No vaccine has a 100% success rate in preventing infection. In fact, many of the standard vaccinations are not nearly as effective on an individual level as one may believe. At the population level, they are pretty effective because a high percentage of the population is vaccinated and/or has natural immunity.
Looking at the natural immunity scenario, one can consider the history of varicella. Prior to widespread vaccination, there were yearly outbreaks amongst children, despite the fact that more than 99% of the adult population had immunity due to prior infection. That is, despite the fact that the adult population was largely immune, there were still outbreaks among children.
Once the varicella vaccine became part of the standard vaccine schedule, those outbreaks essentially ceased. That doesn't mean that the virus isn't still being transmitted. It does mean that if enough people refuse getting the vaccine for their children, then there is a risk of another outbreak precisely because the percentage of people immune to the infection dipped below the herd immunity level.
Similarly, with COVID-19, if we can get the vaccination percentage up to more than 90 to 95% (or whatever percentage the data supports), then outbreaks will cease, but the virus will still be out there.
Looking at the natural immunity scenario, one can consider the history of varicella. Prior to widespread vaccination, there were yearly outbreaks amongst children, despite the fact that more than 99% of the adult population had immunity due to prior infection. That is, despite the fact that the adult population was largely immune, there were still outbreaks among children.
Once the varicella vaccine became part of the standard vaccine schedule, those outbreaks essentially ceased. That doesn't mean that the virus isn't still being transmitted. It does mean that if enough people refuse getting the vaccine for their children, then there is a risk of another outbreak precisely because the percentage of people immune to the infection dipped below the herd immunity level.
Similarly, with COVID-19, if we can get the vaccination percentage up to more than 90 to 95% (or whatever percentage the data supports), then outbreaks will cease, but the virus will still be out there.