Somebody asked me to explain why a vaccine that does not prevent infection but is associated with less severe symptoms among the infected compared to those of the unvaccinated is a problem. “So the vaccine becomes something of a therapeutic,” the person remarked in so many words. “Isn’t this a good thing?”
Not necessarily. The reason is found in grasping basic evolutionary biology, a paradigm that is exquisite in its logic and powerful in its explanatory power. Wherever the discipline of biology is on the question of ontogenesis, there is, outside of a handful of creationists, little doubt among scientists that the theory of evolution explains how, once life appears, all life changes over time. That same exquisite logic applies to microbes, including viruses (even if the question of whether viruses are properly living things is not finally settled).
Here’s how I explain the problem. A virus has no interest in killing or immobilizing its host. The virus needs relatively healthy people to be up and moving about and interacting with other potential hosts to transmits itself. It needs the host’s cellular machinery to reproduce itself. A virus that kills or immobilizes a significant number of its hosts won’t get around as much as a virus that produces mild to moderate symptoms. For example, cold viruses (of which coronaviruses is one type) have evolved via natural selection of random mutation (mistakes occurring in the process of the virus copying itself) to be more contagious but, for those with reasonably intact and experienced immune systems, either asymptomatic or associated with only mild to moderate illness.
This process yields circumstances in which children get half a dozen or more colds a year, while adults get one or two colds annually (having more experienced immune systems), and life goes on. Thanks to the process Darwin describes to explain his discoveries, over time the less dangerous variants of a virus become the dominant variants of the virus and life goes on. Herd immunity works alongside decreasing severity of illness to produce situations in which humans live, work, and go to school in environments teaming with microbes that rarely severely sicken them. (Influenza is a bit different because of the way it ravages cells. But its dominant variants nonetheless present with less severe consequences over time, as well. It makes you a lot sicker than an adenovirus or rhinovirus, but the vast majority of people live to tell about the experience.)
One of the ways this evolutionary process may be disrupted is the development of interventions that allow persons with more dangerous variants of a virus to suffer less severe illness sufficient to allow a greater proportion of the sick to be up and moving about interacting with other potential hosts than they would otherwise. Good for them, of course. They are not as sick as they could be (at least proportionally speaking, since many still get very sick and some die). But this can be bad for a population.
If a vaccine is highly efficacious in preventing infection, and a sufficient number of people are vaccinated, or have acquired immunity through infection, then the virus struggles to find those who are not adequately protected and the vaccine is successful. However, if the vaccine is leaky, that is, the vaccine suffers low efficacy with respect to preventing infection yet continues to enjoy significant efficacy in preventing severe symptoms, then this allows the vaccinated to carry more dangerous variants of the virus to more people, both the vaccinated and unvaccinated. Mutations that would have likely been weeded out during natural selection may indeed thrive in a context where the typical natural trajectory has been derailed. Add to this low durability of a vaccine and those who are vaccinated themselves increasingly begin to suffer severe symptoms.
It may be that, in time, the virus will continue to evolve to become less dangerous, but its path to this end will have been drawn out by the intervention. More troubling, it could also be the case that more deadly mutations survive to produce an even more dangerous situation for everybody.
Imagine a scenario where a population, appearing to have reached herd immunity via natural acquisition to a virus, suddenly sees an explosion of infections with widespread deployment of a vaccine. I hasten to emphasize that the problem is not vaccines per se. The problem is leaky vaccines with low durability. One would expect that the medical-industrial complex would not confront such a development with humility, but would instead double-down on hubris while casting aspersions on the skeptic. The vaccinated could not possibly be the problem. They follow orders.