There are a significant number of babies and children potentially at risk of polio, especially in areas of high deprivation and among Māori, with an outbreak just a plane ride away
Opinion: The recent case of polio in New York State in an unvaccinated young man, and the discovery of type 2 vaccine-derived poliovirus in sewage from London and New York is a stark reminder of the importance of keeping vaccination rates high.
Polio cases are the tip of an infection iceberg. For every case of polio, there are about 200 people infected by this disease, which can cause permanent paralysis, and sometimes death.
Before a polio vaccine was available, New Zealand experienced large recurrent polio epidemics which killed 173 in 1925 – the worst year – and left many with permanent impairments and pain.
Generations of polio survivors have faced post-polio syndrome (renewed muscle weakness, difficulty with breathing and swallowing), which can develop decades after the initial infection.
Vaccine-derived polio occurs when the attenuated (weakened) live virus used in the oral polio vaccine mutates back into a form that can cause disease. Under-immunised populations allow vaccine-derived polio to emerge and spread.
Genetic analyses have linked samples from London and New York to the outbreak in Jerusalem earlier this year. In February, an unvaccinated three-year-old developed polio paralysis, and subsequently eight more children were found to be infected. Only one of those children was up-to-date on their polio vaccinations.
The world is close to eradicating polio for ever. Of the three types of poliovirus, wild types 2 and 3 have been eradicated. Only wild type 1 remains in circulation, primarily in Pakistan and Afghanistan. However, at the time of writing, cases of circulating vaccine-derived poliovirus type 2 have also occurred this past week in Niger, Nigeria, and Yemen, and there have been environmental detections in Benin, DR Congo, Ghana, Niger, and Nigeria.
However, at the time of writing, cases of circulating vaccine-derived poliovirus types 1 and 2 have also occurred this past week in DR Congo, Mozambique, and Yemen, plus environmental detections in Ghana and Madagascar.
The oral vaccine has many advantages – it provides slightly better immunity to polio and it can be administered by volunteers as oral drops, compared with the injected vaccine which needs to be given by a trained health professional.
However, the inherent risk of a live virus vaccine means the global polio eradication initiative is in the process of shifting away from use of the oral vaccine to only the injected, killed-virus version. In the meantime, a newly developed more genetically stable oral vaccine against poliovirus type 2 is also being used in high-risk countries.
The last case of polio caused by wild poliovirus was in 1977, and the last vaccine-derived polio case was in 1998. Since 2002, New Zealand has only used the inactivated (killed) version of the vaccine, and has remained free of further polio cases.
Children in Aotearoa New Zealand are recommended to receive polio vaccinations as part of their routine free childhood immunisations at six weeks, three months and five months old, with a booster dose at age four.
However, since the beginning of the Covid-19 pandemic, childhood immunisation rates for polio, and other ‘childhood’ diseases, have dropped.
As of the end of June, only 67.2 percent of eligible six-month-olds had completed all of their age-appropriate immunisations, which should include their series of three polio shots. In the same period (1 April – 30 June) in 2019, 78.6 percent of six-month-olds were up to date on their immunisations.
For those living in areas at the highest levels of deprivation (NZ Deprivation Index decile 9-10), those stats were already low at 70.0 percent in 2019, but now are at 54.9 percent. Even more alarming, for Māori those stats dropped from only 62.5 percent in 2019 to 45.9 percent now.
That means there are a significant number of babies and children potentially at risk, especially in areas of high deprivation and among Māori, with a potential outbreak – whether from a vaccine-derived strain or wild poliovirus – only a plane ride away.
