The booster, to come eight months after the second COVID shot (or full vaccination), is intended to enhance the effectiveness of the Pfizer and Moderna vaccines over time.

The announcement comes as new coronavirus cases, hospitalizations, and deaths surge across the U.S. due the spread of the highly contagious Delta variant. Through Aug. 17, the country averaged roughly 140,000 new cases daily (calculated from a seven-day period), up 52% from two weeks ago according to New York Times data. Based on a Fortune analysis of New York Times data, the number of new cases reported per day has risen most in South Dakota, where they’ve increased by 545% from two weeks ago, followed by West Virginia, which had a 152% gain. The number of new cases reported has more than doubled during that period in a handful of states including Minnesota, Ohio, and Vermont. The rate of new cases is declining in a handful of states that have been the most hard-hit in recent weeks, including Missouri, Arkansas, and Louisiana.

The number of COVID cases per capita, meanwhile, remains high in those states. Florida, Louisiana, and Mississippi are the virus’ hottest spots, each with more than 100 people per 100,000 residents infected with COVID. Case rates are lowest in the North and Northeast, including Minnesota and Vermont, which have recently experienced a jump in new cases. 

The unvaccinated remain those most vulnerable to the virus, and the possibility of becoming severely ill with COVID. As of Aug 18, 51% of Americans were fully vaccinated (61.9% of adults fall into that category) according to the CDC. Seven states, led by Vermont (67%) and Massachusetts (65%) have more than 60% of their populations fully vaccinated. The vaccination rate in another seven states including Arkansas, Louisiana, and West Virginia is less than 40%. Alabama and Mississippi have the lowest rates at 36%.

14-day increase in the number of new cases

Based on 7-day moving average. Source: New York Times