Thanks to effective vaccination, polio — once considered among the most devastating childhood illnesses — is now nearly eradicated worldwide, with only a few hundred cases each year. However, scientists from the University of Bonn, together with colleagues from Gabon, are reporting alarming new findings: a mutated virus that was able to resist the vaccine protection to a considerable extent was found in victims of a polio outbreak in the Congo in 2010. The pathogen could also potentially have infected many people in Germany, the scientists say.
The polio epidemic in the Congo in 2010 was especially grave. At least 445 people, mostly young adults, were infected, and the disease was fatal for 209 of them. The high mortality rate associated with the outbreak– nearly 50 percent — is surprising, given that the average case fatality rate for polio is just 10 percent.
Another concerning characteristic of the 2010 outbreak was the fact that many of those affected had apparently been vaccinated: Surveys indicated that half of the patients remembered having received the prescribed three vaccination dosages. To date, the vaccination has been considered a highly effective weapon for combating the polioviruses that cause the disease.
Wanting to know more about the causes of the unexpected — and alarming — spread of polio among vaccinated healthy young adults, a team of researchers — led by Drs. Jan Felix Drexler and Christian Drosten, of the Institute for Virology of the University Hospital of Bonn, along with colleagues Dr. Gilda Grard and Dr. Eric Leroy of the West African country of Gabon — analyzed samples of the virus from the 2010 outbreak.
“We isolated polio-viruses from the deceased and examined the viruses more closely,” explains Dr. Drexler. “The pathogen carries a mutation that changes its form at a decisive point.”
The result: the antibodies induced by the vaccination can hardly block the mutated virus and render it harmless. Just to be clear, the mutated virus was not caused by the vaccine, as some have suggested. Rather, the virus mutated because people weren’t getting vaccinated — in fact, the only reason the virus is still around to mutate in the first place is because we have not yet achieved universal immunization.
The researchers then examined the success with which the mutated pathogen evades the immune system. To this purpose, they tested, among others, blood samples from 34 medical students of the University of Bonn. All of them were vaccinated in childhood with the usual methods against polio. And very successfully, as an initial test showed: The antibodies in the blood of the test subjects had no problem combating “normal” polio viruses.
But the situation was different with the mutated virus; the immune reaction was much weaker here. “We estimate that one in five of our Bonn test subjects could have been infected by the new polio virus, perhaps even one in three,” says Dr. Drosten.
‘We can’t afford to sit back and do nothing’
The risk of such polio outbreaks in the future “is very hard to project,” the authors write in the newly published article, which appears in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal. The team said that in areas where not everyone gets vaccinated against polio, conditions are ripe for the emergence of “variant viruses whose fitness is normally impaired in populations with robust immunity.”
Therefore, researchers recommend that people continue to be vaccinated against polio for years to come, potentially with both the inactivated and active vaccine types.
A deadly outbreak of polio in the Democratic Republic of the Congo was caused by a mutated virus that breached vaccine protection, researchers say.
*Side note: This is where the concept of herd immunity comes into play. Herd immunity provides protection against infectious diseases for those who do not have immunity (either because they are too young to be vaccinated or are otherwise ineligible for immunization, or because of a compromised immune system). Once the rate of immunization in the community drops below a certain point, we lose our collective herd immunity. At that point, the rate of disease transmission goes up rapidly. That’s why universal vaccination is so important! For more on that, check out this article.*
Polio is a potentially fatal viral disease that half a century ago killed or crippled hundreds of thousands of people, mainly children. In 1988, the disease was endemic in 125 countries, and 350,000 cases were recorded worldwide, according to the World Health Organization (WHO). Today, the virus is considered endemic in only three countries — Afghanistan, Nigeria and Pakistan — where the vaccination campaign has been attacked by Islamists and tribal leaders.
However, in recent years the virus has started to make a comeback. Polio outbreaks are ongoing across at least 10 countries in Asia, Africa, and the Middle East. Experts are particularly concerned that polio is re-emerging in countries previously free of the disease, such as Syria, Somalia and Iraq, where civil war or unrest now complicates efforts to contain the virus. The vast majority of new cases are in Pakistan, a country which an independent monitoring board set up by the WHO has called “a powder keg that could ignite widespread polio transmission.”
In response to the alarming outbreaks of polio popping up across the globe, the WHO in May issued an unprecedented warning, declaring for the first time ever that the spread of polio is an international public health emergency. The agency further warned that current outbreaks could grow in the next few months and unravel the nearly three-decade effort to eradicate the crippling disease.
The WHO’s polio eradication efforts are modeled after smallpox — thanks to a consistent vaccination strategy, the world has been classified as free of smallpox since 1980. The chances are principally good that something similar could succeed again: The polio virus can also only be transmitted from person to person (predominantly through oral exposure to fecal matter, such as in the case of a diner who contracts the virus from a restaurant worker who did not wash his/her hands before preparing food. In settings with high standards of hygiene, the oral–oral transmission may also be common, says the WHO).
With only one route of transmission, there are no pathogen reservoirs in animals from which the disease could spread repeatedly. Similar to with smallpox, the polio vaccines also offer extraordinary protection. This, however, does not apply when the virus mutates. “When such an altered pathogen encounters a population that has not been consistently vaccinated enough, then things get dangerous,” the scientists warn.
The polio epidemic in the Congo was stopped with a massive vaccination program and hygiene measures. Even the current vaccines thus appear to be good enough to be effective when they are promptly and consistently administered. In fact, new research from the WHO — published earlier this week in the journal Science – indicates that doubling-up on the two available polio vaccines (inactivated and live-attenuated) significantly increases the effectiveness of the immunizations. As a result, the WHO now recommends that both vaccines should be given to children living in areas with ongoing outbreaks.
While scientists are making great strides towards eradicating polio, the new pathogen serves as a reminder of the challenges that lie ahead. “We can’t afford to sit back and do nothing,” the scientists warn. “We need to further increase the vaccination rate and develop new, more potent vaccines. Only in this way do we have a chance of permanently vanquishing polio.”