begun introducing “inactivated” polio vaccine (IPV) in preparation for the introduction of bOPV and to further reduce any risk of children being exposed to polio.

As of February 2, the WHO reports that 156 countries had introduced the IPV to help boost immunisation against all strains of polio and to ensure that children have some baseline immunity against Type two following the switch as a risk mitigation measure.

In Kenya, IPV was introduced in October to boost population immunity.

Booster dose

Dr Collins Tabu, head of the immunisation policy and performance monitoring at Kenya’s Ministry of Health said that close to 600,000 children under one year had received the vaccine so far.

“We introduced the first booster dose of IPV in October and expect to receive the next batch in July,” said Dr Tabu.

The vaccine is given at 14 weeks as the child also gets a third dose of oral polio vaccine.

Uganda recently rolled out IPV with a target of vaccinating about 1.7 million children aged 0-11 months.

Uganda’s director-general of Health Service Jane Ruth Aceng, said that the project would cost the country an estimated $2.6 million, with the Global Alliance for Vaccine Initiative (Gavi) covering $1.4 million of the cost.

“We anticipate to reach 1.67 million children with IPV. This number is expected to rise to 1.73 in 2017 and 1.8 in 2018,” said Dr Aceng.