"Leaky Vaccines" May Result in More Virulent Viruses
A new study looking at the herpesvirus vaccine used to prevent Marek’s disease in poultry has come up with some interesting findings. The vaccine may actually lead to the evolution of more virulent strains, which can threaten unvaccinated birds.
Andrew Read, author of the paper to be published in PLOS Biology, says his study supports what has until now been a controversial idea. But don’t panic – this phenomenon does not apply to most human vaccines. Read states:
“When a vaccine works perfectly, as do the childhood vaccines for smallpox, polio, mumps, rubella, and measles, it prevents vaccinated individuals from being sickened by the disease, and it also prevents them from transmitting the virus to others.”
If the vaccine prevents infection then there is no problem. However, what he calls “leaky” vaccines, like the one for herpesvirus in poultry, do not prevent infection and spread, they just reduce the severity of the infection so that the animal can survive. They can still experience enough of an infection to spread the virus, however.
This is similar to partially treating a bacterial infection with an antibiotic. If you don’t take enough to wipe out the infection thoroughly, some bacteria may survive, and they are likely to be ones with resistance to the antibiotic.
Similarly, partial or leaky protection from a vaccine produces evolutionary pressure for more virulent viruses that can spread despite the vaccine. Another study author, Venugopal Nair, said:
“In our tests of the leaky Marek’s-disease virus in groups of vaccinated and unvaccinated chickens, the unvaccinated died while those that were vaccinated survived and transmitted the virus to other birds left in contact with them,” Nair said. “Our research demonstrates that the use of leaky vaccines can promote the evolution of nastier ‘hot’ viral strains that put unvaccinated individuals at greater risk.”
In other words, their research shows that leaky vaccine have the potential to allow more virulent strains to arise, because they do not block survival and transmission of the virus (no that it actually has).
The study is important for future vaccine research. As we look for vaccines for diseases like HIV, ebola, and malaria, which have proven difficult, we have to be cautious not to create leaky vaccines that can accelerate the evolutionary of greater virulence.
It seems likely that the anti-vaccine movement will exploit this new study for their own propaganda. To address their likely claims ahead of time – this study does not have implications for vaccines in general. As Read listed above, the vaccines currently part of the vaccine schedule for humans are not leaky, they are “perfect” vaccines that prevent the spread of the disease.
Further, Read and Nair emphasize that only the unvaccinated are at risk from leaky vaccines. Getting vaccinated is still the best defense, and it is the unvaccinated who ultimately are the vector for breeding greater virulence.
For example, Marek’s disease is only a minor problem now in the poultry industry because the chickens are vaccinated, significantly reducing the disease.
This study does not in any way question the safety or efficacy of current human vaccines. It simply provides caution and guidance in researching new vaccines – the goal should be to develop vaccines that prevent spread, and not leaky vaccines with only partial protection.