There’s a new argument burgeoning among anti-vaxxers: That it’s a big problem and a giant red flag that there are so many breakthrough cases of Delta among the vaccinated. The fact that the COVID vaccines don’t approach 100% effectiveness against virus spread among the vaccinated has led to no-vaxxer tagging the COVID vaccines with the “leaky vaccine” label – and then to comparisons with Marek’s Disease in chickens.
The idea is that as more people get vaccinated for COVID, the more vaccinated get and transmit COVID. Furthermore, COVID continues to mutate even in vaccinated people. Eventually the mutations get to the point where the vaccines can only mitigate against sickness and worst effects – and no longer have an effect on community transmission at all.
This is where Marek’s Disease is cited. The Wikipedia article has a good quick overview on Marek’s mitigation in chickens. In brief:
Because vaccination does not prevent infection with the virus, Marek’s is still transmissible from vaccinated flocks to other birds, including the wild bird population. The first Marek’s disease vaccine was introduced in 1970. The disease would cause mild paralysis, with the only identifiable lesions being in neural tissue. Mortality of chickens infected with Marek’s disease was quite low.
Current strains of Marek virus, decades after the first vaccine was introduced, cause lymphoma formation throughout the chicken’s body and mortality rates have reached 100% in unvaccinated chickens. The Marek’s disease vaccine is a “leaky vaccine”, which means that only the symptoms of the disease are prevented. Infection of the host and the transmission of the virus are not inhibited by the vaccine. This contrasts with most other vaccines, where infection of the host is prevented.
Under normal conditions, highly virulent strains of the virus are not selected. A highly virulent strain would kill the host before the virus would have an opportunity to transmit to other potential hosts and replicate. Thus, less virulent strains are selected. These strains are virulent enough to induce symptoms but not enough to kill the host, allowing further transmission. However, the leaky vaccine changes this evolutionary pressure and permits the evolution of highly virulent strains. The vaccine’s inability to prevent infection and transmission allows the spread of highly virulent strains among vaccinated chickens. The fitness of the more virulent strains is increased by the vaccine.
Anyway, you can see where this goes when used in anti-vax arguments: Delta breakthroughs = leaky vaccine = COVID vaccines will make future variants worse just like what happened with Marek’s in chickens.
My spidey sense is that this is off the mark, but I struggle to formulate a good, grounded response. I can’t answer why other human vaccines haven’t done what the Marek’s vaccine did in chickens – I only know (?) that this hasn’t yet happened with a human vaccine (?). The best I can offer is human’s aren’t chickens, COVID is not Marek’s, and the COVID vaccines aren’t Marek’s vaccines.
Do we have a better counter to the “leaky vaccines are dangerous” argument?