While polio has never been eradicated globally, recent outbreaks around the world have left experts wondering if vaccination efforts can ever be eased, Michael DePeau-Wilson writes for MedPage Today.

How vaccination campaigns for kids like polio worked in the past–and why they're so hard to make effective today

Background

In 1988, the World Health Organization (WHO) adopted a strategy to globally eradicate poliomyelitis—the disease caused by wild poliovirus and rare cases of vaccine-derived poliovirus (VDPV). Initially, WHO's goal was to achieve global eradication of the disease by 2000.

According to early CDC reports, public health workers quickly progressed toward this goal. From 1988 to 1993, the number of poliomyelitis cases reported globally dropped by 70%—an achievement largely driven by the oral polio vaccine (OPV), which contains a live, weakened poliovirus.

However, recent polio outbreaks around the world stemming from individuals who received the OPV have experts questioning whether the disease will ever be eradicated.

In June, the U.K. Health Security Agency (UKHSA) reported that it had discovered poliovirus in several sewage samples from North and East London in February and April, suggesting that there may have been transmission between individuals. Before this, the last known case of polio in Britain had been in 1984, and the country had been polio-free since 2003.

In July, New York health officials announced that a case of polio had been detected in the state after a unvaccinated young adult from Rockland County became paralyzed from the disease, marking the first reported case of polio in the United States since 2013.

"This is indicative of a transmission chain from an individual who received the OPV," according to a statement from NYSDOH. "This suggests that the virus may have originated in a location outside of the U.S. where OPV is administered, since revertant strains cannot emerge from inactivated vaccines."

There have been several instances of detection of VDPV in wastewater in New York state and the United Kingdom.

Globally, wild poliovirus—which was previously thought to be isolated within Pakistan and Afghanistan—was detected in Mozambique and Malawi early this year. Notably, both countries are located in a region that was confirmed to be polio-free as recently as 2020.

According to Vincent Racaniello, a virologist at Columbia University, this global rise in polio cases has led experts to contend with a new reality—one where countries with high vaccination rates must hope that VDPV does not lead to rare cases of paralysis.

Why some experts 'doubt that poliovirus can be eradicated'

OPV played a key part in early efforts to eliminate poliomyelitis. However, it is now one of the main reasons experts believe vaccination efforts against polio can never stop.

According to DePeau-Wilson, the global eradication of polio "has remained out of reach for the same reason that rates of poliomyelitis have decreased—because of OPV."

"I doubt that poliovirus can be eradicated," Racaniello said. "As long as we keep using OPV in some countries, OPV-derived viruses will continue to circulate and pose a threat to any unvaccinated people."

While OPV is designed to be harmless to the recipient, the "strain can mutate and revert to a version of the virus (VDPV) that can cause poliomyelitis in individuals who are not vaccinated," DePeau-Wilson writes. "While this mutation occurs as rarely as once in approximately 3 million cases, it presents enough of a risk that polio vaccination can never be eased while it is in use."

Since 2000, the United States has exclusively used the injectable poliovirus vaccine (IPV), which contains the inactivated poliovirus. Faced with concern over the limitations of OPV, many experts have called for a global transition to IPV.

"The U.S. would certainly be able to help expand the use of IPV globally," Racaniello said. "It is not easy. Not only would production need to be ramped up, but sterile needles would need to be supplied as well as trained healthcare workers to use them. It is my understanding that WHO would like to globally switch to IPV after 2026."

According to CDC, the IPV protects individuals from paralysis, but it does not prevent an infection in the intestines. While this is harmless to anyone who has been vaccinated with IPV, the vaccine does not prevent further spread of the virus. As a result, unvaccinated individuals still face the risk of paralysis.

"Although challenges remain, it is important to emphasize that the number of polio cases has been reduced by 99.9% in the last three decades thanks to the commitment of frontline staff, affected communities, governments, donors, and partners," a CDC spokesperson said.

But since even IPV does not protect from poliomyelitis completely and "many people in the U.S. are likely infected with poliovirus, but they have no signs of disease … We can keep immunizing with IPV and as long as greater than 90% of the U.S. is immunized, we will not have polio the disease, but the virus will still be here," Racaniello said.

"Even if all three wild poliovirus serotypes are eradicated, we will still have circulation of vaccine-derived strains of poliovirus in humans," Racaniello added. "We will have simply replaced wild polioviruses with vaccine-derived polioviruses. Therefore, to protect the world's population against paralysis caused by vaccine-derived polioviruses, immunization must continue indefinitely."

Ultimately, global immunization campaigns are still "the most effective way to end outbreaks and prevent the emergence of new ones within a community or across borders. When high-quality immunization campaigns and surveillance are coupled with strong government, civil society, and community commitment, we can eradicate polio," the CDC spokesperson added. (DePeau-Wilson, MedPage Today, 10/4)