Public health workers and scientists are on the brink of eradicating polio, a disease that causes incurable paralysis. But total eradication calls for a massive logistical operation “unprecedented” in the history of vaccines: In the next two weeks, 155 countries and territories will safely dispose of hundreds of millions of polio vaccines, switching to a new version of the vaccine.
The goal of the switch is to stop immunizing against Type 2 polio. Since this strain of the virus was eradicated in 1999, the Type 2 component of the oral polio vaccine is no longer needed, according to polio experts. The new vaccine will immunize only against Types 1 and 3.
If the switch is successful, it will dramatically reduce the incidence of vaccine-associated polio.
Polio vaccines are simply weakened strains of the virus. In very rare circumstances — 1 in a million vaccines — the weakened polio virus present in the vaccine mutates to become more dangerous. There were 32 cases of vaccine-associated paralysis last year, and more than 90 percent of those cases were caused by mutant Type 2 strains.
Worse, mutant Type 2 strains can spread from person to person just like wild strains of the virus. There were seven such outbreaks in 2015, resulting in 23 cases. These outbreaks happen where sanitation is poor, as polio is spread through water or food contaminated with infected fecal matter.
The possibility of outbreaks poses a major risk to the upcoming switch.
If workers fail to dispose of the old vaccines properly, then a future outbreak of Type 2 polio could infect a vulnerable population — but it’s expected. Experts estimate at least one such outbreak, which they plan to address with an emergency stockpile of the Type 2 vaccine. And in the lead-up to the switch, health workers upped Type 2 polio vaccinations to bring immunity to a historically high level.
“If a sufficient number of the population are immunized,” the Global Polio Eradication Initiative’s website explains, the virus “will die out and no longer pose a threat, as polioviruses can only survive for a limited amount of time in the environment.”
The switch may hit other road bumps, according to scientists. There’s a risk that glass vials of the vaccines will explode. And since the old and new vials and boxes look almost identical, warehouse managers must mark the old boxes with an “X” to avoid confusion. To mitigate these risks, highly trained monitors will oversee the process in 27 countries.
Though the switch is risky, it’s also necessary. “To eradicate polio it is just as important to stop circulating vaccine-derived polio viruses as it is to stop wild poliovirus,” said Roland Sutter of the Global Polio Eradication Initiative.
Cory Herro is an intern at ThinkProgress.