From chickens to COVID-19
In the 2015 paper, we reported experiments with variants of Marek’s disease virus – the name of the chicken virus we were studying. It is a herpesvirus that causes cancer in domestic chickens. A first-generation vaccine against it went into widespread use in poultry in the early 1970s. Today, all commercial chickens and many backyard flocks are vaccinated against Marek’s.
Chickens with Marek’s disease virus became capable of transmitting the virus about 10 days after they get infected. In our lab experiments, we worked with variants of Marek’s disease virus that were so lethal they would kill all unvaccinated birds in 10 days or fewer. So prior to the vaccine, the birds died before they could transmit the lethal variants to other birds. But we found that the first-generation vaccine protected the birds from dying. In other words, the Marek’s-infected chickens lived and were thus able to spread the highly virulent strains to other birds.
In the case of COVID-19, it’s becoming increasingly clear that even vaccinated people can contract and transmit the highly transmissible delta variant. Since viral transmission from vaccinated chickens is what allowed more lethal variants to spread in Marek’s, it’s reasonable to ask whether COVID-19 transmission from vaccinated people could allow more lethal variants to spread.
In the history of human and animal vaccines, there have not been many cases of vaccine-driven evolution. But in every one of them, individuals and populations have always been better off when vaccinated.
Evolution can go in many directions
As evolutionary ecologist David Kennedy and I have written about previously, the evolutionary path that the Marek’s disease virus took is one of many that are possible – in rare cases where vaccines drive evolution.
Only a minority of human and animal vaccines have influenced pathogen evolution. In nearly all of those cases – which include the hepatitis B virus and bacteria that cause whooping cough and pneumonia – vaccine efficacy was reduced by new variants. But in contrast to Marek’s, there was no clear evidence that the evolved variants made people sicker.
In nature, we know of course that not all viruses are equally lethal. Biological differences in things like the linkage between disease severity and transmission can cause lethality to increase or decrease. This means that the future of one virus cannot be predicted by simply extrapolating from the past evolution of another. Marek’s and SARS-CoV-2 are very different viruses, with very different vaccines, very different hosts and very different mechanisms by which they sicken and kill. It is impossible to know whether their differences are more important than their similarities.
Evolutionary hypotheticals are important to consider. But up against the hugely beneficial impact of COVID-19 vaccines on reducing transmission and disease severity – even against the delta variant – the possibility of silent spread of more lethal variants among the vaccinated is still no argument against vaccination.
As novel variants of the coronavirus spread in the months and years ahead, it will be vital to work out whether their evolutionary advantage is arising because of reduced disease severity among the vaccinated. Delta, for instance, transmits more effectively from both unvaccinated and vaccinated people than did earlier variants. Extrapolating from our chicken work to argue against vaccination because of the delta variant has no scientific rationale: The delta variant would have become dominant even if everyone refused vaccination.
But what if?
If more deadly variants of the coronavirus were to arise, lower vaccination rates would make it easier to identify and contain them because unvaccinated people would suffer more severe infections and higher death rates. But that kind of “solution” would come at considerable cost. In effect, the variants would be found and eliminated by letting people get sick, many of whom would die.
Sacrificing chickens was not the solution the poultry industry adopted for Marek’s disease virus. Instead, more potent vaccines were developed. Those newer vaccines provided excellent disease control, and no lethal breakthrough variants of Marek’s have emerged in over 20 years.