The biggest threat to the effort now is an explosion of vaccine-derived polio outbreaks in Africa that affected almost two dozen countries last year and paralyzed more than 500 children in 2020 and again in 2021.

On 17 February, Malawi’s Ministry of Health announced a nasty surprise: A 3-year-old girl who was paralyzed in November 2021 was infected with the wild poliovirus, which Africa officially vanquished in 2020. The sequence of the virus showed it had somehow made the leap from Pakistan, one of the last two holdouts of the wild virus. A week later came bad news from Afghanistan: Gunmen killed eight polio workers in the country’s northeast.

The incidents are the latest setbacks on the long, bumpy road to global polio eradication. Yet Pakistan has “exported” wild poliovirus before, sparking polio outbreaks that were quickly snuffed out, and the situation in Afghanistan and Pakistan improved dramatically last year, with polio cases tumbling to a historic low.

Instead, perhaps the biggest threat to the effort now is an explosion of vaccine-derived polio outbreaks in Africa that affected almost two dozen countries last year and paralyzed more than 500 children in 2020 and again in 2021. Vaccine-derived strains emerge where children are un- or underimmunized, allowing the live, weakened virus in the oral polio vaccine (OPV) to circulate and accumulate enough mutations to revert to its neurovirulent form and paralyze kids. These outbreaks—which almost always emerge from type 2 poliovirus, one of the three virus strains—are “very worrying” and “front burner” at the Global Polio Eradication Initiative (GPEI), says John Vertefeuille of the U.S. Centers for Disease Control and Prevention (CDC), a partner in the initiative.

A big part of the problem is that countries don’t view vaccine-derived strains as an emergency, says Simona Zipursky, an adviser to the World Health Organization’s (WHO’s) polio program, even though they behave just like the wild virus. “It is not like there is a milder variant as there is with COVID-19,” Zipursky says. Nigeria’s widely lauded victory over the wild virus—it was the last African country to achieve that feat—fed a sense that “the job was done,” says WHO’s Aidan O’Leary, who directs GPEI. The quality of Nigeria’s polio program, once among the best in the world, slipped, and today the country is “the most important generator” of vaccine-derived polioviruses, says Jay Wenger of the Bill & Melinda Gates Foundation, another partner in GPEI. Nigeria accounted for more than half of all vaccine-derived polio cases globally last year and exported the virus to 18 countries.

Other factors have contributed, too. Lately, many African countries have been slow to respond to new outbreaks as they wait for a new vaccine that they think will solve the problems, which has allowed the virus to spread. Many are frustrated with the existing vaccine, monovalent OPV2 (mOPV2); they would use it to quash an outbreak but then, because the vaccine virus occasionally reverts, the response would seed more outbreaks.

Known as novel OPV2 (nOPV2), the new vaccine was engineered to be as effective as mOPV2 but more genetically stable, greatly lessening the chance it will revert. The vaccine, funded by the Gates foundation, was rolled out in a few countries in March 2021 under an emergency use authorization.

Pending its arrival, Senegal waited for almost 1 year before responding to a virus detected in late 2020, instead of using readily available supplies of mOPV2. “If the virus gets a head start for such a long time it is harder to stop,” says Mark Pallansch, who recently retired from CDC but remains involved in GPEI.

Although early data suggest nOPV2 is indeed less likely to trigger outbreaks, Pallansch thinks its promise has been oversold. “Governments thought, if I can just get it, things will be fine,” he says. But countries ran nOPV2 campaigns of poor quality, reaching just a fraction of the target population. Nigeria has burned through about 184 million nOPV2 doses, out of 255 million used so far, and still hasn’t stopped many of its outbreaks. The new vaccine “is not a magic bullet,” Zipursky says.

A new threat
Pakistan and Afghanistan are the last two countries where the wild poliovirus is endemic. Cases in both dropped sharply last year (left). Vaccine-derived outbreaks are now a big threat to polio eradication—especially in Africa (right).

GPEI and other international bodies are hammering home that countries should respond to any outbreak immediately with whatever type 2 vaccine is available. The mantra is “faster, better, bigger,” O’Leary says: Be quicker to detect and respond to outbreaks, improve the vaccination campaigns, and broaden them. “We need to conduct them not where you think the virus is, but, based on migration patterns, where you think it will be,” he says.

The Africa campaign is also suffering from a “self-inflicted wound,” Pallansch says. GPEI has long planned to put itself out of business once polio is gone. As part of a transition plan, many of GPEI’s substantial assets and staff would be integrated into existing WHO programs, for instance, to deal with other infectious diseases—GPEI has already helped with Ebola and COVID-19—and to boost routine immunization. WHO planned to complete this transition in nonendemic countries—including all of Africa—by January 2022.

Accordingly, in February 2021, WHO’s Africa office sent pink slips to all GPEI staff. Unfortunately, the office was slow to say who would be kept on, and some people got nervous and quit, officials say. GPEI soon realized the Africa situation was “too hot right now” to proceed with the plan, Wenger says, and decided to continue to fund the 10 highest risk countries in Africa for another 2 years. But the damage had been done. “Things didn’t have to happen this way,” Pallansch says. “They could have done it in a different sequence and not have viruses all over the continent.”

The new worries come as Pakistan and Afghanistan, the last two endemic countries, are doing surprisingly well, with just five reported cases of wild poliovirus last year, down from 140 in 2020. Pakistan has just gone an entire year without a case. (Vaccine-derived cases in both countries are way down as well.) “It looks better than it ever has,” Wenger says. The low numbers are “absolutely not” an artifact, says Hamid Jafari, who heads the eradication program in the region; surveillance remains “really, really good.” Some of the gains stem from very favorable epidemiology. Polio outbreaks resurged in both countries in 2019 and 2020, and “after a peak we always see a trough,” Jafari says, in part because of increased population immunity. Reduced travel during the COVID-19 pandemic helped.

In Pakistan, vaccination drives already cover most of the target population, and they are improving, Jafari says. Imran Khan, Pakistan’s prime minister, is actively involved. Bill Gates just visited the country to bolster enthusiasm. In Afghanistan, too, “we’ve made more progress than we could have anticipated,” O’Leary says. After resuming power in August 2021, the Taliban rescinded its ban on house-to-house polio vaccination in its strongholds, which had left 3.5 million children out of reach. (In some areas GPEI is still restricted to vaccinating in mosques.) Vaccination campaigns in November, December, and January reached 8.5 million out of 9.9 million children, Jafari says, including 2.6 million who were inaccessible for 3.5 years.

But future campaigns could be hobbled if last week’s killings are a harbinger of further violence. And Jafari suspects the virus may survive in small populations that move back and forth across the border between the two countries. A couple of positive environmental samples detected in December in southern Khyber Pakhtunkhwa in Pakistan show the virus is still lurking there. Jafari worries it could resurge when the weather warms and people begin to travel for Ramadan and Eid al-Fitr. The recent spread to Malawi underscores the risk of further delays, he says: “We want to kill the virus now.”

Source: Science

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