Which of the following is NOT one of the six links in the chain of infection?

The correct answer is "C. Vaccine" because vaccines are not considered a part of the chain of infection. The chain of infection consists of six essential components: the causative agent, reservoir, portal of exit, mode of transmission, portal of entry, and susceptible host. Each link in this chain represents a stage in the process that leads to an infection.

A vaccine, on the other hand, is a preventive measure used to stimulate the immune system to protect against specific diseases. While vaccines play an important role in public health by preventing infections, they do not fit into the chain of infection framework, which focuses on how infections are transmitted and spread. Understanding the chain of infection is crucial for implementing effective infection control strategies, while the role of vaccines is to interrupt the chain by providing immunity to susceptible hosts.