Imperfect Vaccination Can Enhance the Transmission of Highly Virulent Pathogens

You're right, but that does not preclude the current mRNA based vaccines from being undermined by viral evolution driven by mass vaccination.

In general a leaky vaccine is one that keeps the host alive without necessarily stopping further infection and transmission. These conditions contribute to an environment that can allow the virus to accumulate mutations which eventually lead to immune escape [1][2].

Because the current mRNA vaccines induce an immune response highly targeted toward the spike protein RBD, there is concern that mass vaccination may create tremendous selective pressure which further enhances the fitness of the virus [3]. It's difficult to predict if such selective pressure will lead to enhanced virulence or infectiousness, but so far many variants of concern have mutations impacting the spike protein RBD. These mutations result in partial immune escape and manifest as increased infectiousness, reduced vaccine efficiency, and also reduced immunity in naturally infected individuals [4][5].

[1] The adaptive evolution of virulence: a review of theoretical predictions and empirical tests https://www.cambridge.org/core/services/aop-cambridge-core/c...

[2] Why does drug resistance readily evolve but vaccine resistance does not? https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2016...

[3] Risk of rapid evolutionary escape from biomedical interventions targeting SARS-CoV-2 spike protein https://pubmed.ncbi.nlm.nih.gov/33909660/

[4] SARS-CoV-2 immune evasion by the B.1.427/B.1.429 variant of concern https://science.sciencemag.org/content/early/2021/06/30/scie...

[5] mRNA vaccine-elicited antibodies to SARS-CoV-2 and circulating variants https://www.nature.com/articles/s41586-021-03324-6