From Quora:
Viruses have not evolved to become “resistant” to vaccines the way that bacteria have evolved to become resistant to antibiotics. Vaccines don’t attack viruses directly, they just prime our immune systems to do so. Our immune systems and viruses have been battling it out for hundreds of millions of years, and the likelihood that either side could evolve to defeat the other is zero. If it was more than zero, it would already have happened.
That said, there are some viruses (and also bacteria) that pose difficult problems for vaccine development. Here is a good summary:
From Refueling the innovation engine in vaccines. Viruses are in red boxes.
None of the viruses in the high-income group are new or have become more-difficult vaccination challenges. They’ve always been hard.
The low-income group is different. These viruses have only emerged in the last decade as public health threats. While I don’t think that viruses are evolving farther or faster than they have in the past, it’s quite possible that they are emerging faster. Several factors could accelerate the emergence of new viruses into human populations: expansion of trade networks, ecological disruption, climate change, and population growth.
But there doesn’t really seem to be any accelerating trend in the discovery of new human viruses.
Discovery curves for human viruses. (a) Virus discovery curve by species. Cumulative number of species reported to infect humans. Statistically significant upward breakpoints are shown (vertical lines). (b) Virus discovery curve by family. Cumulative number of families containing species reported to infect humans. From Human viruses: discovery and emergence
One could argue that there has been a slowdown in new virus emergence since 1990, especially at the family level. Certainly there is no evidence of an explosion of new viruses.
The implication is that our failure to develop vaccines to new viruses is not driven by the biology of viruses but by the failures of markets. Vaccine development is not very profitable, particularly for diseases that primarily afflict low-income countries. Organizations like GAVI are doing much to correct the failures of market-driven drug discovery, but their resources are not equal to the need. We are not being overwhelmed by new threats, we are just responding to them too slowly.