Estimating SARS‐CoV‐2 infections and associated changes in COVID‐19 severity and fatality

Abstract

BackgroundThe difficulty in identifying SARS‐CoV‐2 infections has not only been the major obstacle to control the COVID‐19 pandemic but also to quantify changes in the proportion of infections resulting in hospitalization, intensive care unit (ICU) admission, or death.MethodsWe developed a model of SARS‐CoV‐2 transmission and vaccination informed by official estimates of the time‐varying reproduction number to estimate infections that occurred in Italy between February 2020 and 2022. Model outcomes were compared with the Italian National surveillance data to estimate changes in the SARS‐CoV‐2 infection ascertainment ratio (IAR), infection hospitalization ratio (IHR), infection ICU ratio (IIR), and infection fatality ratio (IFR) in five different sub‐periods associated with the dominance of the ancestral lineages and Alpha, Delta, and Omicron BA.1 variants.ResultsWe estimate that, over the first 2 years of pandemic, the IAR ranged between 15% and 40% (range of 95%CI: 11%–61%), with a peak value in the second half of 2020. The IHR, IIR, and IFR consistently decreased throughout the pandemic with 22–44‐fold reductions between the initial phase and the Omicron period. At the end of the study period, we estimate an IHR of 0.24% (95%CI: 0.17–0.36), IIR of 0.015% (95%CI: 0.011–0.023), and IFR of 0.05% (95%CI: 0.04–0.08).ConclusionsSince 2021, changes in the dominant SARS‐CoV‐2 variant, vaccination rollout, and the shift of infection to younger ages have reduced SARS‐CoV‐2 infection ascertainment. The same factors, combined with the improvement of patient management and care, contributed to a massive reduction in the severity and fatality of COVID‐19.

Risk of breakthrough infection and hospitalisation after COVID-19 primary vaccination by HIV status in four Italian regions during 2021

“…However, our estimates lacked precision due to the low number of events observed, particularly for COVID-19 hospitalisations, which reflects both the low prevalence of HIV in Italy and the high degree of protection induced by COVID-19 vaccination. Our analysis occurred in a period where underascertainment in Italy was lower than in omicron-prevalent periods [ 40 ], as testing availability was high and virus circulation was contained. However, if underascertainment occurred and was differential according to HIV status (e.g., PLWH being more aware of health risks may have been more likely to get tested) it could have biased our estimates probably towards an overestimation of the infection risk differences.…”

Relative effectiveness of bivalent Original/Omicron BA.4-5 mRNA vaccine in preventing severe COVID-19 in persons 60 years and above during SARS-CoV-2 Omicron XBB.1.5 and other XBB sublineages circulation, Italy, April to June 2023

“…Firstly, as for other observational studies, although the analysis was adjusted for several covariates, a residual bias because of uncontrolled confounders might have affected our estimates. Secondly, under-reporting because of missed notification of self-diagnosed or unascertained cases has likely caused an overestimation of the number of susceptible people exposed to the risk of severe COVID-19, especially in the comparator group (first booster of an mRNA vaccine received at least 120 days earlier) [20]. This could have led to underestimate rVE, especially at later time intervals, even in the event that the proportion of under-reported cases did not differ between the compared groups.…”

Complexity and Diversity of the Neurological Spectrum of SARS-CoV-2 over Three Waves of COVID-19

“…Since 2021, changes in the dominant SARS-CoV-2 variant, the implementation of vaccination programs, and the shift of infection to younger ages have decreased the detection of SARS-CoV-2 infections. These same factors, along with improved patient management and care, have significantly reduced the severity and mortality of COVID-19 [ 11 ].…”