Children show their ink-marked fingers after being administered with polio drops by healthcare workers as part of the Pulse Polio Program, at New Gardiner Road Hospital in New Delhi.
The gains of science can easily be frittered away, much faster than they were earned. Eternal vigilance is the price we pay to inhabit a planet like Earth, with the range of biodiversity it has. Any letup, and it’s quite likely that years of achievement might be undone. As in the case of polio, which is seeing a resurgence worldwide, with environmental samples and human cases throwing up both wild polio virus as well as vaccine-derived polio virus.
The World Health Organization (WHO) has reported that poliovirus had been detected through routine surveillance of wastewater systems in five countries in the WHO European Region (Finland, Germany, Poland, Spain, and the United Kingdom) since September this year. “While no cases have been detected to date, the presence of the virus underscores the importance of vaccination and surveillance, as well as the ongoing risk that any form of poliovirus poses to all countries everywhere,” the WHO says.
In Pakistan, four WPV1 cases and eight WPV1-positive environmental samples were lifted. In Cameroon and Cote D’Ivoire, one case each of cVDPV2 was detected, while in Chad and Nigeria, four and three cases of cVDPV2 were detected recently, according to the WHO. Additionally, positive environmental samples were found in Finland, Germany, and the United Kingdom.
In the past 3 months, circulating vaccine-derived poliovirus type 2 (VDPV2) was detected in sewage samples in Barcelona, Spain; Warsaw, Poland; Cologne, Bonn, Dresden, Duesseldorf, Mainz, Hamburg, and Munich in Germany; Tampere in Finland; and Leeds, London, and Worthing in the U.K., according to the WHO. The detected virus was genetically linked to a strain that emerged in Nigeria. In none of the five countries was there confirmed local circulation of the virus at this time, the health body said.
“WHO continues to support national and local public health authorities in their investigations and monitoring of the situation, including identification of any potential subnational immunity gaps that may need to be addressed,” added Robb Butler, Director, Communicable Diseases, Environment and Health at WHO/Europe. “Vaccination of every vulnerable child is essential to ensure that the virus cannot lead to lifelong paralysis or even death.”
All five countries where the virus was detected maintained strong disease surveillance and high levels of routine immunisation coverage, estimated at 85–95% nationally with 3 doses of inactivated polio vaccine (IPV), which provides excellent protection from paralysis caused by poliovirus. Earlier, polio was also picked up in Gaza on July 16, 2024. It was confirmed in a 10-month-old child who became paralysed in August 2024.
However, pockets of undervaccination exist in every country. In response to the detections, investigations are ongoing, including through continued disease surveillance. Subnational immunity levels have been examined to identify any potential gaps and immunisation of unvaccinated children, the WHO says.
Vaccine controversy
In the context of the surge, the debate about what vaccine to use has once again come to be of interest. Even back in the early 20th century, initial theories about poliovirus transmission pointed to respiratory transmission based on the epidemiological features of polio before later studies shifted the focus to the faecal-oral route, primarily due to research on virus shedding in the gut. The polio Sabin vaccine, also known as the oral polio vaccine (OPV), is a live-attenuated vaccine that was developed by physician and microbiologist Albert Sabin. It was adopted by the WHO because of the efficacy in administering the vaccine and given the mammoth task in front of the world at that time. The flip side, however, was that oral polio vaccine allowed vaccine-derived polioviruses to infect children.
A recent paper, The Respiratory Route of Transmission of Virulent Polioviruses, by T. Jacob John, Dhanya Dharmapalan, Robert Steinglass, and Norbert Hirschhorn, in the peer-reviewed journal Infectious Diseases, presents a thorough analysis of the transmission routes of poliovirus, particularly wild polioviruses (WPVs) and circulating vaccine-derived polioviruses (cVDPVs). They make the case that the transmission of virulent polioviruses (WPV and cVDPV) is via the respiratory route, like other contagious childhood infectious diseases such as measles, rubella, varicella, diphtheria, and pertussis. But the systems of virus amplification and transmission are different in that silently reinfected individuals can transmit virulent polioviruses but not the pathogens of these other childhood diseases.
“Poliovirus does not replicate in the environment nor in contaminated food — hence, faeces has to be consumed to become exposed. If faeces were consumed at high dosage and frequency to saturate children or cause outbreaks of polio, certainly one would expect many other diseases to occur pari passu with polio. They don’t. Conversely, one would expect polio to be a regular part of outbreaks of enteric diseases following contamination of drinking water; it isn’t,” the paper rationalises.
Respiratory transmission?
The paper explores the long-standing debate over whether poliovirus is primarily transmitted via the faecal-oral route or the respiratory route. Historically, the faecal-oral route was widely accepted, particularly after the introduction of the oral polio vaccine. The authors argue that available evidence leans strongly towards respiratory transmission as the primary route. While poliovirus is known to shed in both the throat and stool, the paper challenges the assumption that the faecal-oral route is the dominant means of transmission. Instead, the authors emphasise that virus shedding in the throat is critical for transmission, and this shedding supports the respiratory transmission theory.
The authors say continuing to distribute OPV is the wrong path to polio eradication. On the other hand, vaccination with Injectible Polio Vaccine will expedite the eradication of WPV and cVDPVs, Dr. Jacob John argues. The earlier OPV is replaced with IPV, the faster the world will reach global polio eradication. Since IPV is non-transmissible, the OPV-to-IPV switch should be planned/implemented country by country until OPV is no longer used anywhere. That is the only way the Global Polio Eradication Initiative (GPEI) can avoid causing polio in the name of eradication.
The paper discusses how initial studies pointed to respiratory transmission based on the epidemiological features of polio before later studies shifted the focus to the faecal-oral route, primarily due to research on virus shedding in the gut. Dr. John, a retired professor of virology from Christian Medical College, who also served as an expert member on the National Technical Advisory Group on Immunisation, has always been forceful about his view that OPV could be used only for “polio control” but not for “polio eradication”. Without global use of IPV and without global withdrawal of OPV, the polio eradication project cannot even begin.
The authors argue that the widespread assumption of faecal-oral transmission has led to the exclusive use of the oral polio vaccine (OPV) in the GPEI. However, the authors suggest that, based on their findings, a shift towards theinactivated poliovirus vaccine (IPV)would be more effective in preventing polio transmission, particularly in regions where respiratory transmission is the main route.
Published – December 26, 2024 10:48 pm IST