To date, none of the available vaccines prevent transmission from one individual to another. Most focus on decreasing the severity of symptoms. In the short term, that is a very effective strategy. In the longer term it can turn into a double-edged sword.
Consider the situation globally where COVID vaccines convert overtly symptomatic infections to asymptomatic ones without also decreasing viral shedding and transmission potential. Paradoxically, that could increase transmissions, given that viral testing and isolation are less likely in asymptomatic persons.
It may be that we are going to have to learn to live with COVID and it is going to have to learn to live with us.
Endosymbiosis, the existence of one organism within another, is not uncommon. The evolution of these relationships occurs across many lineages and plays a significant role in allowing adaptation of both species to new ecological niches.
Many microorganisms infect us in sterile environments such blood, central nervous system, and muscle tissue, and invade tissues in organs colonized by billions of microorganisms, for example the intestinal tract.
Humans are essentially symbiotic organisms. There are an estimated 100 trillion microorganisms in the microbial ecosystem of the human body. They can contribute to a range of functions and disfunctions, unique to each one of us, from assisting metabolism in the gut, to immune stimulation in the bone marrow. For example, patients receiving bone marrow transplants who have a high diversity of gut microorganisms have an 11% mortality rate compared with a 50% rate in those with low gut microbiome diversity. Similar improvements in survival were found with low gut microbiome diversity in non-small cell lung cancer, renal cell carcinoma and melanoma.
Of the 300 or so dominant microorganisms that can be identified in humans, fewer than 20 are present in any one individual, indicating that there is massive inter-individual variation. Some we rarely hear of, such as Panthera species, because life in our bodies is so toxic to Panthera, and Panthera is so toxic to humans that neither live long enough to reproduce. Other species such as Toxicodendron would happily live on in humans – if only they didn’t kill us.
Not all species have a detrimental effect. Lactobacillus lives quite passively in the human vagina and protects many women from other, more symptomatic infections such as Candida. This is a kind of mutualism.
The transmission dynamics of SARS-CoV-2 virus are not yet fully understood. Unfortunately, the ability of infected individuals to transmit infection when asymptomatic or in a presymptomatic period means that infection control strategies that focus solely on preventing transmission from symptomatic individuals will be insufficient alone to interrupt the transmission.
Evolving intranasal vaccines may provide the answer to virus shedding.
Or perhaps we will have to wait for the more benign, Lactobacillus kind of mutualism to occur. Evolving new strains maybe more beneficial than detrimental. Just as occurred with Influenza strains, the dynamics of new COVID strains are likely to be less lethal to the general population. From a scientific standpoint, if COVID continues to kills us, its host, it cannot survive indefinitely. In the meantime, the virus needs to reproduce itself much more slowly both within individuals and between individuals, so as not to overrun us. And we must continue to vigilantly wear masks, practice social distancing and limit travel until new, less lethal, strains of COVID prevail.