Subscribe
The fight against polio through massive vaccination efforts since 1988 has helped reduce the number of cases by more than 99 per cent worldwide — but there remains a worry. Instances of vaccine-derived poliovirus (VDPV) — caused by the virus strains used in the polio vaccines regaining their ability to cause polio — have seen a spike between 2010 and 2019.
As of May 2020, a total of 149 immunodeficiency VDPV (iVDPV) cases had been reported to the World Health Organization (WHO) between January 1961 and December 2019, according to the WHO’s Weekly Epidemiological Record.
The number of reported cases increased over time: at least two out of three cases (66 per cent) reported so far were detected between 2010 and 2019, according to the WHO report. Out of these, 59 per cent occurred in children aged less than two years. At least 60 per cent cases were reported in males; and 64 per cent patients had Acute Flaccid Paralysis (AFP), the most severe sign on polio, as the first symptom.
During July 2018-December 2019, 16 new iVDPV cases were reported from five countries — Argentina, Egypt, Islamic Republic of Iran, Philippines and Tunisia, according to WHO.
These cases were detected during AFP surveillance — which allows for rapid detection of polio outbreaks.
According to the report, the increase in the number of infections in 2010–2019 was “a consequence of increased activity for identification of infection among PID patients and better methods for detecting polioviruses”.
What are VDPVs?
Vaccine-derived polioviruses are rare strains of poliovirus that have genetically mutated from the originally strain in the oral polio vaccine. Oral polio vaccine (OPV) contains an attenuated (weakened) vaccine-virus, which activates an immune response in the body.
When a child is vaccinated, the weakened vaccine-virus replicates in the intestine and triggers a protective immune response. But when the child excretes the vaccine-virus (for about six to eight weeks), some of the vaccine-virus may no longer be the same as the original one: It gets genetically mutated during replication. This is called a VDPV.
There are four types of VDPVs — circulating vaccine-derived poliovirus (cVDPV); immunodeficiency-related vaccine-derived poliovirus (iVDPV); and ambiguous vaccine-derived poliovirus (aVDPV).
According to WHO, there is evidence of community transmission in case of cVDPVs — if a population is poorly vaccinated, there would be enough susceptible children for the excreted vaccine-derived polioviruses to begin circulating in the community.
If the vaccine-virus is able to circulate for a long time, it can mutate and regain virulence. These viruses are called cVDPVs.
iVDPVs are observed in children with rare immune deficiency disorders. Children with iVDPVs are unable to mount an immune response and are, therefore, unable to clear the intestinal vaccine-virus infection. The two major risks that iVDPV pose are that of progression to paralysis (AFP) and death.
Most countries with AFP surveillance detected iVDPV in paralysed children, who then received a diagnosis for a primary immunodeficiency (PID), the report said. However, instances of iVDPV were also found in PID patients with no paralysis, the report added.
For example, in Egypt, a PID surveillance project during July-December 2018, identified six iVDPV infections, one in a patient who developed AFP, according to the report.
In all 149 cases reported to WHO between 1961 and 2019, AFP occurred before a primary immunodeficiency was diagnosed.
According to Anant Bhan, researcher, bioethics and global health, under-immunisation is a looming concern for countries, for it has a direct impact on herd immunity and the need for special rounds of immunisation.
“The vDPV cases are a risk factor with oral polio vaccine (OPV), especially for PID patients. When immunisation coverage (OPV) is sub-optimal, circulating VDPVs can revert to neurovirulence, and causes paralytic polio outbreaks. These are major concerns for disease control efforts and, of course, is debilitating for children who have been impacted,” he said.
Increase in cases
Half of the detected cases during 2010-2019 were in the WHO Eastern Mediterranean Region; the report attributed recent higher PID surveillance and a high rate of consanguineous (when partners are from the same kinship) marriages, leading to a higher prevalence of PID, as the reason of increase in reported cases there.
More surveillance to get a better sense of VDPV outbreaks, including iVPDV, should be carried out, according to Bhan.
He added:
We need global efforts to enhance vaccine coverage in all parts of the world, which will lessen the risk of VDPV in the community. The report also points to the need to hasten the transition of phasing out OPV usage in all countries to avoid the risk of VDPV, which can stopping wild polio virus transmission. There is a need to shift to inactivated polio vaccine to maintain population immunity levels.
WHO has earlier supported pilot projects for iVDPV surveillance in children with PID in several countries, including Egypt, Islamic Republic of Iran, Pakistan, Sri Lanka, Tunisia and, more recently, China and India.
Additional countries are being identified in other WHO regions in which to implement systematic surveillance for PID-affected children without paralysis, according to the report.