Poliomyelitis
Eradication: The Final Hurdles
Margery Kennett
As a result of global cooperation among the partners of the Global Polio Eradication Initiative, which includes the countries affected by polio, WHO, Rotary International, CDC, UNICEF and many donors, the world is on the brink of eradicating wild poliovirus. Poliomyelitis has been known since biblical times, but its causative agent, wild poliovirus, was only identified in the early 20th century. Although 90% of infected people have no clinical illness, 10% may initially have fever, headache, vomiting, neck stiffness and pain in the limbs. In 1 in 200 infections, irreversible paralysis occurs, usually in the lower limbs, and fatalities may occur if the breathing muscles become immobilised. Two safe and effective poliomyelitis vaccines, Salk inactivated IPV) and Sabin live attenuated (OPV) were introduced in the mid 1950s and early 1960s respectively. As a result, poliomyelitis had almost disappeared from developed countries by the late 1960s. Further gains in the elimination of poliomyelitis and other infectious diseases were made when the World Health Organisation WHO) introduced the Expanded Program on Immunisation (EPI) in the early 1970s, providing the means to introduce routine infant immunisation to all children in the world.
Poliomyelitis was chosen as the second virus for eradication after a successful campaign culminated in global smallpox-free certification in 1980. In 1988 the 41st World Health Assembly resolved to eradicate poliomyelitis globally by the end of the year 2000. Poliovirus is a suitable candidate as it occurs naturally only in humans, does not survive for long in the environment and effective vaccines are available. An eradication strategy involving routine infant immunisation, supplementary immunisation, surveillance and “mop-up” operations was set in place, with funding provided to poorer countries.
However, some children do not have access to routine vaccination programs. There are countries with areas of conflict, some villages or families are too remote from the central administration, births are not registered and some parents are either unaware of, or object to, vaccinations. Therefore, supplementary doses of OPV are given to every child under 5 years, regardless of their previous immunisation history, National Immunisation Days (NIDS) or Sub National Immunisation Days (SNIDS). Since viruses are not restricted to national borders, several rounds of NIDS may be coordinated across several countries to reach hundreds of millions of children. Children in conflict areas have been immunised during “days of tranquillity” negotiated by the United Nations. The herd” of susceptible children is thus reduced to level at which circulation of wild poliovirus ceases.
Surveillance is essential in an eradication campaign. Unlike smallpox where every infected person has symptoms, most poliovirus infections are silent. Acute flaccid paralysis (AFP) surveillance has been proven to be a sensitive indicator wild poliovirus activity. Case managers are now in place in every country to investigate all reports of AFP. AFP has many causes apart from wild poliovirus infection so each case must be rapidly and fully investigated and virological testing performed. After polio eradication, the work of this network can be extended to cover measles and other vaccine-preventable disease agents.
Once wild poliovirus circulation has ceased or been reduced to a few small foci in a country, mop-up” campaigns are utilised in areas around known case and in pockets of high-density urban populations. When a country is certified free of wild poliovirus, immunisation must continue until several years after the last virus has been eradicated globally as the risk of imported cases remains. Numbers of susceptible children can build up very rapidly and negate earlier efforts.
Before a region can be certified polio-free, four conditions must be met. There must be no evidence of wild poliovirus transmission for three years, surveillance must be of certification standard, each country must have a plan in place detect, report and respond to an importation wild poliovirus and all pre-eradication tasks on the safe handling of wild poliovirus and potentially infectious materials must be complete. Although the year 2000 target was not met remarkable progress has been made. In 1988 there were an estimated 350,000 cases of poliomyelitis in 125 countries. By the end of 2001 only 453 cases were reported from 10 countries. The Americas were declared polio-free 1994, the Western Pacific in 2000 and Europe 2001. In 2002, poliovirus remained endemic in seven countries with 98% of cases confined to India, Nigeria and Pakistan.
Pakistan’s program has been excellent. With the planned rounds of NIDS and SNIDS in 2003, Pakistan may be the first of the remaining endemic countries to stop transmission. Unfortunately, although 10 NIDS or SNIDS were carried out in India in 2000, there were only three rounds in 2001. Cases of wild poliovirus in India increased from 268 in 2001 to 1599 in 2002. The failure to reach adequate immunisation coverage meant that transmission of wild poliovirus was not interrupted. In Nigeria the National government’s commitment is high, however, improvements are needed at the state and local government level to eliminate the virus. OPV coverage was still too low to interrupt transmission in 2002.
There are several “end game” issues that are still being addressed. Once human-to-human transmission ceases, environmental sources of poliovirus, including laboratories and vaccine production facilities need to be considered. If there were an escape from one of these facilities into the community once immunisation ceases, we could be back where we started. WHO began consultations in 1997 with scientists from diagnostic, research, control, environmental and reference laboratories, vaccine manufacturers, biosafety experts and the polio laboratory network staff to reach consensus in preparing a global action plan for containment of wild poliovirus infectious materials. These include not only virus cultures but also tissue, faecal or sewage samples collected in a country at a time when poliovirus was endemic. One year after the last polio case has been identified, countries must ensure that all institutions working with wild poliovirus do so using appropriate containment, destroy the material, or remove it to a WHO accredited repository.
The other end game issue concerns the cessation of polio immunisation. The timing and method of doing so is crucial. Several scenarios are being investigated, including indefinite use of OPV, immunisation of everybody then withdrawal of OPV worldwide on the same day, or to continue with IPV alone. In recent years three small outbreaks of paralytic disease caused by “drifted” vaccine-derived strains have been reported. Although 97-99% identical with OPV strains, these viruses have the transmissibility and paralytic properties of wild polioviruses, and have emerged and circulated where OPV coverage has dropped to low levels. If one country persisted with OPV while neighbouring countries did not, there may be more drifted vaccine strains and thus paralytic disease as levels of immunity fell. More studies into these effects are required before the World Health Assembly can determine a post-eradication policy for polio immunisation.
The strategies employed in eradicating poliomyelitis over the last fifteen years have been proven to work when implemented correctly. The greatest challenge is to ensure that countries are properly resourced to implement these strategies. As is usual towards the end of any race, there are still a few foreseen and perhaps unseen hurdles to jump such as the US $275 million shortfall in funding through to the end of 2005, due mainly to the depressed global economy. These issues must be addressed or there will be a risk of poliomyelitis re-occurring.
Margery Kennett has worked as a diagnostic virologist since 1961 at Fairfield Hospital and at the Victorian Infectious Diseases Reference Laboratory at the Royal Melbourne Hospital. In recent years, she was the director of both the WHO Collaborating Centre for Biosafety, and the Western Pacific Regional and Australian National Polio Reference Laboratories. The Western Pacific Region was certified wild poliovirus free in October 2000, just prior to her retirement in early 2001. In January 2003 Margery Kennett was awarded the Medal of the Order of Australia for service to public health in laboratory biosafety and polio eradication.