Answer:
This happens because viruses mutate and vary according to the environment they face. That is why different strains occur and appear every day, which must be updated in the vaccines, in order to have immunity up to date and keep up with the viral mutation without making us sick.
Step-by-step explanation:
Viruses are acellular structures that do not have their own metabolism or replication method, it is because they must dominate and invade the human body so that its DNA or RNA is coupled to the human and this uses the human machinery to be able to replicate.
On the other hand, as infections occur and viruses appear, they can associate with bacteria forming bacteriophages, they can mutate within an organism that is infected by more than one viral infection and they can even adopt retrovirus mechanisms, which day by day. These new viruses are called as new strains, which evolve and will have to be coupled with vaccines.
Vaccines are a method of natural ACTIVE immunity, that is, part of the virus is inoculated, so that it generates the formation of a specific antibody in the body.