To understand how a vaccine can help stop the spread of a virus, it is essential to know how a virus behaves and how variants develop. An R0, pronounced as R naught, is a mathematical value used to indicate how contagious or reproductive a virus is in a population. If an R0 value is calculated to be over 1, meaning each existing infection will cause one new infection, it will result in a growth of active cases. The R0 of COVID-19 was initially calculated to be 1.4 - 2.4 by the World Health Organization (WHO), though more recent studies have concluded higher.   By understanding the importance of this value, we can begin to understand how COVID-19 has spread to a global level. Populations adhering to COVID-19 precautions, such as social distancing, isolation, and mask-wearing, have reduced the R0 value. The goal with pandemic outbreaks is to reach herd immunity, in which a majority of the population has immunity to a virus through vaccinations.   With herd immunity, a virus is being transmitted less, mutated less, and does not survive in a host for as long. Over time, the virus will slowly die out as it cannot survive as well in hosts. As long as a virus has time to reproduce and mutate, variants are expected to rise. Mutations that put a virus at an advantage, as seen with the Delta-variant and Omicron-variant, will wreak more havoc on the population by increasing illness severity and virulence.   Most COVID-19 mutations affect the spike protein, a virus structure that binds it to a healthy cell in a host. The majority of COVID-19 vaccines work by targeting proteins in the virus, such as the spike protein. Because of this, vaccines may be less effective if the targeted protein has been mutated, though it does not make the vaccine ineffective. The diversity in administered vaccinations and their efficacy also slows the reaching herd immunity. However, through herd immunity, the spread of variants is guaranteed to be decreased.