From the Document: "A population can establish herd immunity to an infectious disease once a large enough portion of the population--typically 70 to 90 percent--develops immunity. [...] For diseases where no vaccination is available, it is possible to develop herd immunity through exposure to, and recovery from, the disease. However, if COVID-19 [coronavirus disease 2019] runs its natural course, this approach would entail the risk of severe disease or death. Given the risk associated with COVID-19 infections, achieving herd immunity without a vaccine could result in significant morbidity and mortality rates."
Government Accountability Office: https://www.gao.gov/