Although polio was officially declared eradicated in 2004, the virus was discovered last week in sewage samples from north and east London. The United Kingdom’s Health Security Agency (UKHSA) believes that the virus arrived with someone who was recently vaccinated overseas with the live, attenuated (weakened) form of the virus. Although the vaccine-derived poliovirus can spread to unvaccinated people, the main message from the UKHSA is to make sure your child’s vaccinations are up to date.
Most people in the US and the UK have been vaccinated with a different kind of vaccine—an inactivated poliovirus vaccine (IPV) which is a killed version of the virus and cannot cause polio. However, the pandemic has caused disruptions getting routine vaccinations, and some parents are now panicking because they do not recall whether their young child is up to date or their adolescent got the teen booster. The US immunization schedule frontloads all the doses, meaning children are fully vaccinated by age 6.
What is polio caused by?
Polio is caused by a virus passed through sneezing or coughing but it can also be caught from contaminated food or water and passed from person to person. The virus circulates in three types: type 1, type 2 and type 3. With widespread use of effective childhood vaccines, polio has not circulated in the US since 1979. Worldwide, two types of polio have been eradicated leaving only Type 1 in circulation, which means cases can be imported, as happened in London. Polio is highly infectious, spreading more easily when hand hygiene and sanitation are poor. In a household with vulnerable (unvaccinated) family members, secondary cases may be as high as 90-100%.
What symptoms does polio cause?
Like many viruses, most people don’t actually feel sick, and in fact 70% of children have no symptoms. However, others feel like they have the flu for about 10 days, and disease severity increases with age. Approximately 1 in 4 children develop symptoms which may include a fever, extreme fatigue, headaches, vomiting, and muscle aches or a stiff neck. Rarely (approximately 1% of cases), if the virus infects the brain and spinal cord, it can cause muscle weakness and paralysis. Most people are able to regain movement over time, but rarely the virus can affect the muscles used to breathe. When this happens, polio is life-threatening.
Can polio be treated?
Yes, treatment includes hospitalization where pain can be controlled and breathing support provided if needed. Therapy can help restore movement with stretching and strengthening exercises.
Polio is preventable
Two types of vaccines prevent polio: inactivated poliovirus vaccine (IPV) and oral poliovirus vaccine (OPV). In the US, the regular childhood vaccine schedule includes IPV. By showing the body an inactivated (killed) version of the virus, the immune system develops antibodies to all three types of poliovirus. In the US, children get four doses of inactivated polio vaccine (IPV) at 2 months, 4 months, 6 to 18 months and a booster between age 4 and 6. Children who got a combination vaccine (IPV with DTaP, for example) may need to get a fifth dose. In the UK, children receive a total of five doses at 8, 12 and 16 weeks, between 3-4 months, and at age 14.
If polio was nearly eradicated, how did it get to London?
The emergence of polio in London sewage is not entirely unexpected—cases pop up every now and then, but are not detected over time. This is a testament to the excellent public health surveillance capacity of the National Health Service. In this case, the vaccine-derived polio viruses found in the sewage appear to be closely linked, meaning transmission has been happening among closely connected people. The agency is now trying to sort out whether any community spread is happening. But how does a strain that was declared eradicated manage to resurface?
The answer lies in how the oral polio vaccine works. The OPV vaccine is a live but weakened version of the virus and induces immunity by replicating in the intestinal tract. For this reason, the weakened virus is shed over several weeks after vaccination and can be spread to other people when sanitation is poor and handwashing infrequent. If the weakened virus does not mutate, this spread helps to immunize close contacts. However, if the weakened virus mutates and gains strength, an unvaccinated person can become sick. Rarely, vaccine-derived poliovirus (VDPV) can cause paralytic polio. The global polio eradication program wanted to reduce harms associated with vaccines, so as soon as it was possible, a change was made to the OPV.
The Type 2 strain was dropped from the oral vaccine because it was the strain most likely to cause vaccine-associated polio. When serotype 2 was declared eradicated in 2015, the OPV became a bivalent strain (Types 1 and 3 only) and the IPV was added to the schedule. When a child receives IPV first, they develop antibodies to all three types and thus cannot get vaccine-derived polio later. However, a combination of low immunization coverage and the switch to a vaccine with only two serotypes left a gap open for type-2 VDPV to resurface.
The vaccine virus can circulate in London among people who were never vaccinated. Given the pandemic disruptions, vaccination coverage has fallen to 86%, leaving 14% unprotected. These individuals can be infected with the vaccine-associated strain and pass it to others. For this reason, the two key messages are: 1) sewage surveillance is extremely helpful for identifying emerging pathogens, and 2) vaccination remains the key to prevention. Check your child’s immunization schedule for IPV or OPV and count the doses. If you are unsure of your child’s vaccination status, call your primary care provider and schedule an immunization appointment to catch up on any missed doses during the pandemic.
Resources
Routine Polio Vaccination | CDC
Polio Disease and Poliovirus | CDC
Vaccine-derived polio is on the upswing. Can a new vaccine stop the spread? : Goats and Soda : NPR
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