One reason why there is no vaccine for the common cold attests to how viruses adapt to evade the immune system. When we are exposed to a new virus or a variant that is much different than a virus that caused a previous infection, we do not have antibodies against that virus or variant. So, even if we have had a prior infection to an earlier version of the virus, the variant has changed enough to make it essentially a new virus as far as our immune system is concerned.

Once exposed to a new virus or variant, it takes between 10 to 20 days to produce an antibody response. Antibodies bind to proteins on invading organisms to neutralize and destroy them, but that process takes time. And in most cases, by the time an antibody response happens in an infection caused by a new virus or variant, the infection has pretty much been dealt with 100% by other ways in which our immune system fights viral infections. 

Antibodies provide a head start for the immune system when exposed to a virus that caused a previous infection. In an acute infection due to a new virus or variant other components of the immune system must be functioning correctly, particularly non-specific and cell-mediated immunity, since there is not yet an antibody response.

When it comes to fighting an infection to a new virus or variant, the functioning of non-specific and cell-mediated immunity is critical. Antibodies bind to organisms that are "extracellular," meaning they are not inside human cells. Once inside a cell, an organism is somewhat shielded from antibody attack by our cell membranes. Most viruses are "intracellular "pathogens. Meaning they wreak the most havoc because they reside inside our cells. Cell-mediated is much better at recognizing and destroying intracellular pathogens than antibody-based or humoral immunity. 

The thymus gland is the master control of cell-mediated immunity. Inside the thymus, immature white blood cells formed in the bone marrow transform into T lymphocytes (the T stands for "thymus-dependent"). There are several types of T lymphocytes or T cells. Particularly deadly to viruses are cytotoxic T cells. These killer T cells directly kill virus-infected cells.