Healio
Healio
Issue: April 2016
ByDonald Kaye, MD, MACP
ByMarjorie P. Pollack, MD
April 19, 2016
6 min read
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Poliomyelitis: Are we there yet?

Issue: April 2016
ByDonald Kaye, MD, MACP
ByMarjorie P. Pollack, MD
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In May 1985, the director of the Pan American Health Organization proposed that polio be eradicated from the Americas region by 1990. Seeing the progress of the efforts in the Americas, the World Health Assembly passed a resolution launching the Global Polio Eradication Initiative in May 1988.

The last case of polio documented in the Americas had a date of onset of illness of Sept. 5, 1991, and the region was declared free of local transmission of the wild poliovirus (WPV) on Aug. 20, 1994. The Western Pacific Region of WHO was certified as polio-free in 2000, the European Region in 2002, and the South-East Asia Region in 2014.

Marjorie P. Pollack

Folks may recall the days of smallpox eradication when vaccinating all individuals in a circle surrounding a case — a strategy termed “surveillance and containment” — was the successful approach. This followed the suboptimal success associated with the “mass vaccination” strategy. From the launch of the surveillance and containment strategy in 1967, WHO officially declared smallpox eradicated in 1980. But smallpox was less challenging to eradicate as the infected individuals were easily recognizable, unlike polio where an average of only one out of every 200 infected individuals has paralytic manifestations, and the remainder present with either mild viral syndrome-like symptoms or no symptoms at all. Hence, national and subnational vaccination campaigns have been the basis of the eradication activities in addition to enhanced campaigns in areas with documented wild poliovirus circulation.

The three WPV types — types 1, 2 and 3 — require a trivalent vaccine, and there are challenges inherent in getting tritypic seroconversion. Complicating matters is the existence of two different types of polio vaccines available — a live-attenuated virus vaccine (the oral polio vaccine [OPV], or “Sabin” vaccine, which is administered orally), and the killed virus vaccine (inactivated poliovirus vaccine [IPV], or “Salk” vaccine, which is injectable). After one dose of OPV, approximately 50% of recipients have tritypic seroconversion; after three doses, approximately 95% of recipients have tritypic seroconversion. The OPV vaccine does not require highly trained personnel or the use of syringes, and the production of immunoglobulin A against the poliovirus prevents transmission of a WPV by a vaccinated individual. Given the ease of its administration, OPV became the obvious choice as the vaccine for eradication activities.

Donald Kaye

Vaccine-derived polioviruses

The last reported case of WPV type 2 (WPV2) was isolated from a child in 1999 in Aligarh, Uttar Pradesh, India. WPV2 was certified as eradicated in September 2015. The last reported case of WPV type 3 (WPV3) was isolated from a child on Nov. 10, 2012, in Yobe, Nigeria. Theoretically, that leaves just WPV type 1 (WPV1) as the key player in WPV circulation. Simple, right? Just use monovalent OPV type 1 and all will be well ...

Not so easy, though. On rare occasions, paralytic polio-like illness has been associated with the OPV. In approximately one in every 2.7 million first doses of OPV administered, a vaccine recipient will develop paralytic disease known as vaccine-associated paralytic poliomyelitis. Since 1999, cases of polio due to circulating vaccine-derived polioviruses (cVDPV) began appearing. These outbreaks have occurred in areas with suboptimal vaccination coverages and where the vaccine virus has undergone genetic changes resulting in a more neurovirulent virus, the cVDPV. Since 2000, there have been 103 cases of polio due to infection with cVDPV type 1, another 694 cases of polio due to cVDPV type 2, and 12 cases of polio due to cVDPV type 3.

Current progress in polio eradication

So, where are we today with respect to polio cases? According to the Polio Eradication Initiative, as of the week of Feb. 10, the most recent date of onset of a WPV1-associated case was Jan. 17, in a child from Gadap Town area of Karachi, Pakistan. Cases with dates of onset in 2015 are still being identified (eg, in mid-February, there was a report of a WPV1 case with onset of paralysis on Dec. 20, 2015, in the Shah Wali Kot district of Kandahar, Afghanistan). As of Feb. 18, the global total number of polio cases due to WPV1 infection in 2015 is now at 74, all in the two remaining polio endemic countries — Pakistan and Afghanistan. As of this same date, the total number of polio cases due to cVDPV infection in 2015 is now at 28, of which 25 cases were reported in countries that are considered nonendemic, where transmission of wild poliovirus has been interrupted for at least 3 consecutive years.

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Given the challenges inherent with the use of the trivalent OPV (ie, producing cVDPVs), and with the certification of eradication of WPV2, there will be a global shift this month from the use of trivalent OPV (tOPV) to the use of a bivalent OPV (bOPV) containing types 1 and 3, thereby eliminating the risk for creating new cases of cVDPV type 2. In addition, a stated goal of the Polio Eradication Initiative is to have all countries introduce at least one dose of the IPV by the end of this year, so that WPV2 protection will be provided even with the removal of the OPV type 2. As of Aug. 15, 2015, 97 countries were known to be using one or more doses of IPV as part of routine immunization activities. (And there were 111 countries that had introduced the IPV, according to a presentation prepared by WHO on countries using or planning to introduce IPV and the global status of bOPV registrations, as of Oct. 1, 2015.)

Existing barriers to eradication

What are the remaining challenges? First is the interruption of WPV circulation in the two remaining endemic countries. In both Pakistan and Afghanistan, there are areas with civil unrest and intermittent violence against polio vaccinating teams. As recently as Feb. 17, there were two attacks on polio vaccinators in Lahore, Pakistan, and just before, on Jan. 13, a suicide bomber killed 16 people and injured more than 10 near a polio center in Quetta, Pakistan. In addition, in areas controlled by fundamentalist groups, rumors abound on the use of these vaccines to sterilize children. With the continued circulation of the WPV in these two countries, there exists the potential for the introduction of the WPV into countries that have interrupted transmission. In 2014, there were cases of WPV identified in Ethiopia, Syria, Iraq, Equatorial Guinea, Cameroon and Somalia — all considered to have interrupted WPV transmission, with virus introduced from the three then-endemic countries (Nigeria, Pakistan and Afghanistan).

Second, the problem remains of the occurrence of outbreaks associated with cVDPVs in countries that have interrupted WPV circulation. In 2015, there were outbreaks of cVDPV1 in Myanmar, Laos and Ukraine, with last known WPV cases in each country in 2007, 1993 and 1996, respectively. As long as the OPV is being used for routine immunizations, the dangers of polio outbreaks associated with cVDPVs in areas with suboptimal immunization coverages are present. However, a transition to a pure IPV schedule carries significant economic as well as logistic challenges.

The third problem is the need to recognize that there can be “silent” transmission of the poliovirus (both WPV and cVDPV); there is the need for environmental sampling to check for the presence of these viruses. An example of this was the finding of WPV1 in environmental samples in Egypt in 2012, followed by the discovery of WPV1 in environmental samples in Israel and the West Bank in 2013 and 2014, and outbreaks of WPV1 in Syria and Iraq in 2013 and 2014.

To quote the impatient child: “Are we there yet? When will we get there?” It has been 28 years since the Global Polio Eradication Initiative was launched, and no, we are not there yet, but we are well on our way.

  • For more information:
  • Donald Kaye, MD, is a professor of medicine at Drexel University College of Medicine, associate editor of ProMED-mail, section editor of news for Clinical Infectious Diseases and an Infectious Disease News Editorial Board member.
  • Marjorie P. Pollack, MD, is deputy editor of ProMED-mail and an independent consultant medical epidemiologist with a focus on developing world issues following CDC training. She is based in New York.

Disclosures: Kaye and Pollack report no relevant financial disclosures.

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