The term for this week is antigenic drift. This is the term used for a virus that evolves over time. The evolution involves small changes to the composition of the virus. The new virus is called a different strain.
This is the kind of thing that we see happening with the COVID-19 virus. It is part of the natural course of viral infections.
The most common virus that does this is influenza A. It is constantly changing. That is one of the reasons that we have an annual flu vaccine. It is an attempt to keep up with those changes.
Antibodies form against the original virus. That is true if they come from a natural infection. That is true if they come from a vaccine.
Most of the time the changes to the virus are small. The result is that the antibodies can still work against the new virus.
Sometimes the changes can be multiple. Sometimes they can be large. In those cases the antibodies will not recognize the new virus.
This is why some influenza seasons are more serious than others.
Repeated exposure will give the body the opportunity to make new antibodies to the small changes. This exposure can occur naturally. It can occur with annual flu vaccine boosters.
If someone does not get a booster for a few years and happens to avoid the annual natural exposure, there is a good chance that their antibodies will not work as well with the next encounter.
We are seeing this of COVID-19 already. Therefore, it is likely that there will be a need for booster immunizations for the same reason.
A second concept is something called antigenic shift. This means that a virus undergoes a radical change. The change can allow the virus to jump from one species to another. The new species will have no antibodies against the virus. That is because it had never occurred in nature.
This is what happened with SARS. This is what happened with MERS. This is what happened with COVID-19.
Antigenic drifts tend to be less deadly than antigenic shifts. That is because there are still many people in the population with enough immunity to the old virus.
We already know that some of the new strains are less susceptible to preventing illness in vaccinated individuals. However, they do not produce severe disease. There is still protection. Exposure of vaccinated individuals to these strains will allow them to make antibodies to the newer changes in the virus. That is how our natural immune system keeps up with antigenic drift.