Figure 1.

Potential mechanisms of vaccine-induced strain replacement.(A) In cases without a vaccine, the dominant strain outcompetes other strains and infects the majority of the population. The differential effectiveness of a vaccine creates opportunities for nontargeted strains to proliferate. Vaccines affect both the basic and invasion reproduction numbers of the targeted strain, leading to a decrease in its competitiveness within the population. As the infection rate of the dominant strain decreases, resources are made available for other strains in terms of susceptible hosts. (B) Elevated vaccination rates diminish the susceptible hosts for both strain A and strain B and decrease their infection rate when the pathogens exist independently. In the presence of a perfect vaccine that can target both strain A and strain B, the infection rate will drop for both strains. However, in the scenario where strain A can infect individuals who are already infected with strain B (co-infection or super-infection), strain replacement may occur. With the increase in vaccination rates initially, the vaccine supports the coexistence of both strains. With higher vaccination levels, strain A is eventually eliminated, and strain B becomes dominant. (C) There are three levels of bacteria categorization based on their polysaccharide capsule (serotyping), metabolic profile including substrate transporters (metabolic typing), and genomic sequence (genetic sequence typing). Under the selective pressure of vaccination, bacteria are found to initiate genetic recombination to transfer virulence-related factors. Genetic recombination may occur via either conjugation or transformation between vaccine-targeted and untargeted strains.