On July 21, 2022, New York State health officials announced the first case of polio in the US since 2013. The US resident, who suffers from muscle weakness and paralysis, had not been vaccinated.
Before safe and effective vaccines were invented in the mid-20th century, polio was a common cause of paralysis in children. Thanks to global vaccination campaigns, polio is almost eradicated. Only 13 cases of endemic wild poliovirus have been reported in 2022 to date worldwide.
The New York patient reportedly contracted a form of polio traced back to the live, but weakened, poliovirus used in the oral polio vaccine. This version of the vaccine has not been used in the US since 2000. Health officials said the virus that affected the male patient likely originated somewhere abroad, where oral vaccines are still administered.
William Petri is an infectious disease specialist and chairman of the World Health Organization’s Polio Research Committee. Here he explains what vaccine-derived poliovirus is and why the inactivated polio vaccine currently given in the US cannot cause it.
What are the two types of polio vaccine?
Vaccines introduce a harmless version of a pathogen into your body. The idea is that they train their immune system to fight off the real germ if they ever find it.
Originally developed by Albert Sabin, the oral polio vaccine uses a live but weakened poliovirus that is ingested in a sugar cube or drop. Scientists weaken, or attenuate, the virus so that it can no longer cause disease.
The other type of polio vaccine that is given by injection was originally developed by Jonas Salk. Contains inactivated, dead virus.
At 2, 4, and 6 months of age, children in the US receive the inactivated polio vaccine. Provides almost complete protection against paralytic poliomyelitis.
How can the live vaccine lead to a case of polio?
The weakened form of the live virus in the oral vaccine cannot directly cause illness. However, because the vaccine is administered orally, the weakened virus is excreted in the feces. From there it can spread from someone who is vaccinated to their close contacts. If the weakened virus circulates from person to person long enough, it can mutate and regain its ability to cause paralysis.
The mutated virus can infect people in communities with poor sanitation and low vaccination rates, causing illness and even paralysis.
Fortunately, this is an extremely rare occurrence. With more than 10 billion doses of oral polio vaccine administered since 2000, fewer than 800 cases of vaccine-derived polio have been reported.
Apparently, the current patient in New York was somehow exposed to a mutated poliovirus that had been transmitted after vaccination abroad. Earlier this summer, routine surveillance detected vaccine-derived poliovirus in London’s sewage system, but no cases have been reported there.
Why use the oral vaccine anywhere if it carries this risk?
Actually, there is a silver lining in the fact that weakened live virus can circulate in the community once recipients of the oral vaccine excrete it in their faeces. Traveling by the faeces-to-oral route, it can help induce immunity even in people who were not directly vaccinated. The oral polio vaccine is also cheaper and easier to administer than inactivated polio vaccines.
Most importantly, the live virus vaccine stops the transmission of wild poliovirus in a way that the inactivated virus vaccine does not. The eradication of polio in the Americas, Europe, and Africa has been achieved solely through the use of the live oral vaccine. Once polio has been eliminated from a continent, then it is safe to stop using the live oral vaccine and use only the inactivated vaccine, which prevents disease in recipients and does not pose the rare risk of vaccine-derived paralytic polio .
A new, safer oral polio vaccine that has been designed not to mutate is now replacing the earlier live virus vaccine. Therefore, even this extremely rare complication of polio vaccination should soon become a thing of the past.
WHO: Polio eradication: reaching every last child.
How close is the world to eradicating polio?
Thanks to a tremendous global effort, two of the three viruses that cause polio have been eradicated. The world is now on the verge of eradicating the last one, wild poliovirus 1 (WPV1).
Today, endemic polio is only found in Pakistan, with 12 cases of paralytic polio so far in 2022, and Afghanistan, with just one case this year. Africa has two cases, imported from abroad, which are being contained through additional vaccination campaigns.
Once wild poliovirus has been eradicated from the planet, vaccination efforts can switch to inactivated polio vaccine, eliminating the risk of future vaccine-derived cases.
Written by William Petri, Professor of Medicine, University of Virginia.
This article was first published on The Conversation.
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