Vaccines save millions of lives each year by teaching the immune system how to fight against certain viruses or bacteria. But a new study suggests that, paradoxically, they could sometimes teach the pathogens become more dangerous too.
The study is controversial. It was made in chickens, and some scientists say there is little interest for human vaccination; they fear it will strengthen doubts about the appropriateness or safety of vaccines. It should not, says lead author Andrew Read, a biologist at Pennsylvania State University, University Park: The study provides no support for the anti-vaccine movement. But it suggests that some vaccines may be monitored more closely, he argues, or supported with additional measures to prevent unintended consequences.
evolutionary science suggests that many pathogens are not life threatening or even not very virulent, because if they kill their host too quickly that they can not spread to other victims. Now enter the vaccination. Some vaccines do not prevent infection, but they do reduce how sick patients become. As supported Read the first time in a document Nature 14 years ago, keeping alive their guests as "imperfect" or "leaks" could give vaccines deadly pathogens from one side, it allowing them to spread when they would normally burn quickly.
now, Read has published a paper showing that what seems to have happened with Marek's disease, a viral infection in chickens. Marek's disease is spread when infected birds shed virus from their feather follicles, which is then inhaled dust by other chickens. Poultry farmers to vaccinate against the disease, maintaining their healthy flocks but keeps chickens from becoming infected and spreading the virus regularly. In recent decades, Marek's disease has become much more virulent that some scholars believe is the result of vaccination.
Read chickens and researchers from Pirbright Institute in Compton, UK, infected with Marek's disease virus of different strains known to cover the spectrum of low to high virulence. When birds are not vaccinated, infection by highly virulent strains killed them so fast that they lose very little virus-orders of magnitude less than when they were infected with less virulent strains. But in vaccinated birds, the opposite was true: People infected with the more virulent strains of virus shed more than birds infected with less virulent strain
In one experiment unvaccinated birds infected the most virulent strains were housed together. with healthy birds. Again, the infected chickens had died in a short time, leaving no chance of spreading the disease to their healthy cage mates. But when the vaccinated birds were infected with the highly virulent strain, they lived longer and all healthy birds housed with them became infected and died. So "immunization allowed the onward transmission of the virus otherwise too deadly for transmitting, unvaccinated people at risk of severe disease and death," the authors write online today in PLOS Biology .
study is compelling, said Michael Lässig, a physicist at the University of Cologne in Germany, who studies the evolution of flu. "But it is a very special set of circumstances" he warns. "I would be careful to draw general conclusions."
Adrian Hill, a vaccine researcher at the University of Oxford in the UK, said the experiments support the idea that vaccines have made of Marek's disease deadly, but do not prove many other things have changed in the poultry industry in the last decades. flocks became much larger, for example, which could also encourage more virulent strains. But Read said these "hot strains" would disappear very quickly if vaccines were taken
Hill did not doubt that some vaccines may lead to increased virulence. the real issue is the probability to occur. His response: It is very unlikely, and not something we should be worried. "They took 15 years to do an experiment on the only example of what happens."
Read counters that there may be other examples. Feline Calicivirus, which causes a respiratory infection in cats, is a strong candidate, he said, "there have been outbreaks of" superhot Watch is particularly concerned about the bird flu in Europe and the United States, whole poultry flocks are generally slaughtered to stop an epidemic "strains in vaccinated populations." ;. Asian farmers often use birds vaccine against influenza. "You might have the emergence of strains superhot," as a result, they say it. Ab Osterhaus, a virologist at Erasmus MC in Rotterdam, the Netherlands, said it is "very unlikely, but a scenario can not be excluded."
But what about human diseases? Most human vaccines used today are not "leaky"; they are very good at stopping the transmission of the disease. But researchers are turning to diseases that are more difficult to protect against, such as malaria or HIV, they set their sights lower, to vaccines that prevent severe disease, but not infection. "We are entering the era of vaccines fleeing humans," says Read. Candidate vaccines against Ebola and malaria which recently received an important stamp of approval in Europe should certainly be used if they are safe and effective, he said, but could lead to more virulent pathogens. "We need to have a responsible discussion about it."
but for Hill, the comments themselves are irresponsible. Read "no more evidence that this will happen with an Ebola vaccine that will happen with another vaccine in humans," he said. "He should stop scaremongering." The whole distinction between vaccines and fleeing nonleaky is flawed, Hill argues: ". Each vaccine is pierced, in that some people are not protected by it, some people are partially protected, some people have the disease prevention, and other infection prevention not " millions worldwide receive shots of each month and there is no evidence that this has never led to a deadly disease become, said Hill.
moreover, the natural immunity should have the same effect, he adds :. After we recover from illness, we usually end up with a limited "leaky" protection against a pathogen that is not very different from what vaccines reach, said Hill "for malaria, regardless of the fact today vaccine is a drop in the ocean of any immunity which is in Africa of all infections to all people."
hill worried that the reading task will play into the hands of antivaxxers. Read but said that even if a human vaccine is never shown to cause dangerous changes in the pathogen, it would not be a reason not to vaccinate. The most important thing would be to support immunization with other vacuum transmission measurements, such as bed nets against malaria.
Ironically, the increase in virulence, it would be even more important to vaccinate everyone, he said, because that universal vaccination would prevent the most dangerous strains of harming anyone. This is in fact what happened in Marek's disease, Read said. "I think because of these vaccines, the industry has created strains superhot, but the vaccine still works beautifully because it can be delivered to any single vulnerable bird."
* Correction, July 28, 4:07 p.m. :. a quote by Adrian Hill, this story has been corrected
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