Friday August 22 2014
Polio is a viral infection caused by the poliovirus
Double vaccines "could hasten the end of polio", BBC News reports. Researchers in India found that using a combination of the oral and injected vaccines provided enhanced protection against the disease.
Polio is a viral infection that can cause paralysis and death. Thanks to initiatives such as the NHS Childhood Vaccination Schedule, it is now largely a disease of the past, found in only three countries: Afghanistan, India and Nigeria. It is hoped that the disease could be entirely eradicated in the same way as smallpox.
There are two types of polio vaccine: the oral polio vaccine, which contains weakened strains of polio, and a vaccine known as the Salk inactivated poliovirus vaccine (IPV), which contains chemically inactivated poliovirus and is given by injection.
A new study, performed in India, found that giving a booster injection with the Salk IPV to children who had already been given the oral vaccine boosted gut immunity. This was demonstrated by the fact that fewer children had virus in the faeces after they received a challenge dose (an additional dose) of oral vaccine.
On the basis of this study’s results, the World Health Organization (WHO) is recommending that at least one dose of Salk inactivated poliovirus vaccine is added to routine vaccination schedules, instead of the all-oral vaccination schedule that many countries have.
Hopefully, the ambition of eradicating polio will be achieved in the coming years.
Where did the story come from?
The study was carried out by researchers from the WHO, the US Centers for Disease Control and Prevention, Imperial College London, the Enterovirus Research Centre in India and Panacea Biotech Ltd. Funding was provided by the Rotary International Polio Plus Program.
The study was published in the peer-reviewed journal Science. This article is open-access, so is free to download and read.
The results of the study were well reported by BBC News. Additional insight into the challenges of vaccinating children in conflict-ridden areas, such as Taliban-dominated areas of Afghanistan, was also provided.
What kind of research was this?
This was a randomised controlled trial. The researchers wanted to see if giving children a booster injection with the Salk inactivated poliovirus vaccine (IPV) could boost “mucosal” immunity, which includes immunity in the gut. This is because poliovirus can replicate in the guts of people who have been vaccinated but who don’t have strong mucosal immunity, and can therefore continue to be spread in faeces.
What did the research involve?
To do this, they randomised 954 children in India (in three age groups: infants aged 6 to 11 months, children aged 5 and children aged 10) who had already been vaccinated with the oral polio vaccine to booster injections with:
- the Salk IPV
- another dose of the oral polio vaccine
- no vaccine
Four weeks later, children received a challenge dose of the oral polio vaccine, and the researchers measured the amount of poliovirus that was in their faeces after 3, 7 and 14 days. The researchers were interested in two types of poliovirus: poliovirus type 1 and poliovirus type 3. They wanted to see if the booster injection with the Salk IPV reduced the number of children with either of these two polioviruses in the faeces.
What were the basic results?
Infants aged between 6 and 11 months
- Booster injections with the Salk IPV significantly reduced the proportion of infants with type 3 poliovirus in their faeces compared to no vaccine, but did not significantly alter the proportion of infants with type 1 poliovirus in their faeces.
- Another dose of the oral polio vaccine did not significantly alter the proportion of infants excreting poliovirus, compared to no vaccine.
Children aged 5
- Booster injections with the Salk IPV significantly reduced the proportion of children aged 5 with type 1 or type 3 poliovirus in their faeces, compared to no vaccine.
- Another dose of the oral polio vaccine did not significantly alter the proportion of children excreting poliovirus, compared to no vaccine.
Children aged 10
- Booster injections with the Salk IPV significantly reduced the proportion of children aged 10 with type 1 or type 3 poliovirus in their faeces, compared to no vaccine.
- Another dose of the oral polio vaccine also significantly reduced the number of children aged 10 with type 1 or type 3 poliovirus in their faeces, compared to no vaccine.
When all the age groups were considered together, booster injections with the Salk IPV significantly reduced the proportion of children with type 1 or type 3 poliovirus in their faeces compared to no vaccine, while another dose of the oral polio vaccine had no significant effect.
How did the researchers interpret the results?
The researchers conclude that their study "provides strong evidence that IPV boosts intestinal immunity among children with a history of multiple [oral poliovirus vaccine] doses more effectively than an additional [oral poliovirus vaccine] dose".
They go onto report that “as a result, the WHO is no longer recommending an all-[oral poliovirus vaccine] schedule; rather, it recommends that all-[oral poliovirus vaccine] using countries introduce [at least] one dose of the IPV into routine vaccination schedules".
This randomised control trial has found that a booster vaccination with the Salk inactivated poliovirus vaccine (IPV) can boost gut immunity against polioviruses in infants and children who have already received multiple doses of the oral vaccine.
It appears that receiving both vaccines is key, as the researchers report that the ability of the Salk IPV to induce gut immunity is limited. They say that studies in countries that do not use oral vaccine show that more than 90% of children given the IPV excrete challenge poliovirus. However, the researchers also say the oral vaccine has been reported to give incomplete intestinal immunity that does deteriorate.
Polio is transmitted by the faecal-oral route, either by exposure to faecally contaminated food or water, or by person-to-person contact. These findings are important, as in many of the parts of the world where polio is a problem, the standards of sanitation are poor. This means the potential for children to contract the disease by coming into contact with infected faeces passed by someone with weakened intestinal immunity is high.
The researchers also note one limitation to their study: it was performed in one district of India, and therefore extrapolation or generalisation of these findings must be done with caution. Despite this, on the basis of the results of this study the WHO is recommending that at least one dose of Salk IPV is added to routine vaccination schedules instead of the all-oral vaccination schedule that many countries have.
The UK vaccination schedule will remain unchanged, as all children should be given the IPV vaccinations as part of the routine vaccination schedule.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.