UK health officials announced on Wednesday that a vaccine-derived version of the poliovirus has repeatedly surfaced in London’s sewage over the past several months, suggesting there may be a latent or hidden spread among some unvaccinated people. Is.
So far no case of polio has been reported and no case of paralysis has been reported. But sewage sampling at a treatment plant in London has repeatedly detected the related vaccine-derived poliovirus between February and May. The UK Health Protection Agency (UKHSA) said: “This suggests that there has been some spread between closely related individuals in north and east London and that they are now shedding the type 2 poliovirus strain in their feces.”
Although the current situation raises alarm, the agency notes that it is common for a small number of vaccine-like polioviruses to pop up in sewage from time to time, usually from people who have recently been vaccinated outside the country. This is because many countries use oral polio vaccines that contain weakened (attenuated) polioviruses, which can still replicate in the intestines and thus be present in feces. They can also be spread through poor sanitation and hygiene (ie, unwashed hands and water contaminated with food or sewage), which may be among poor vaccination rates.
how and why does this happen
In short, there are two types of polio vaccines: attenuated oral vaccines and inactivated vaccines. Many high-income countries that are considered polio-free—including the UK and US—use inactivated vaccines, in which the virus is not able to replicate or spread. These vaccines are highly effective in preventing paralytic polio, but they do not produce a high level of local immune response in the gut. Therefore, if a vaccinated person encounters wild poliovirus, the virus may still be able to replicate and spread in their gut. In areas affected by wild polio outbreaks, this means the virus can continue to spread.
Oral polio vaccines, on the other hand, can not only prevent paralytic polio, they can also induce strong local immune responses in the gut that prevent the virus from replicating there, thus inhibiting its spread. These vaccines can also be up to five times cheaper than the inactivated type. For all these reasons, oral polio vaccines are the major vaccines used in the long, drawn-out fight to eradicate wild polio. Currently, wild polio is still found in Afghanistan and Pakistan, and Malawi and Mozambique have recently reported single cases.
But, one of the disadvantages of oral polio vaccines is that vaccinated people can shed attenuated vaccine virus in their stool for several weeks after vaccination. If it occurs in a community with poor sanitation, hygiene and low vaccination coverage, the vaccine virus can spread from person to person. Over time, as the vaccine virus spreads to more people, it can pick up mutations that make it similar to wild-type polio, allowing it to acquire the ability to cause disease and, in rare instances, without Vaccinated people develop paralysis. At this point, the mutated vaccine virus is called a “vaccine-derived poliovirus,” or VDPV. Recently, cases of VDPV have been reported from several African countries and Israel.