From 2020 to early 2022 in England, the COVID-19 infection-fatality ratio (IFR) decreased from 0.67% to 0.10%, and the infection-hospitalization ratio (IHR) declined from 2.60% to 0.76%, with increases during the SARS-CoV-2 Alpha and Delta variant surges, estimates a study published yesterday in PLOS Biology.
Imperial College London researchers estimated COVID-19 infections in England each month from May 2020 to March 2022 to analyze the evolving relationship between case rates and deaths and hospitalizations. They also estimated the case-ascertainment rate, or the ratio of detected infections to the true number of cases, using data from the country's mass testing program.
Reliable IFR and IHR estimates, "along with the time-delay between infection and hospitalisation/death can inform forecasts of the numbers/timing of severe outcomes and allow healthcare services to better prepare for periods of increased demand," the study authors wrote.
Cases dropped amid vaccination, Omicron
The COVID-19 IFR declined from 0.67% in 2020 to 0.10% in 2022, and the IHR fell from 2.60% to 0.76% over the same period.
The average case-ascertainment rate during the entire study period was estimated at 36.1%—signifying that actual cases were more than a third higher than reported—but the authors noted significant variation in continuous estimates of this rate. Continuous IFR and IHR estimates rose during the Alpha and Delta variant waves and fell during the COVID-19 vaccine rollout and the emergence of the Omicron variant.
In 2020, the interval between a positive COVID-19 test and hospitalization was 19 days, and the lag between positivity and death was 26 days. The intervals declined to 7 days for hospitalization and 18 days for deaths by late 2021 and early 2022.
As investments in community surveillance of SARS-CoV-2 infection are scaled back, alternative methods are required to accurately track the ever-changing relationship.
"Even though many populations have high levels of immunity to SARS-CoV-2 from vaccination and natural infection, waning of immunity and variant emergence will continue to be an upwards pressure on the IHR and IFR," the researchers wrote. "As investments in community surveillance of SARS-CoV-2 infection are scaled back, alternative methods are required to accurately track the ever-changing relationship between infection, hospitalisation, and death and hence provide vital information for healthcare provision and utilisation."