16 Poliomyelitis

16.3 Epidemiology

16.3.1 Global burden of disease

In the pre-vaccination era, cases of poliomyelitis occurred sporadically and in epidemics in developed countries of temperate zones. In tropical countries, where the virus still circulates, there is no seasonal pattern.

Classically, poliomyelitis is a disease of young children and adolescents. However, with improvements in living standards, a greater number of cases have occurred in older individuals, with an associated higher frequency of paralytic disease. Paralytic disease is a particular risk in early adult life. In countries where polio was endemic, most children acquired antibodies to all three subtypes by age 5 years and most paralytic disease occurred in children aged under 3 years.

The resurgence of polio in some countries occurred because of the introduction of wild-type polio virus into poorly immunised populations.

In 2012 the lowest number of new polio cases (223), from the lowest number of countries, were reported than at any previous time in history.1 Cases increased in 2013 (385), mainly due to an outbreak in the Horn of Africa (207 cases) affecting previously polio-free countries (Somalia, Kenya, Ethiopia and South Sudan). Cases have also emerged in Syria, and a comprehensive outbreak response has been implemented there. Polio remains endemic in Afghanistan, Nigeria and Pakistan. Compared to 2012 there were 28 percent fewer cases in these endemic countries in 2013 (157 cases).3

Vaccine-associated paralytic poliomyelitis (VAPP) with OPV

After receiving oral poliomyelitis vaccine (OPV), most infants excrete the polio vaccine virus for about six weeks. Their family and other contacts are exposed to the vaccine virus and the contacts may then excrete the virus in faeces. There is a small risk that the vaccine virus may revert to neurovirulence and cause VAPP in a vaccine recipient or non-immune contact. VAPP presents with acute flaccid paralysis (AFP) from 7 to 30 days after vaccination in the recipient and from 7 to 60 days in the contact of a vaccine recipient. The immunosuppressed are more likely to suffer VAPP, whether they receive vaccine or acquire infection as a contact.

VAPP presenting in New Zealand can only occur from contact with people vaccinated in countries still using OPV. The risk of importing wild-type or neurovirulent oral vaccine-derived strains means that maintaining high inactivated poliomyelitis vaccine (IPV) coverage in New Zealand is essential.

Once the wild virus became uncommon and restricted to specific countries, the risk of VAPP became higher than the risk of imported wild virus disease. This led New Zealand to change from OPV to IPV in 2002 to eliminate the risk of VAPP (see Appendix 1). The last case of VAPP in New Zealand occurred in 1999.4

16.3.2 Global poliomyelitis eradicationTop

In 2012 the World Health Assembly declared the persistence of polio ‘a programmatic emergency for global public health’ and called on the WHO to develop a comprehensive polio end game strategy. The Polio Eradication & Endgame Strategic Plan 2013–20181 was developed by the Global Polio Eradication Initiative. Its goal is ‘the complete eradication and containment of all wild, vaccine-related and Sabin polioviruses’ by 2018.

The Americas were certified polio-free in 1994. The Western Pacific, which includes New Zealand, was the second region to be certified polio-free, in October 2000, with no indigenous polio cases reported since March 1997. Vaccination against polio will continue worldwide until the disease has been eradicated.

16.3.3 New Zealand epidemiologyTop

New Zealand has been free of wild-type polio for about 50 years (see Figure 16.1). Since 1962 only six polio cases have been reported.5 Four of these cases were laboratory confirmed as VAPP and two were classified as probable VAPP. There were no polio notifications in 2013.

Figure 16.1: Numbers of cases of poliomyelitis, 1915–2013

Figure 16.1: Numbers of cases of poliomyelitis, 1915–2013

Key: IPV – inactivated polio vaccine; OPV – oral polio vaccine.

Source: Ministry of Health and the Institute of Environmental Science and Research

The New Zealand Paediatric Surveillance Unit carries out active surveillance of AFP. In 2012 there were eight cases of AFP notified to the Unit. All cases have been reviewed by the New Zealand National Certification Committee for the Eradication of Polio (NCCEP) and all have been classified as non-polio.5