  Background: Trends in hospitalised case-fatality risk (HFR), risk of
intensive care unit (ICU) admission and lengths of stay for patients
hospitalised for COVID-19 in England over the pre-vaccination era are unknown.
  Methods: Data on hospital and ICU admissions with COVID-19 at 31 NHS trusts
in England were collected by Public Health England's Severe Acute Respiratory
Infections surveillance system and linked to death information. We applied
parametric multi-state mixture models, accounting for censored outcomes and
regressing risks and times between events on month of admission, geography, and
baseline characteristics.
  Findings: 20,785 adults were admitted with COVID-19 in 2020. Between March
and June/July/August estimated HFR reduced from 31.9% (95% confidence interval
30.3-33.5%) to 10.9% (9.4-12.7%), then rose steadily from 21.6% (18.4-25.5%) in
September to 25.7% (23.0-29.2%) in December, with steeper increases among older
patients, those with multi-morbidity and outside London/South of England. ICU
admission risk reduced from 13.9% (12.8-15.2%) in March to 6.2% (5.3-7.1%) in
May, rising to a high of 14.2% (11.1-17.2%) in September. Median length of stay
in non-critical care increased during 2020, from 6.6 to 12.3 days for those
dying, and from 6.1 to 9.3 days for those discharged.
  Interpretation: Initial improvements in patient outcomes, corresponding to
developments in clinical practice, were not sustained throughout 2020, with HFR
in December approaching the levels seen at the start of the pandemic, whilst
median hospital stays have lengthened. The role of increased transmission, new
variants, case-mix and hospital pressures in increasing COVID-19 severity
requires urgent further investigation.
