We are in the midst of a global polio eradication campaign launched by the Global Polio Eradication Initiative (GPEI), which has reduced the number of polio cases by 99.9%, saving an estimated 15 million children from paralysis. Only 3 countries in the world still have endemic polio (meaning transmission is constantly maintained): Afghanistan, Pakistan and Nigeria. However, since 2017, there has been an outbreak of polio in the Democratic Republic of Congo, which has resulted in 29 children being paralyzed. In February 2018, the Congolese government declared a national public health emergency due to the outbreak.
Key Things to Know about Polio:
- Polio (technically known as poliomyelitis) is a very infectious virus which mainly affects young children
- Most cases of polio don’t have serious symptoms, slight fever or nothing at all
- 1 in 200 cases results in irreversible paralysis, and some die if the paralysis affects breathing
- The virus is transmitted from person to person
- Polio is excreted through feces (poop). It mostly spreads through the fecal-oral route (i.e. contaminated water from fecal matter that is ingested)
- There are 3 strains of wild poliovirus (1,2 & 3)
- There is no cure for polio, it can only be prevented by vaccination
The outbreak in the Congo was not caused by wild poliovirus, but by vaccine-derived poliovirus (VDPV).
How can that be true?
First, we need to understand the two different vaccines for polio: oral polio vaccine (OPV) and inactivated polio vaccine (IPV).
IPV contains a dead virus. It was introduced in 1955 by Jonas Salk, who championed a mass immunization test in the US which eventually involved 1.8 million children. IPV has some downsides: it’s administered as an injection and is fairly expensive to produce. On the plus side, there are no risks of contracting disease from its vaccine strain.
By contrast, OPV is an attenuated (live) vaccine which is taken as an oral drop on the tongue. OPV was introduced in 1961 by Albert Sabin. OPV is cheap to produce and because it is a live vaccine, it provides a strong immune response with contact vaccination. Contact vaccination is explained in the video from the WHO, below, but basically means the weakened vaccine virus can multiply in the gut and be passed from person to person just like any other virus. This effectively vaccinates everyone who comes into contact with the vaccinated individual. Unfortunately, OPV carries a small risk where in fewer than 1/10,000 cases, the weakened viral strains can mutate in the intestines to revert to disease-causing polio virus. This is known as vaccine-derived poliovirus (VDPV).
Ultimately, OPV came to replace IPV as the preferred polio vaccine in the US in part due to the Cutter Incident. Given high demand for the polio vaccine, in 1955, Cutter Laboratories, one of the labs rushing to produce Jonas Salk’s IPV vaccine, incorrectly manufactured a vaccine which contained live virus. ~400,000 children were vaccinated with the faulty Cutter vaccine in April of 1955, which led to 94 cases of paralytic polio, 126 cases among family contacts, and 40 cases among community contacts. Unfortunately, this one lab’s mistake did permanent damage to the reputation of the recently heralded Salk vaccine, and paved the way for Sabin’s OPV to replace it in the US as the preferred vaccine. In a way, the Cutter incident was an important regulatory milestone, illustrating the need for increased regulation of vaccine production and ushering in systems such as the National Vaccine Injury Compensation Program.
So what does all this mean?
Type 2 poliovirus has been eradicated in the wild since 1999, but it is type 2 polio virus which is involved in the outbreak in the Congo (and which is the most common strain seen in VDPV outbreaks). Because of this, in 2016, the GPEI phased out the use of the polio type-2 oral vaccine because all type 2 cases were coming from the vaccine. Unfortunately, VDPV can circulate and mutate for years, and taking out type 2 from the vaccine means that there is less immunity to type 2 in the population.
How could a vaccine strain cause an outbreak? This video from GPEI can help explain.
Basically, weakened type 2 polio virus in the vaccine can mutate in the gut after someone is vaccinated. If enough people are vaccinated in the community against type 2, this shouldn’t be a problem, because “If a population is fully immunized against polio, it will be protected against the spread of both wild and vaccine strains of poliovirus.” But if immunization rates aren’t high enough, or after trivalent OPV is phased out, and the community is only vaccinated against types 1 and 3, this vaccine-derived polio can spread, causing the very outbreak it was supposed to be preventing.
What can be done?
The protocol to address this outbreak is to introduce the very vaccine which is responsible for the mutated strain: OPV type 2. While this should be a simple fix, operational quality deficits and implementation problems create high risk that the outbreak will continue to spread in the Congo and even perhaps beyond.
While OPV is a tremendously important, inexpensive, easy-to-administer, lifesaving vaccine, as we near global eradication, the very small chance that vaccine mutates becomes more dangerous to eradication efforts, and relies upon very high levels of vaccination in the community. To solve this problem, the GPEI is targeting a phase-out all oral vaccines by 2020, replacing them with IPV.
A little perspective…
It’s important to note that circulating VDPVs are extremely rare. From 2000 to 2011, over 10 billion OPV doses were given worldwide but only 20 VDPV outbreaks occurred (causing 580 polio cases).
How many paralytic polio cases would there have been in the absence of vaccination?
The GPEI estimates over 6 million.
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