The trivalent oral polio vaccine has been instrumental in the near-eradication of the disease worldwide, but the vaccine still has some challenges that can be addressed by switching to a newer version. Starting this Sunday, the 155 countries that use the oral polio vaccine will move to the bivalent vaccine, the first time a changeover of this scale has been attempted.
The Global Polio Eradication Initiative, a public-private partnership spearheaded by the WHO, is leading the charge. The project is set to conclude on May 1, leaving a two-week window for the world to simultaneously make the switch.
The trivalent vaccine protects against poliovirus types 1, 2 and 3, but type 2 was declared eradicated in 1999. The advantages of cutting type 2 out of the vaccine are twofold: it reduces vaccine-associated polio and creates a stronger vaccine as it does not contain an extraneous component.
Sanofi ($SNY), Serum Institute of India, Bharat Biotech and GlaxoSmithKline ($GSK) are among the companies that manufacture WHO-prequalified trivalent polio vaccines. None of them will lose out in the switch, however, since every company that makes the trivalent vaccine also produces WHO-prequalified bivalent polio vaccines.
All oral polio vaccines are made with weakened live poliovirus, so there is a one-in-a-million risk of a weakened strain mutating and sickening people. However, according to the Global Polio Eradication Initiative, poliovirus type 2 is responsible for about 90% of paralysis cases caused by vaccine-associated polio. The switch will tamp down on the circulation of new strains of type 2 polio. Meanwhile, the Gates Foundation and the WHO are funding the development of a synthetic polio vaccine, which could eliminate the risk of vaccine-associated polio for good.
Despite the benefits of switching to the bivalent vaccine, there is a risk of declining population immunity to type 2 polio. The Global Polio Eradication Initiative has been scrambling to boost type 2 polio immunity levels ahead of the switch. It is also calling for a global stockpile of a monovalent vaccine against type 2 in case there is an outbreak of that polio type following the switch.