Last steps in polio eradication prove challenging

Oral polio vaccine
Oral polio vaccine

Administering the oral polio vaccine., CDC Global / Flickr cc

Many experts hope wild polio virus (WPV) will be eradicated from the world by the end of next year. But the last steps in banishing the virus from the earth may prove to be the most difficult, even though a recent shift from the oral polio vaccine (OPV) to an injected one brings hope.

Since 1952, when Jonas Salk developed the first effective polio vaccine, eradication of the devastating childhood illness has been a global public health goal. In 1988, the World Health Assembly stated that global eradication of polio was an attainable goal, prompting researchers to look ahead 30 years to 2017 and 2018 as a time when every child could be inoculated against poliomyelitis.

Now, a report by the Independent Monitoring Board (IMB) of the Global Polio Eradication Initiative (GPEI) sheds light on the last hurdles researchers and scientists must jump before they can say polio is banished from the earth. The report, "Polio will not end everywhere until everywhere ends it," was published in August and describes logistical and organizational difficulties reaching the three remaining WPV reservoirs in the world: Nigeria, Pakistan, and Afghanistan. It is the IMB's 13th report since April 2011.

Last WPV reservoirs

"There are very serious weaknesses that are placing the entire endeavour in jeopardy," the new report said. "They are crying out for definitive solutions and incisive action, implemented with speed and dynamism."

So far this year, there have been three cases involving WPV in Nigeria, where the terrorist organization Boko Haram has blocked vaccine efforts and surveillance. Those are the first cases in that country since 2011.

Pakistan has reported 14 cases and Afghanistan 9 so far this year. In 2015, Pakistan had 54 cases of WPV, and Afghanistan reported 20.

Both countries, according to the report, suffer from shaky political leadership and trust issues with vaccinators. As stated in the report, "The number of inaccessible children in the Eastern region of Afghanistan has increased from 26,000 in March 2016 to 130,000 in May 2016. In this region, there are substantial issues of trust."

The report also notes that one fifth of the polio cases in the world occur in the Shigal District of Afghanistan, where 1,000 people live. Vaccinators have not been able to reach children there for 4 years.

Vincent Racaniello, PhD, of Columbia University in New York City, has worked on polio since 1979. He said the reservoirs represent the biggest challenge to eradication: Reaching children where they live.

"Our only obstacle is getting in these places to vaccinate, and if you can't get into the kids at all, getting [there] three times is really hard," said Racaniello. The current vaccine used in Nigeria, Pakistan, and Afghanistan requires three doses. Racaniello said the goal is not impossible—it was a task completed in India in 3 years—but the political upheaval of the three reservoirs makes access to children nearly impossible.

Ending vaccine-derived cases

There are three types of WPV, and in 2015 type 2 was eradicated. The success prompted the GPEI to switch to using a bivalent vaccine in April of this year, a move that infectious disease and vaccine expert Walter Orenstein, MD, said was one of the most astounding things he has witnessed in his career. Fully 155 countries that used OPVs switched to an inactivated bivalent vaccine by May 12.

"I can't think of another time in history when the world has united on such a goal," said Orenstein, associate director of the Emory University vaccine center in Atlanta and the former director of the US National Immunization Program,

Orenstein and Racaniello explained that polio cannot be fully eradicated until vaccine-derived polio virus is eliminated. Vaccine-derived cases are rare, occurring in 1 out of 1.5 million vaccine recipients, and they occur only when the attenuated vaccine that includes the Sabin-derived strain is used. Unfortunately, that's the strain used in OPVs. From 2006 to 2016, there were 721 vaccine-derived polio cases.

As Racaniello said, "We cannot eradicate polio with the vaccine that has almost eradicated the virus. It's a catch 22."

A report in the Sep 9 Morbidity and Mortality Weekly Report (MMWR) said the switch has gone smoothly, and as of Aug 31, no new vaccine-derived polio viruses have been detected.

Orenstein said he hoped the wild virus can be eradicated by the end of 2017. Then the world would enter a 3-year period of heightened surveillance to make sure no WPV is circulating.

"Eradication is an unforgiving goal," said Orenstein. "A handful of cases means you've failed. You have got to get to the last chains of the disease, wherever they may be."

See also:

GPEI IMB report

GPEI case Web site

Sep 9 MMWR report

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