How can an eradicated disease like polio suddenly come back?

Currently, the last wild strain of polio can only be found in Pakistan and Afghanistan. In the rest of the world, it's kept at bay by strong immunisation programs where three injections of the polio vaccine are considered to bestow the recipient 99 per cent effective disease prevention.

Such programs have run across the world for over three decades. That's why the reports of the disease in developed nations with strong immunisation history are so surprising. Cases have been reported in New York State, US, and the virus has been detected in London and Jerusalem wastewater tests.

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Polio is usually transmitted person-to-person by faecal-oral transmission, highlighting the importance of basic public hygiene, including even the simplest act of hand washing.

Initially, the virus may infect the throat and increase the risk of respiratory transmission, but it then enters the digestive system where it replicates in the gut and is shed in faeces for at least six weeks.

Nine in 10 people infected with poliovirus will be asymptomatic and recover. Some might have nothing more than a fever and sore throat. These symptoms may be indistinguishable from those of other diseases.

It's when the virus gets into the spinal cord that the most severe symptom - paralysis - can occur. Around one percent of infected people will experience this severe symptom. But polio management isn't a case of prevention being better than cure. There is no cure.

Having no cure available for polio is why health authorities around the world use immunisation programs to prevent the disease spreading.

Largely, these have been successful: wild poliovirus type 1 remains only in Pakistan and Afghanistan. Types 2 and 3 have been globally eradicated.

In the United States, over 92 per cent of children under two years of age are immunised against polio. Among the standard suite of seven childhood vaccines recommended for various infectious diseases, polio has the highest rate of uptake among US children.

That figure is similar in other nations - around 93 per cent in the UK and Israel are immunised against the virus. And yet cases and wastewater detection in these countries indicates it is spreading again.

The response has been swift: New York's governor declared a state of emergency to address the emergence of the disease, mobilising more health services to administer the vaccine to those who need it. The UK has implemented a campaign to get vaccine into the arms of unprotected children.

It would be easy to assume that wild polio has hitched a ride to these places from a nation where it remains active, and it would be possible - travelling to or from Pakistan or Afghanistan does carry the risk of transmitting Type 1 polio.

But authorities aren't finding the wild virus in the US, UK or Israel. Rather, a "vaccine-derived" form of polio has emerged. While that might seem surprising, vaccine-derived poliovirus (VDPV) is somewhat common, and it's due to the survival tactics employed by all viruses.

A live, attenuated (diminished) form of the virus is used in oral polio vaccines (OPVs). These are administered through the mouth and widely used in nations which need to swiftly stifle an outbreak. Once eradication has been achieved, authorities generally implement injected polio vaccine programs (IPVs), which use an inactive version of the virus.

Because OPVs are administered as droplets in the mouth, they eventually mimic a natural polio infection in the gut, which provides mucosal immunity. IPVs trigger a less strong - but still very effective - antibody response.

It's the presence of live virus in OPVs that carries some risk of eventual viral escape.

Even though the attenuated virus in OPVs are weaker, they're still capable of doing the one thing viruses are renowned for: Mutating.

As Royal Melbourne Hospital's Associate Professor Bruce Thorley, director of the World Health Organization's Polio Regional Reference Laboratory at the Doherty Institute told Cosmos, this is why the disease has been discovered among unvaccinated people in the US.

"The normal part of the replication cycle of the polio virus, whether it's a wild polio virus or vaccine strain, can have mutations during the replication cycle, where the sequence within the viral genome may change as it is growing in the gut," Thorley says.

"What can occur is in areas of lower vaccine coverage, you can get the vaccine virus being shed in the stool [of vaccinated people].

"And then if, through person-to-person contact, it infects someone who's not immunised, that virus will continue to grow and may accumulate another mutation.

"The vaccine strain can incorporate a number of mutations that then lose its weakened form, and then it can actually cause polio.

"That is what has been described in New York."

This is partly why successful immunisation programs rely on hitting a base level of vaccine coverage to prevent serious infectious diseases spreading.

On face value, coverage of over 90 per cent vaccination among a national population should be more than enough to prevent the spread.

In Australia, estimates published in Communicable Diseases Intelligence (CDI) suggest around 82 percent of people are immunised against type 1 wild poliovirus. These are "sufficient to prevent outbreaks of this type."

Other countries have similar levels of immunisation.

But reports suggest that polio has remerged in New York State by spreading among vaccine-resistant communities, where some immunisation rates in these areas could be as low as 37 per cent .

This is where the concept of 'herd immunity' takes a hit.

"We estimate polio vaccination coverage to be about 80 to 85 per cent of the Australian population," says Dr Daryl Cheng, who is the medical lead at the Melbourne Vaccine Education Centre

While polio has not re-emerged in Australia, another infectious disease - diphtheria - has.

How can this happen, when diphtheria vaccination rates among children have been over 90% nationally since 2000?

Some age demographics - in this case, people born before 2000 - might be 'under-vaccinated'.

Similarly, some geographic areas may have lower rates of vaccination masked by the high national average. This is what parts of New York state are seeing with polio's return - while a nation's population may have a strong level of overall immunisation, herd immunity may not necessarily exist in smaller communities.

Earlier his year, two diphtheria cases were reported in children for the first time since 1992 in northern New South Wales. This region has that state's lowest level of fully immunised people under five years.

Diphtheria immunisation rates in this region are some of the lowest in Australia - around 85% in some pockets.

"The diphtheria rates - or the childhood vaccination rates in general - were below the herd immunity threshold, putting them at risk," says Cheng.

"If you looked at that population, let's hypothetically say their coverage was 40 per cent, or 30 per cent , or 60 per cent below the heavy immunity rates - or the calculated herd immunity rate - it would place that population at risk of a specific vaccine preventable disease."

While no Australian jurisdiction reports childhood vaccination rates as low as the 37 percent levels touted in some parts of New York State, fewer than 4 in 5 children in some areas and age brackets are fully vaccinated.

Cheng says the high level of public discussion around COVID-19 vaccines over the past two years may have resulted in broader reluctance for parents to immunise their children, and that is something health authorities and educators are trying to repair.

"Just because the COVID vaccines are new doesn't mean the measles vaccine is new, the measles vaccine has been around for many years, the polio vaccine has been around for years and years," Cheng says.

"People who previously did not question, or would get their kids vaccinated on time, because of the impacts of the discussions around COVID vaccines, have thought twice about bringing their kids to get vaccinated."