New York health officials announced in July that an unvaccinated adult man from Rockland County had been diagnosed with polio-the first case of the life-threatening disease in the United States since 2013. The virus that causes polio was later detected in New York City wastewater, and city and state health officials now say the virus is probably circulating in the city.
The virus identified in New York is a vaccine-derived poliovirus. Wild polioviruses were eliminated from most of the world and now circulate only in Afghanistan and Pakistan.
But vaccine-derived viruses, which emerge when the weakened viruses in the oral polio vaccine mutate and spread in unvaccinated populations, still occasionally cause outbreaks.
VOA spoke with three polio experts about vaccine-derived polioviruses and the oral polio vaccine. Here's what you need to know.
What are vaccine-derived polioviruses?
Vaccine-derived polioviruses are related to the active viruses in the oral polio vaccine (OPV).
OPV works by infecting cells in the gut with weakened polioviruses, allowing the body to safely develop immunity to polio without the risk of paralysis posed by the real disease.
"[The weakened viruses] would still infect you. They would still replicate in your gut.You will develop a lifelong immunity, but you will not get paralyzed,' said virologist Konstantin Chumakov, a Global Virus Network Center of Excellence director and an adjunct professor at The George Washington University.
'But the [vaccine-derived] virus will still be able to transmit [from person to person]," Chumakov added. "It was considered a big advantage of the vaccine, because basically, you can immunize several kids with one dose."
This transmission becomes problematic in communities with low vaccination rates. If the virus can spread for a long time, it has many chances to mutate and revert to a dangerous paralytic form.
Why is the oral polio vaccine (OPV) used?
Although OPV is linked to vaccine-derived polioviruses, it also has a number of advantages over the inactivated polio vaccine (IPV) - the injected vaccine - which is still used in most of the world.
People who receive OPV cannot be 'silent spreaders." After developing an immune response to the vaccine, they are immune to polio for life. Polioviruses cannot replicate in their gut and infect others.
In contrast, IPV protects against paralysis, but does not prevent the virus from replicating in the gut. People who receive IPV can spread polio, even though they won't get sick from it.
'In populations where you want to stop the spread, that 'gut immunity' that OPV confers is essential,' said Captain Derek Ehrhardt, a polio incident manager at the Centers for Disease Control and Prevention (CDC). 'If you have just IPV, it's possible that the child will be protected, but they could still spread [paralytic polio] to their neighbor, their brother, their sister, whomever."
This made OPV instrumental in the eradication of wild polio from most of the world, including in countries such as the United States that now use IPV exclusively.
OPV also has other advantages.
"We work with oral polio vaccine because it is low cost," said Richter Razafindratsimandresy, head of the National Reference Laboratory for Poliomyelitis at the Institut Pasteur de Madagascar. "And it is easy to administer, because it is oral - they don't need to inject the children, and it is not a problem for the parents to accept."
If OPV prevents silent spread, why are the U.S. and other nations using only IPV?
After the United States and other wealthy countries eliminated wild polio, they stopped using OPV because it carries the risk of creating vaccine-derived polioviruses in undervaccinated communities, and because it has a slight risk of causing paralysis. This happens between roughly two and four times per million births, according to the Global Polio Eradication Initiative (GPEI). Because there are so few cases in the U.S., thanks to the very successful campaigns with OPV, that risk is now seen as unacceptable.
The U.S. and other wealthy countries have robust health care systems and can effectively acquire the more expensive IPV and get it into every arm. So, using IPV means the well-vaccinated populations in these countries are well-protected from paralysis from polio, despite being less protected against catching and spreading polioviruses.
How do vaccine-derived poliovirus cause outbreaks?
Countries with weak vaccination systems are more likely to experience outbreaks of vaccine-derived poliovirus. That's happened several times in Madagascar, Razafindratsimandresy said.
"[The virus] is not from another country. Because in Madagascar, we have ... immunization coverage [that] is very, very low," he said.
Imported vaccine-derived polioviruses can also cause outbreaks in countries with high vaccination rates, especially if IPV is used instead of OPV.
'Basically, it can transmit from person to person to person without causing any symptoms, because everybody is protected from paralysis,' said Chumakov. 'But at some point, the virus can hit an unvaccinated person or a person with immune deficiency, and then it can paralyze this person. And this is exactly what happened in New York.'
Is OPV safe?
"Our vaccines are safe and effective," said Ehrhardt. "We need to vaccinate our under- and unimmunized children to stop the ongoing spread of these viruses."
Chumakov said that developing better versions of OPV could reduce risks while also keeping the gut immunity provided by OPV, which he said is important for preventing silent spread. He was previously involved in a GPEI effort to develop a safer oral vaccine for Type 2 polioviruses. Clinical trial data suggest the novel OPV is less likely to revert to a dangerous form.
While the future may bring improved vaccines, vaccinating as many children as possible with existing OPV remains a priority in much of the world.
According to the CDC, global polio vaccination coverage sunk to 81% in 2021, the lowest in a decade. This was largely due to the COVID-19 pandemic, but not all countries have enough resources for vaccination campaigns even in normal times.
Razafindratsimandresy said that even though Madagascar aims to vaccinate all children with OPV, the country doesn't have enough personnel and often runs out of vaccine.
"These immunity gaps must be closed for us to stop these diseases," Ehrhardt said. "If you have polio anywhere, then children everywhere are still at risk."