By Bunmi Yekini
Researchers have called on the World Health Organization to revise its definition of vaccine safety and phase out the use of the live oral polio vaccine in favour of the inactivated version, arguing that the current standard has delayed global eradication of the disease.
In an analysis published in The Lancet, the authors say vaccine safety has effectively been defined as the absence of serious adverse events following immunisation occurring at an “unacceptable frequency,” a threshold they argue is subjective and has allowed the continued widespread use of the oral poliovirus vaccine (OPV).
The live OPV, widely used since the launch of the Expanded Programme on Immunization in 1974, can in rare cases cause vaccine-associated paralytic polio (VAPP), a serious adverse event that occurs in roughly one out of every 100,000 to 750,000 infants receiving one or more doses, the study said.
During the 1980s and 1990s, about 500 children globally developed VAPP each year, according to the researchers.
By contrast, the inactivated poliovirus vaccine (IPV) does not cause paralytic polio or other serious adverse events, the authors said, arguing that it should have been the preferred option where feasible.
The study also highlights a second risk linked to OPV: vaccine viruses can mutate and regain virulence, producing circulating vaccine-derived polioviruses (cVDPVs) that can spark outbreaks in under-immunised communities.
Such outbreaks have been recorded in dozens of countries since 2000, the paper said. Between 2018 and 2024, there were 3,955 cases of cVDPV-related polio worldwide, an average of about 565 cases annually.
According to the researchers, these vaccine-derived outbreaks and rare vaccine-associated cases represent one of the key reasons global polio eradication has been delayed by more than two decades.
Scientists have attempted to reduce the risk by developing genetically modified versions of the oral vaccine known as novel OPVs, but these too have occasionally reverted to vaccine-derived strains, the paper said.
The authors argue that vaccine safety definitions should also account for serious disease, disability or death occurring in unvaccinated bystanders infected through the spread of vaccine viruses.
They said adopting such a broader definition could encourage countries to replace OPV with IPV, which they describe as the safer alternative.
The call comes as the global eradication drive led by the Global Polio Eradication Initiative seeks to eliminate both wild and vaccine-derived polioviruses.
“Once the world exclusively uses IPV,” the authors wrote, “we will have reached the runway for taking off towards a world without polio caused by wild, vaccine, or vaccine-derived polioviruses.”