A new vaccine shows promising results to stamp out a mutated strain of poliovirus responsible for causing sporadic outbreaks across Africa and Asia, researchers have said after conducting a trial on 1,200 adults and children.

The World Health Organization (WHO) has given Emergency Use Listing to the vaccine after Phase two trial for using it during health emergencies. The new vaccine, named nOPV2, aims to deal with vaccine-derived poliovirus, which has become a serious concern in Afghanistan, Pakistan, the Philippines, Malaysia, and other countries.

"Millions of people potentially have no immunity to the spread of vaccine-derived virus, which is caused when the weakened live virus in oral polio vaccines mutates and regains its ability to become infectious, cause disease, and spread in communities with low vaccination rates," said Professor Pierre Van Damme from the University of Antwerp in Belgium and lead author of the study in adults.

"The nOPV2 vaccine appears at least as safe and effective as the Sabin vaccine and genetically more stable, and could be a key breakthrough towards a polio-free world," Damme added. The Sabin vaccine or oral poliovirus vaccine (OPV) containing live, weakened poliovirus has been extensively given to children worldwide, especially in developed countries, to eradicate the disease.

When a child is administered OPV, the weakened virus replicates in the intestine, generating antibodies. But the virus can be present in the child's stool, and it can infect other children in areas where sanitation facilities and vaccine coverage are inadequate.

With polio immunization drives suspended because of the novel coronavirus pandemic, health professionals fear an increase in vaccine-derived polio cases in some countries. The three strains of wild poliovirus – Type 1, Type 2, Type 3 – causes paralysis among children and, in some cases, leads to death.

Extensive vaccination campaigns almost eradicated the wild poliovirus globally, leading to a sharp drop in cases to 33 in 2018 from 350,000 in 1988. But more than 90 percent of these cases were caused by Type 2 strain. To prevent transmission, the type 2 component was removed from the trivalent vaccine in 2016.

But the withdrawal couldn't help deal with the Type 2 strain as the virus was being passed through excreta, increasing the cases of vaccine-derived polio cases, which shot up from 71 in 2018 to 739 cases in 2020. The sudden spiral of cases forced the WHO to declare vaccine-derived polio infection as a Public Health Emergency of International Concern on October 22, 2020.

"The nOPV2 candidates also had lower stool shedding rates 28 days after vaccination. The results indicate that in infants, one or two doses of both nOPV2 and mOPV2 candidates were safe and well-tolerated," wrote the researchers. There were a few cases of mild illness, and four cases of adverse events occurred during the trial.

(Cover: A child receives free polio vaccine during a government-led mass vaccination program in Quezon City, Metro Manila, Philippines, October 14, 2019. /Reuters)