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 deadly disease in the United States since 2013. The virus that causes polio was later found in New York City’s 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 eradicated from most parts of the world and now spread only in Afghanistan and Pakistan.
But vaccine-derived viruses, which are exposed when weakened viruses in the oral polio vaccine mutate and spread to unvaccinated populations, still occasionally cause outbreaks.
VOA spoke with three polio experts about vaccine-derived poliovirus and 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 intestine with weakened polioviruses, allowing the body to safely develop immunity to polio without the risk of paralysis resulting from the actual disease.
,[The weakened viruses] will still infect you. They will still repeat in your stomach. You will develop a lifelong immunity, but you will not be paralyzed,” said virologist Konstantin Chumakov, a Global Virus Network Center of Excellence director and an assistant professor at George Washington University.
“But, he [vaccine-derived] the virus will still be able to transmit [from person to person]Chumakov said.
This transmission becomes problematic in communities with low vaccination rates. If the virus can spread for a long time, there are many possibilities for it to mutate and return in a dangerously paralyzed form.
Why is the oral polio vaccine (OPV) used?
Although OPV is closely related to the vaccine-derived poliovirus, it has several advantages over the inactivated polio vaccine (IPV) – the injected vaccine – that is still used in most parts of the world.
People receiving OPV cannot be “silent spreaders”. After developing an immune response to the vaccine, they remain immune to polio for life. Polioviruses cannot replicate in their own gut and cannot infect others.
In contrast, IPV prevents paralysis, but does not prevent the virus from replicating in the intestine. People who get IPV can spread polio even if they don’t get sick from it.
“In populations where you want to stop the spread, the ‘gut immunity’ that OPV provides is essential,” said Capt. Derek Ehrhart, polio event manager at the Centers for Disease Control and Prevention (CDC). “If you just have IPV, it’s possible that the child will be protected, but they can still be spread. [paralytic polio] Your neighbor, your brother, your sister, whoever.”
This allowed OPV to be instrumental in eradicating wild polio from much of the world, including countries such as the United States that now exclusively use IPV.
There are other advantages of OPV.
“We work with the oral polio vaccine because it’s low-cost,” said Richter Razafindersimandresi, head of the National Reference Laboratory for Poliomyelitis at the Institut Pasteur de Madagascar. “And it’s easy to administer, because it’s oral—they don’t need to be injected to children, and parents have no problem accepting it.”
If OPV silence prevents spread, why are the US and other countries only using IPV?
After the United States and other wealthy countries wiped out wild polio, they stopped using OPV because it carries the risk of creating a vaccine-derived poliovirus in less vaccinated communities, and because it carries little risk of paralysis. it happens. According to the Global Polio Eradication Initiative (GPEI), it occurs between about two and four times per million births. Because there are so few cases in the US, thanks to very successful campaigns with OPV, that risk is now seen as unacceptable.
The US and other wealthy countries have robust health care systems and can effectively afford more expensive IPV and take it in every hand. Therefore, using IPV means that well-vaccinated populations in these countries are well protected from being paralyzed by polio, although less protected from catching and spreading the polio virus.
How do vaccine-derived polioviruses cause outbreaks?
Countries with weak vaccination systems are more likely to experience outbreaks of vaccine-derived poliovirus. This has happened several times in Madagascar, Razafindratsimandresi said.
,[The virus] Not from another country. Because in Madagascar, we have… immunization coverage [that] Very, very little,” he said.
Imported vaccine-derived polioviruses can cause outbreaks even in countries with high vaccination rates, especially if IPV is used instead of OPV.
“Basically, it can be transmitted from person to person without any symptoms, because everyone is protected from paralysis,” Chumakov said. “But at some point, the virus can kill an unvaccinated person or a person with an immune deficiency, and then it can paralyze this person. And that’s exactly what happened in New York.”
Is OPV safe?
“Our vaccines are safe and effective,” Ehrhart said. “We need to vaccinate our underage and unvaccinated children to stop the ongoing spread of these viruses.”
Chumakov said that developing improved versions of OPV could reduce the risk, as well as maintain the immunity provided by OPV, which he said is important to stop the silent spread. He was previously involved in the GPEI effort to develop a safe oral vaccine for type 2 poliovirus. Clinical trial data suggest that novel OPVs are less likely to return in a dangerous form.
While better vaccines may come in the future, 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 fell to 81% in 2021, the lowest in a decade. This was largely due to the COVID-19 pandemic, but not all countries have sufficient resources for vaccination campaigns even in normal times.
Razafindratsimandresi said that even though Madagascar aims to vaccinate all children with OPV, the country does not have enough personnel and often runs out of vaccines.
“These immune gaps must be closed to prevent these diseases,” Ehrhart said. “If you have polio anywhere, kids everywhere are still at risk.”