Why I got my one-year-old vaccinated against polio

It's a Thursday afternoon, and I should be at work. Instead, I'm chasing my toddler around the small, disheveled garden behind my doctor's office, along with around 15 other parents. We're all here for the same reason-to get our young children vaccinated against polio. We're doing about 200 children today," the nurse tells me.

My youngest daughter-who is currently running around the garden pointing at piles of stones and yelling That's poo!"-will be two in October. She's already had three doses of polio vaccine and is scheduled to receive another when she's three years and four months old. But thanks to the detection of polioviruses in sewage in North and North East London, where we live, she is one of hundreds of thousands of children between one and nine in the city who are being offered a booster dose.

It's not the first time the poliovirus has been found in London's sewage. But this time, there are signs that it may be spreading. No cases have been diagnosed in the UK since 1984, but in the US, a 20-year-old man in Rockland County, New York, has developed paralysis caused by polio-the country's first diagnosed case of the disease since 2013. What's going on? And can booster vaccination campaigns like London's help?

The virus spreads incredibly easily-usually via contaminated food or water, or through close contact. Once it enters a person's body, the virus makes its way to the gut, where it replicates and can cause an upset stomach.

For many people, that's the end of the story-symptoms are mild in up to 90% of cases, according to the World Health Organization. But in some cases, the virus attacks the nervous system, which can lead to paralysis of the limbs or-more rarely-the muscles we use to breathe. When this happens, an infection can be fatal. And while many people recover from severe disease, they can be left with some degree of permanent paralysis.

All but two countries have managed to eliminate wild poliovirus since vaccines were introduced in the 1950s and 1960s. The virus is still circulating in Afghanistan and Pakistan, though vaccination efforts are underway in both places.

Little virus factories

The UK Health Security Agency says it is urgently investigating the spread of the virus in London. Genetic sequencing of the samples found so far shows that the virus closely resembles that found in a type of polio vaccine.

Only one of the two vaccines in use today can lead to vaccine-derived polio. The oral polio vaccine, delivered via drops, contains a live, but weakened, form of poliovirus. When this virus reaches the gut, it replicates for a limited amount of time and can generate a strong immune response, protecting that person from future infections. This virus is also shed in the person's feces.

But, as we all know, viruses can mutate. Very rarely, the weakened virus can mutate into a form that can cause disease. Extremely rarely, it can cause polio in people who receive the vaccine and also have a weak immune system. And because it is shed in feces, it has the potential to spread-and can cause outbreaks among unvaccinated people, even if the vaccinated individual didn't get sick. Most people who receive the oral vaccine only shed the virus for a few days, but some who don't clear the virus as quickly can shed it for years, says Nicola Stonehouse, a virologist at the University of Leeds in the UK.

In these cases, the virus has more opportunities to mutate in the person's body, while continually being shed. They're basically being used as a little virus production factory," says Stonehouse.

This doesn't happen with the injected vaccine, which contains a form of the virus that is essentially dead and unable to replicate at all. That's why many countries have switched to injected polio vaccines-the UK moved away from oral polio vaccination in 2004, for example. The injected vaccine is delivered in four or five doses, the first at two months of age.

How did it get here?

So why is the virus in London's wastewater? The transmission chain probably started with a child-someone who had recently been vaccinated with the oral vaccine in another country, says Stonehouse. That child may be absolutely fine, and may have stopped producing vaccine-derived polio by now," she says. But they may have passed it on to somebody else, who has passed it on to somebody else." The virus appears to have spread through a small, but unknown, number of people in London.

I say London because that's where the virus has been spotted. Sewage is routinely screened for a range of viruses at a couple of sites there and one in Scotland, says Stonehouse. But many virologists think the virus could be spreading more widely in the UK and beyond-it's just that we haven't looked.

Why now? Stonehouse puts it down to bad luck." It's possible that this kind of spread has happened before and we just didn't notice. That's no reason to be blase, though. The virus spreads so easily that any indication of transmission is really worrying," she says.

Just how worried should we be? Adults can develop severe disease, but it's rare, and they should still be protected by any vaccinations they received in childhood. I'm feeling grateful for the vaccine-dosed sugar lump I was fed when I was a child.

It's children under five who are at greatest risk of polio and its complications, so it's important to make sure children are up to date with their routine vaccinations. My daughter should be protected already-she's already had the three doses recommended for a child her age.

Children are more likely to become infected with and spread the poliovirus than adults are, and they can do so even if they've received the injected vaccine. A statement from the UK's Joint Committee on Vaccination and Immunisation (JCVI) mentions unpublished evidence that the whooping cough vaccine given to pregnant women can lower their babies' immune response to their first vaccinations, suggesting that those vaccines might not be as protective in early life. That is partly why even vaccinated children like mine are being offered the vaccine now. And because the vaccine is so safe, even if you're fully covered, it won't hurt to get another one," says Stonehouse.

Polio can't be cured, but it can be prevented. Which is why I packed my daughter, along with toddler-friendly biscuits and a ready-loaded YouTube Cocomelon video on my phone, off to our local doctor on a sunny Thursday afternoon.

My eldest daughter has not been invited for a booster dose-vaccines aren't being offered to children who have received their preschool shot (at three years and four months) within the last year. But she'll be eligible in a couple of months' time. I'm hoping we won't need one by then.