Over the last few months, poliovirus 2 derived from the strain used in the Sabin vaccine (oral polio vaccine) has been repeatedly detected in environmental samples in the USA as well as in the UK. What does this signify? The World Health Organization (WHO) discusses the importance of this situation in a brief report.
Polio is an infectious disease with high transmissibility. It is mostly caught by young children below five years, and in 0.5% of cases, it results in permanent paralysis. In up to a tenth of those who become paralyzed, death occurs.
Polio spreads via the feco-oral route but uncommonly via contaminated food or water. Notably, over 90% of infections are asymptomatic or very mild.
In symptomatic cases, the virus replicates in the gut and then enters the nerves. After an incubation period of 7-10 days (though this may stretch up to 35 days), it presents initially with fever, tiredness, vomiting, neck stiffness, and limb pain due to gut infection and, later, aseptic meningitis.
Polio is a preventable disease, and polio immunization is almost universal. Both injectable and oral polio vaccines are available. The latter contains the Sabin strain of poliovirus, a live attenuated virus that can replicate in the gut of the immunized child to cause infection and thus elicit an immune response against the wildtype poliovirus when it is encountered.
However, reversion to the wildtype strain has been reported with the Sabin strain, causing vaccine-derived poliovirus to emerge. This may spread in settings with low polio vaccination rates, encouraged by poor sanitary facilities, overcrowding, or lack of adequate hygiene. With the spread of the virus, more mutations occur.
In a small proportion of such cases, the vaccine-derived virus becomes a paralytic poliovirus – VDPV.
The first time the Sabin-like type 2 poliovirus (SL2) was detected in sewage samples in the UK was in February 2022, when it was picked up by the WHO Global Polio Laboratory Network (GPLN) at the National Institute for Biological Standards and Control (NIBSC) in London. Further samples were collected in the last week of May this year. These showed mutations in the virus that led to its being identified as vaccine-derived poliovirus type 2 (VDPV2).
Again, on August 8, 2022, the virus was detected in a new sample, leading to its being considered circulating VDPV2, though no human polio cases have been reported in this country.
Conversely, in the USA, SL2 has been detected in several environmental samples from April 21 to August 26, 2022, from Rockland County in New York, as well as adjacent counties.
During this period, one paralytic polio case was also reported in Rockland County, an unvaccinated individual who had not traveled abroad recently. Importantly, no polio has been reported in the USA since 2013.
Viral sequences from the first half of August showed five or more mutations and were, in both samples, linked to this clinical case. This led to their being classified as circulating VDPV2 as well.
Currently, these two instances are linked genetically to virus detected in sewage samples collected in Jerusalem, Israel, over the six months from January to June 2022. “The detection of VDPV in at least two different sources and at least two months apart, that are genetically linked, showing evidence of transmission in the community, should be classified as ‘circulating’ vaccine-derived poliovirus type 2 (cVDPV2).”
Over 92% and 93% of children have received three doses of the polio vaccine by one year of life in the USA and the UK, respectively.
What is to be done?
Careful monitoring of the situation by genetic and epidemiologic tools is being continued to find out how the virus may spread and what degree of danger is posed to the world’s people at large by the spread of these isolates detected in different parts of the world.
In June 2022, the International Health Regulations (IHR) Emergency Committee decided that this posed a Public Health Emergency of International Concern (PHEIC).
All those involved in the evaluation of patients and laboratory samples were given instructions to watch out for VDPV2 in London, and all samples positive for the virus are now being sent to the UKSHA. Sewage sampling areas are also being expanded across the UK and the USA.
Besides this, catch-up immunization for children below the age of 5 years was carried out in London in June 2022. Children in London from 1-9 years are encouraged to have one dose of inactivated polio vaccine as a booster. Children who might be under-immunized, such as immigrants or refugees who have recently entered the country, should be checked for routine immunization status.
In the USA, an immunization campaign is also being planned for Rockland County residents who might have been exposed to the virus. Surveillance for polio across the country, especially where the virus has been identified in sewage, is being strengthened and coordinated. Continuing health education on this disease and the vaccine is being imparted via health advisories across the state.
On September 9, New York State declared a state disaster emergency. This means that polio vaccine can be given by pharmacists and related health workers in addition to medical professionals, as well as permitting standing orders for polio vaccination to be issued by the latter.
The importance of global surveillance to detect the emergence of VDPV or its importation, cannot be emphasized enough. This includes maintaining figures for acute flaccid paralysis (AFP), the proportion of cases evaluated within 48 hours, and the proportion for which a good sample was achieved.
Moreover, in each district, polio immunization must be achieved and kept at high rates (>95%) worldwide. In addition, national polio response plans must be kept current to ensure that the presence of VDPV is rapidly detected and transmission foiled as soon as possible.
“The emergence of cVDPV2 in the United Kingdom and in the United States of America is a reminder that until polio is eradicated, polio-free countries will remain at risk of polio re-infection or re-emergence.”