No 34 - 2012

Eradicating poliomyelitis: the final countdown?

In June 2012, ten years had passed since the certification of the WHO European region as polio-free. At this time, the number of polio-endemic countries has never been lower and one of the world’s most populated countries, India, is now considered non-endemic, having seen its last polio case in January 2011.

Despite tremendous progress achieved in reducing the global number of cases, continued circulation and a rising number of cases in endemic countries still threatens the final goal of eradicating poliomyelitis. The global number of reported wild polio virus (WPV) cases was 1604 in 2009, 1352 in 2010, 650 in 2011 and 121 cases as of 15th of August 2012. Although the global number of reported cases has decreased, poliomyelitis remains a threat until eradication has been achieved, even for non-endemic countries including Denmark.

Background on the disease

Poliomyelitis is a communicable disease caused by 3 poliovirus serotypes (serotypes 1, 2 and 3). The main route of transmission is fecal-oral. In the pre-vaccine era, virtually every child was infected with poliovirus and 1/200 infected children developed paralytic poliomyelitis. Progress in poliomyelitis control has been achieved mainly owing to the widespread use of the live attenuated oral polio vaccine (OPV) and the inactivated poliovirus vaccine (IPV). The OPV has been administered in many countries because of its ease of administration, its low cost compared with IPV and its higher capability of inducing intestinal mucosal immunity.

The Global Poliomyelitis Eradication Initiative and recent trends in countries with on-going transmission

Since the launch of the Global Poliomyelitis Eradication Initiative by the 41st World Health Assembly in 1988, the global incidence of poliomyelitis has been reduced by more than 99%. Tremendous efforts have resulted in the certification of 3 of the 6 WHO regions as free of wild-strain polioviruses (the Americas, Europe and Western Pacific). Furthermore, the WPV type-2 has not been detected worldwide since 1999.

However, the eradication process has been facing many challenges and obstacles including geopolitical impediments, social obstacles, weak political commitment, distrust towards poliovirus vaccines and emergence of virulent vaccine-derived polioviruses (VDPV). Monovalent and bivalent vaccines have proven to be more effective than the trivalent OPV. Use of the monovalent vaccine for the Supplementary Immunization Activities (SIA) has resulted in better control of both WPV and virulent VDPV. Despite dramatic progress achieved towards the goal of polio eradication, re-emergence of the disease in polio-free countries remains a major concern and poses a threat at the global level.

The number of endemic countries has been reduced from 125 in 1988 to 3 in 2012: Nigeria, Pakistan and Afghanistan. Following importation, transmission of wild poliovirus has been on-going for more than 12 months in Chad. In contrast, no new cases were reported in the Democratic Republic of Congo nor in Angola in 2012. In fact, Angola has now passed a year without polio, and in India no new cases have been reported since the 13th of January 2011. This represents a “monumental milestone” in the history of poliomyelitis eradication.

Pakistan: In 2011, the number of reported cases accounted for 30% of the total number of reported cases worldwide. Serotypes 1 and 3 were both circulating in the country and as of 15th of August 2012, 29 cases have been reported (year-to-date 2011: 69 cases).

Nigeria: The Northern States of the country have a highly persistent, on-going transmission and serotypes 1, 3 and Vaccine Derived Polio Virus (VDPV) cases are circulating in the states of Borno and Kano. As of 15th of August 2012, 70 cases have been reported (year-to-date 2011: 23 cases).

Afghanistan: The number of cases rose three-fold in 2011 (80 cases) compared with 2010 (25 cases) and as many as 270,000 children are not accessible to vaccination teams in the south of the country because of the conflict situation. Polio has spread outside its traditional reservoirs in the south reaching up to the northern border which increases the risk of exportation to the neighboring countries of Turkmenistan, Uzbekistan and Tajikistan. As of 15th of August 2012, 17 cases have been reported (year-to-date 2011: 12 cases).

Chad: Chad reported only 5 cases as of 15th of August 2012 compared with 100 cases for the same period in 2011.

Poliomyelitis eradication completion: a programmatic emergency for global public health in 2012

Recent major outbreaks occurring in some countries previously declared free of poliomyelitis (Congo and Tajikistan in 2010) have demonstrated the necessity to maintain high immunity levels until reaching the last unimmunized child in the world.

In 2012, achieving eradication is seen as an emergency by the WHO. The Executive Board of the WHO meeting in Geneva on the 21st of January 2012 and the World Health Assembly on the 25th of May 2012 declared that the completion of poliomyelitis eradication constitutes a “programmatic emergency for global public health”. The current WHO eradication goal is cessation of all wild poliovirus transmission by end-2012.
 
After eradication, use of all live polio vaccine should cease in order to prevent outbreaks of virulent VDPV. Encouraging studies have proven that IPV can be used successfully in resource-poor countries. However, OPV cessation will only be decided when certain conditions are fulfilled, EPI-NEWS 19-20/01 and EPI-NEWS 48/05.

Vaccination against poliomyelitis and polio surveillance in Denmark

OPV has been used in Denmark from the beginning of the 1960s until 2003 when this vaccine type was replaced by an IPV booster dose at 5 years of age, EPI-NEWS 50/01. In the childhood vaccination programme, an IPV primary vaccination series is administered at the age of 3, 5 and 12 months (DTaP-IPV/Hib).

In Denmark, an intensified surveillance of enterovirus, including poliovirus, has been implemented since 2001 to demonstrate continued evidence that the country is free of poliomyelitis and to be able to detect and confirm any suspicious case of poliomyelitis occurring in the country following importation, EPI-NEWS 19-20/01.

Despite the current low case number globally, the re-emergence trends seen in past years in some countries emphasize the risk that all countries are facing until poliomyelitis is eradicated. Denmark therefore needs to remain vigilant to keep up with a high level of population immunity, EPI-NEWS 23a/12, as well as with continued surveillance of enteroviruses.
(F. Dorleans, P.H. Andersen, Dept. of Infectious Diseases Epidemiology)

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