When people talk about the impact of vaccines, they usually mean the millions of humans saved from disease and death. But Andrew Read, an evolutionary biologist at Pennsylvania State University, University Park, likes to think about what vaccination does to pathogens.
In 2001, he published a theory in Nature suggesting that some vaccines may cause viruses and bacteria to become more deadly. Now, Read has some evidence to back that up—at least in animals.
A paper published in PLOS Biology this week suggests that widespread vaccination against Marek's disease, a viral infection in chickens, explains why it has evolved to become more lethal the past few decades. Something similar might happen with certain human vaccines, Read cautions.
But other researchers say the study has little relevance for public health. Read “should stop scaremongering,” says vaccine researcher Adrian Hill of the University of Oxford in the United Kingdom. He and others worry that the paper—and news stories like this one—will only play into the hands of the antivaccine movement.
Read's ideas are built on the widely accepted idea that pathogens often evolve to become less lethal over time. After all, killing their host quickly reduces their chances of being passed on, whereas causing mild symptoms, or none at all, should aid their spread. So-called leaky or imperfect vaccines, which don't prevent infection but merely reduce symptoms, upend that notion, Read argues. They allow the spread of deadlier pathogens that would normally burn out quickly. Leaky vaccines are common for animal infections, including Marek's disease.
Most human vaccines, on the other hand, actually prevent infection, but that may soon change. With diseases like malaria or HIV, for which protection is very hard to achieve, researchers may settle for vaccines that save lives by preventing severe disease, but not infection. In the study, Read and his co-workers, working at the Pirbright Institute in Compton, U.K., showed that unvaccinated birds infected with highly virulent strains of Marek's disease didn't shed much virus; they also died too fast to pass the disease on to healthy, unvaccinated birds. But just as Read predicted, the opposite occurred in vaccinated birds: They shed more virus when infected with a virulent strain, readily infecting and killing unvaccinated cagemates. To Read, the result suggests that vaccines can favor strains that would otherwise be too lethal to spread.
It's a convincing study, says Michael Lässig, who studies influenza evolution at the University of Cologne in Germany, “But it's a very special set of circumstances … I would be careful about drawing general conclusions.” Hill also thinks that Marek's disease may be a special case; nothing suggests that human vaccines have ever made a disease more virulent, he says. What's more, natural immunity is “leaky,” too, Hill argues, allowing infected people to survive and transmit a disease that is deadly to others. “For malaria, whatever today's vaccine does is a drop in the ocean of all the immunity that is happening in Africa from all the infections,” he says.
Read suspects the phenomenon is more widespread. Feline calicivirus, which causes a respiratory infection in cats, also appears to have increased in virulence as a result of vaccination, Read says, and he is worried about the same thing happening with avian influenza, which some countries keep at bay with poultry vaccines. “You could have the emergence of super-hot strains,” he says.
As for human disease, the study offers no support whatsoever for those who oppose vaccination, Read stresses. And if leaky vaccines are proven safe and effective, they should be used, he adds, but perhaps with closer monitoring and additional measures to reduce transmission, such as bed nets for malaria. “We need to have a responsible discussion about this.”
Definitely worth a discussion! But definitely true that natural immunity is leaky and can cause a similar effect. Why it happens with these few animal diseases is an interesting question.
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