Vaccine-derived poliovirus 2 detected in environmental samples in the UK and USA

Over the past 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 US as well as in the UK. What does this mean? The World Health Organization (WHO) discusses the importance of this situation in a brief report.

Introduction

Polio is an infectious disease with high transmissibility. It is mostly caught by young children under the age of five, and in 0.5% of cases it results in permanent paralysis. Death occurs in up to a tenth of those who become paralyzed.

Polio is spread via the feco-oral route, but uncommonly via contaminated food or water. In particular, over 90% of infections are asymptomatic or very mild.

In symptomatic cases, the virus replicates in the gut and then penetrates the nerves. After an incubation period of 7-10 days (although this may extend up to 35 days), it presents initially with fever, fatigue, vomiting, neck stiffness and limb pain due to intestinal infection and later aseptic meningitis.

Polio is a preventable disease and polio vaccination 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 wild-type poliovirus when encountered.

However, reversion to the wild-type strain has been reported with the Sabin strain, causing vaccine-derived poliovirus to emerge. This can spread in settings with low polio vaccination rates, encouraged by poor sanitation, 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. Additional samples were collected in the last week of May this year. These showed mutations in the virus that led to its identification as vaccine-derived poliovirus type 2 (VDPV2).

Again, on August 8, 2022, the virus was detected in a new sample, leading it to be considered circulating VDPV2, although no human polio cases have been reported in this country.

Conversely, in the United States, 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, a case of paralytic polio was also reported in Rockland County, an unvaccinated person who had not traveled abroad recently. Importantly, there have been no reports of polio in the United States since 2013.

Viral sequences from the first half of August showed five or more mutations and in both samples were associated with this clinical case. This led them to also be classified as circulating VDPV2.

Currently, these two cases are linked genetically to viruses detected in sewage samples collected in Jerusalem, Israel, during the six months from January to June 2022.Detection of VDPV in at least two different sources, at least two months apart, that are genetically related, that show 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 within one year of life in the US and UK, respectively.

What needs to be done?

Careful monitoring of the situation using genetic and epidemiological tools continues to find out how the virus can spread and the degree of danger posed to the world population as a whole 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 constituted a Public Health Emergency of International Concern (PHEIC).

Everyone involved in the evaluation of patients and laboratory samples was given instructions to look after VDPV2 in London and all samples positive for the virus are now being sent to UKSHA. Wastewater sampling areas are also being expanded across the UK and US.

In addition, in June 2022 a catch-up immunization was carried out for children under 5 years of age in London. Children in London aged 1-9 are encouraged to receive one dose of inactivated polio vaccine as a booster. Children who may be underimmunized, such as immigrants or refugees who have recently entered the country, should be checked for routine immunization status.

In the United States, an immunization campaign is also planned for Rockland County residents who may have been exposed to the virus. Polio surveillance across the country, particularly where the virus has been identified in sewage, is being strengthened and coordinated. Continuing health education about this disease and the vaccine is passed on through health advisories throughout the state.

On September 9, New York State declared a state disaster emergency. This means that polio vaccine can be given by pharmacists and allied health workers in addition to medical professionals, as well as allowing 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 acute flaccid paralysis (AFP) figures, the proportion of cases evaluated within 48 hours, and the proportion for which a good sample was obtained.

Furthermore, polio immunization must be achieved in every district and maintained at high rates (>95%) worldwide. In addition, national polio control plans must be kept up to date to ensure that the presence of VDPV is quickly detected and transmission is prevented as soon as possible.

“The emergence of cVDPV2 in the United Kingdom and in the United States is a reminder that until polio is eradicated, polio-free countries will continue to be at risk of reinfection or resurgence of polio.”