(Bloomberg) -- Mortality rates among adult Covid-19 patients in U.S. hospitals rose in July from all-time lows, a slight setback in what has generally been a trend of steadily improving outcomes. 

Among all age groups 18 and older, patients hospitalized with Covid-19 in July were slightly more likely to die than they were in June, according to Premier Applied Sciences data distributed by the Centers for Disease Control and Prevention. 

Those 65-74 died 12.3% of the time, up from 10% in June. About 14.9% of patients 75 and over died, up from 13.2%. Among those 18-24, the youngest cohort for whom data was available, the mortality rate rose to 1.4% from 0.6% a month earlier.

The U.S. has seen massive improvements in care since the early days of the pandemic, but the data suggest that outcomes tend to worsen somewhat at moments of acute stress for hospitals -- including December 2020 and now July, when the fast-spreading delta variant began to drive patients into emergency departments.

The Premier Healthcare Database draws on submissions from more than 800 facilities nationwide.

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