“Phillip, stop running!” When I think about poliomyelitis (polio for short), these words of my mother—shouted out the window at the back of my home as I played baseball in the alley— immediately come to my mind. It was 1954, polio was rampant, and there was the perception (backed by little if any evidence) that intense exercise fostered the development of this frightening disease. While none of us playing baseball that day developed polio, I remember visiting Jerry Nelson’s dad who was a polio victim. He was being cared for at home in an iron lung. And my mom’s oldest sister, Aunt Esther, had a peculiar gait due to legs paralyzed by the polio virus. (Something like President Franklin Roosevelt’s handicap, I thought.)
The introduction of the Salk polio vaccine one year later was an unbelievable game changer. (World Polio Day is coming up on October 24, a day established by Rotary International to commemorate the birth of Jonas Salk, the virologist who led the first team to develop a polio vaccine.) It is perhaps difficult for readers born after 1955, when Salk’s injectable, inactivated polio virus vaccine (IPV) appeared, to comprehend the importance of this development. And the same can be said regarding the attenuated oral polio vaccine (OPV), introduced in 1961 by the Polish American researcher Albert Sabin.
But consider these facts, before 1955 polio was one of the most feared diseases of the 20th century. Since its introduction in 1955, polio was eradicated in the U.S. (the last case occurred in 1979). Because of the Global Polio Eradication Initiative (GPEI), spearheaded by the World Health Organization (WHO), Rotary International, the US Center for Disease Control and Prevention, and national governments (with support by the Bill & Melinda Gates Foundation), cases of polio have plummeted by over 99.99% since 1988, from an estimated 350,000 cases that year to 33 cases reported in 2018. And it is estimated that more than 18 million people are able to walk today, who otherwise would have been paralyzed.
In 2017, only 22 cases of polio were reported, and it looked like we were on the cusp of eradicating this dreaded disease globally. (You may remember from an earlier Germ Gems blog on measles that only one infectious disease in humans has been eradicated to date—smallpox.) But in 2018 the downward trend was halted when 33 cases were reported: 21 in Afghanistan and 12 in Pakistan. And as of October 2, 2019, the number of cases totaled 85—16 in Afghanistan and 69 in Pakistan. Sadly in April, Pakistani officials suspended their nationwide anti-polio campaign after a health worker and two policemen escorting vaccination teams were killed by militants. Underscoring the critical importance of vanquishing polio in these last holdout countries, some experts suggest that the failure to stop polio in these last remaining areas could result in as many as 200,000 new cases every year within 10 years, all over the world.
Before explaining why it is necessary to maintain a zero-tolerance approach to achieve poliomyelitis eradication, it is essential to understand something of the causative agent—three stereotypes of poliovirus (type 1, type 2, and type 3). The virus is usually transmitted person-to-person, spread mainly through the fecal-oral route. When infected, most people don’t develop any symptoms, although they can shed the virus in their stools. In those who become sick, initial symptoms are fever, fatigue, headaches, vomiting, and neck stiffness. Polioviruses pose a big health threat mainly because of their ability to attack the nervous system (spinal cord and brain). One in two of these infections leads to irreversible paralysis (usually in the legs). Among those paralyzed, 5% to 10% die when their breathing muscles become immobilized.
Both the Salk and Sabin polio vaccines confer immunity to all three poliovirus types. But a major difference between the two is that the viruses in the Salk vaccine are inactivated (killed), whereas those in the Sabin vaccine are “attenuated,” which are weakened but live viruses. At the time they were introduced, Sabin argued that his vaccine would prove superior. Both turned out to be equally effective in preventing polio.
While the Sabin vaccine is better tolerated in that it is given orally, the viruses it contains are live and therefore can be spread through stools to others. Infections caused by these attenuated viruses are called circulating vaccine-derived poliovirus (cVDPV) cases as opposed to those acquired by ingestion of naturally occurring polio viruses, dubbed wild polio (WPV) cases. (The only case of polio that I personally diagnosed was in the 1990s in an infant with paralyzed legs who was infected with VDPV.)
In 2000, the use of Sabin OPV was discontinued in the US, but it’s still the mainstay of polio prevention in the developing world, where cVDPV cases can outnumber WPV cases. For example, two cases of cVDPV poliovirus type 2 were reported in September 2019 in the Philippines. It should be pointed out that VDPVs are genetically changed from the original attenuated virus allowing it to cause disease. However, when the population is fully immunized with both OPV and IPV, this kind of transmission cannot occur.
