Live attenuated vaccines, which use a weakened form of the pathogen, offer several advantages. They typically provide strong and long-lasting immunity, often with a single dose. The immune response they elicit closely resembles the response to a natural infection, involving both the cellular and humoral arms of the immune system. However, there are disadvantages as well. They are usually not suitable for people with compromised immune systems, as there is a small risk that the attenuated pathogen can revert to a more virulent form. Additionally, these vaccines often require careful handling and refrigeration to maintain their effectiveness, which can be challenging in areas with limited healthcare infrastructure.

Multiple doses of some vaccines are required to achieve and maintain effective immunisation for several reasons. The primary dose of a vaccine introduces the immune system to the antigen, initiating an immune response and the production of memory cells. However, this initial response may not be strong enough to provide long-term immunity. Subsequent doses, often referred to as booster shots, re-expose the immune system to the antigen, enhancing the immune response and ensuring the production of a sufficient number of memory cells. This process results in a more robust and durable immunity. The requirement for multiple doses is especially common in inactivated and subunit vaccines, which generally induce a weaker immune response compared to live attenuated vaccines.

Antigenic variation refers to the changes in the antigens of a pathogen, often as a result of mutations. This variation can affect vaccine development and effectiveness because vaccines are designed to target specific antigens. If these antigens change, the immune response elicited by the vaccine may not effectively recognise and neutralise the pathogen. This is a significant challenge in developing vaccines for viruses like influenza, which undergo frequent antigenic changes, requiring the annual update of flu vaccines. Understanding and monitoring antigenic variation is crucial in vaccine research, as it informs the selection of relevant strains to include in vaccine formulations.

Herd immunity is a form of indirect protection from infectious diseases that occurs when a large percentage of a population becomes immune to an infection, thereby providing a measure of protection for individuals who are not immune. Vaccination contributes significantly to herd immunity. When a sufficient proportion of the population is vaccinated and becomes immune to a disease, the spread of the pathogen is reduced or even halted, thereby protecting those who cannot be vaccinated, such as individuals with certain medical conditions or weakened immune systems. Herd immunity thresholds vary by disease, depending on factors like the infectiousness of the disease and the effectiveness of the vaccine. Achieving herd immunity through vaccination is a key strategy in controlling and eliminating infectious diseases.

mRNA vaccines represent a novel approach in vaccine technology. Unlike traditional vaccines that introduce antigens to stimulate an immune response, mRNA vaccines use messenger RNA to instruct cells in the body to produce a piece of the pathogen, typically a protein. This protein piece then triggers the immune response. The benefit of this method is that it allows for a rapid development process, as the mRNA sequence can be designed as soon as the genetic information of the virus is known. Unlike live attenuated or inactivated vaccines, mRNA vaccines do not contain a live virus, reducing the risk of an attenuated virus reverting to a pathogenic form. Their development marks a significant advancement in vaccine technology, offering a versatile and rapid response to emerging infectious diseases.