After initiating its own wastewater testing for polio last year, Canada has detected two samples of vaccine-derived polio virus type 2 (VDPV2).
According to the World Health Organization (WHO) and Pan American Health Organization (PAHO)'s epidemiological update released on Dec. 30, 2022, the virus strain was found in two wastewater samples collected between Aug. 20 and Aug. 30 2022.
The Public Health Agency of Canada (PHAC) confirmed the positive samples in an email statement to CTVNews.ca on Monday, clarifying the specimens found may not be due to local spread of the virus.
"A vaccine-derived poliovirus detection in wastewater doesn’t necessarily mean there is local spread of polio," PHAC spokesperson Tammy Jarbeau said. "It can mean someone who travelled to an area where vaccine-derived poliovirus was present was infected and returned to Canada."
The report said the sampling sites chosen to be tested were in communities connected to New York state, but it did not provide exact locations. The report said there have been no suspected or confirmed cases of acute flaccid paralysis, which is the most common syndrome caused by polio, in the areas where the Canadian wastewater samples were taken.
In August 2022, New York state reported emerging samples of the virus in its waste water after an unvaccinated patient was diagnosed with polio, marking the first time a polio case was reported in the U.S. after nearly a decade.
After the emergence of cases abroad last summer, the Public Health Agency of Canada (PHAC) said it would begin wastewater testing and surveillance for the virus. Canada has not reported a case of polio since 1994. The presence of VDPV2 in the two samples collected in August was confirmed on Dec. 23, 2022.
The Canadian National Microbiology Laboratory, which collected the specimens in Canada, did not find polio in wastewater samples collected between Oct. 31 and Nov. 9. PHAC confirmed the samples collected in September, October and November 2022 were negative for poliovirus, but said that high-risk areas will continue to be tested.
The report said the WHO/PAHO will be working with PHAC to follow up on the findings and advised other nations to continue efforts to increase population immunity through vaccine coverage against polio. The Canadian Immunization Guide recommends Canadians be immunized against polio as children, in combination with vaccines that protect against tetanus, diphtheria and whooping cough at the ages of two, four, six and 18 months. PHAC advises Canadians to ask their family doctor about immunization for children or adults who have not been previously vaccinated against the virus or require a booster.
"Immunization is the best way to protect ourselves and prevent the spread of polio," Jarbeau said. "Reach out to a healthcare provider to learn more about polio immunization, including questions about vaccination for adults not previously vaccinated against polio and the possible need for a booster for those at high risk of exposure."
Since September 2022, the U.S. has reported 94 wastewater samples that tested positive for polio, of which 87 are linked to the confirmed case in Rockland County, N.Y., according to the report.
The WHO and the U.S. Centers for Disease Control say VDPV is a rare strain of poliovirus, genetically changed from the original, weakened strain contained in the oral polio vaccine.
“If allowed to circulate in under- or unimmunized populations for long enough, or replicate in an immunodeficient individual, the weakened virus can revert to a form that causes illness and paralysis,” the CDC says on its website, while stressing that the oral polio vaccine is safe and has been “instrumental in eradicating wild polioviruses around the world.”
