How evasion undermines each intervention
For vaccines, evasion acts at two levels. First, if the virus blocks the innate sensing that is needed to prime an adaptive response, the vaccine may fail to generate strong memory. Second, if the virus downregulates MHC class I or mutates the antibody target, the effector responses that are generated may not recognize the circulating strain. For antivirals, evasion is not the right term; the relevant process is resistance, which is the same evolutionary logic applied to a drug rather than an antibody. A mutation that reduces drug binding while preserving enzyme function is the antiviral equivalent of an escape mutation.
The rationale for broadly protective designs
A broadly protective vaccine targets a region of the virus that is conserved across strains because it performs a function that cannot tolerate mutation. The influenza hemagglutinin stem is one example: it mediates fusion and is structurally constrained, so antibodies that bind it can neutralize many subtypes. The HIV envelope receptor-binding site is another: it must bind CD4, so mutations that disrupt antibody binding there may also disrupt receptor binding. The trade-off is that these conserved regions are often shielded by glycans or conformational masking, making them poor immunogens. Broadly protective strategies therefore often use engineered immunogens that present the conserved region in a more accessible conformation.
The unifying principle
Antivirals and vaccines are both attempts to impose a selection pressure on the virus. The virus responds by mutating. The durability of an intervention depends on how many mutations are required to escape it and how much fitness the virus loses in the process. Broadly protective designs aim to make escape costly by targeting regions where mutation is constrained by function.