How more COVID-19 vaccines, how more COVID-19 mutations

Just as antibiotics breed resistance in bacteria, vaccines can incite changes that enable diseases to escape their control. Marek’s disease a highly contagious, paralyzing and ultimately deadly ailment that costs the chicken industry more than $2 billion a year. So they created a vaccine, which took care of the disease ... that was in 1970. It worked well ... for a while, until 'suddenly' the vaccine failed. In 1983 they created a second version of the vaccine. Today, the poultry industry is on its third vaccine. It still works, but scientists are concerned it might one day fail, too — and no fourth-line vaccine is waiting. Worse, in recent decades, the virus has become more deadly. Some scientists were thinking about the reason why the vaccines were failing. So a little test was setup in 2015. They took 200 chickens, and vaccinated only half of them. They then infected all the birds with strains of Marek’s that varied in how virulent — as in how dangerous and infectious — they were. They learned that the vaccinated chickens produced much more mutations than those who were not vaccinated!

The findings suggest that the Marek’s vaccine encourages more dangerous viruses to proliferate. This increased virulence might then give the viruses the means to overcome birds’ vaccine-primed immune responses and sicken vaccinated flocks.

The principle of a vaccine is simple. A vaccine exposes your body to either live but weakened or killed pathogens, or even just to certain bits of them. This exposure incites your immune system to create armies of immune cells, some of which secrete antibody proteins to recognize and fight off the pathogens if they ever invade again.

That said, many vaccines don’t provide lifelong immunity, for a variety of reasons. A new flu vaccine is developed every year because influenza viruses naturally mutate quickly. Vaccine-induced immunity can also wane over time. After being inoculated with the shot for typhoid, for instance, a person’s levels of protective antibodies drop over several years, which is why public health agencies recommend regular boosters for those living in or visiting regions where typhoid is endemic.

But the above effectiveness of vaccines are caused by the mutation of viruses by themselves in a natural way. It's different when we talk about mutations of viruses caused by vaccines, and that's what we talk about in the case of COVID-19 viruses and vaccines.

Vaccines stimulate the immune system to make antibodies that attack features on a pathogen. Because pathogens naturally mutate, random changes in a pathogen’s features can make a vaccine ineffective over time. But scientists are now seeing signs that, in some cases, the vaccines themselves can drive pathogen populations to become resistant. It's important to realize what exactly happens when we're talking about viruses infecting a host. A host like ... yourself.

Let's take a host, which is infected by a virus. After three days, one milliliter of its blood contains 100 billion viral particles. That's roughly the number of stars in the Milky Way. When an RNA virus replicates, the copying process generates one new error, or mutation, per 10,000 nucleotides, a mutation rate as much as 100,000 times greater than that found in human DNA.

That said, take a look at the Hepatitis B vaccine. The current vaccine, which principally targets a portion of the virus known as the hepatitis B surface antigen, was introduced in the U.S. in 1989. A year later, in a paper published in the Lancet, researchers described odd results from a vaccine trial in Italy. They had detected circulating hepatitis B viruses in 44 vaccinated subjects, but in some of them, the virus was missing part of that targeted antigen. Then, in a series of studies conducted in Taiwan, researchers sequenced the viruses that infected children who had tested positive for hepatitis B. They reported that the prevalence of these viral “escape mutants,” as they called them, that lacked the surface antigen had increased from 7.8 percent in 1984 to 23.1 percent in 1999. Most of the vaccines we get in childhood prevent pathogens from replicating inside us and thereby also prevent us from transmitting the infections to others. But scientists have so far been unable to make these kinds of sterilizing vaccines for complicated pathogens like HIV, anthrax and malaria.

To conquer these diseases, some researchers have been developing immunizations that prevent disease without actually preventing infections — what are called “leaky” vaccines. And these new vaccines may incite a different, and potentially scarier, kind of microbial evolution.

The more pathogens a host is infected with, the more change the pathogens have to infect others. But the setback is that it might kill the host, which does not support infecting others. The problem with leaky vaccines is that they enable pathogens to replicate unchecked while also protecting hosts from illness and death, thereby removing the costs associated with increased virulence.