Stratification of the risk of developing severe or lethal Covid-19 using a new score from a large Italian population: a population-based cohort study

Abstract

Objectives: To develop a population-based risk stratification model (COVID-19 Vulnerability Score) for predicting severe/fatal clinical manifestations of SARS-CoV-2 infection, using the multiple source information provided by the healthcare utilisation databases of the Italian National Health Service.

Design: Retrospective observational cohort study.

Setting: Population-based study using the healthcare utilisation database from five Italian regions.

Participants: Beneficiaries of the National Health Service, aged 18-79 years, who had the residentship in the five participating regions. Residents in a nursing home were not included. The model was built from the 7 655 502 residents of Lombardy region.

Main outcome measure: The score included gender, age and 29 conditions/diseases selected from a list of 61 conditions which independently predicted the primary outcome, that is, severe (intensive care unit admission) or fatal manifestation of COVID-19 experienced during the first epidemic wave (until June 2020). The score performance was validated by applying the model to several validation sets, that is, Lombardy population (second epidemic wave), and the other four Italian regions (entire 2020) for a total of about 15.4 million individuals and 7031 outcomes. Predictive performance was assessed by discrimination (areas under the receiver operating characteristic curve) and calibration (plot of observed vs predicted outcomes).

Results: We observed a clear positive trend towards increasing outcome incidence as the score increased. The areas under the receiver operating characteristic curve of the COVID-19 Vulnerability Score ranged from 0.85 to 0.88, which compared favourably with the areas of generic scores such as the Charlson Comorbidity Score (0.60). A remarkable performance of the score on the calibration of observed and predicted outcome probability was also observed.

Conclusions: A score based on data used for public health management accurately predicted the occurrence of severe/fatal manifestations of COVID-19. Use of this score may help health decision-makers to more accurately identify high-risk citizens who need early preventive or treatment interventions.

Keywords: COVID-19; health policy; public health.

Figure 1

Relationship between categories of COVID-19 Vulnerability Score and (A) the risk of occurrence of severe/fatal forms of COVID-19, (B) its distribution among National Health Service beneficiaries. Columns indicate the observed values (of risk and prevalence, respectively). Solid and dashed lines, respectively, represent the fitted cubic spline with the corresponding 5th and 95th percentiles. The analysis was based on the cohort of 7 655 502 beneficiaries of the Lombardy Region Health Service for at least 2 years, who on 21 February 2020 were alive, aged between 18 and 79 years and did not reside in a nursing home. During the first epidemic wave (until June 2020), this cohort experienced 9160 severe (intensive care unit admitted and mechanically ventilated via intubation) and/or fatal outcomes. The average incidence rate during the first wave was therefore 12.0 cases per 10 000 people at risk.

Figure 2

Receiver operating characteristic (ROC) curves comparing discriminant power (A) of COVID-19 Vulnerability Score (CVS), Charlson Comorbidity Index (CCI) and Multisource Comorbidity Score (MCS) from the derivation set (B) of CVS from several validation sets. Derivation set (left box) was based on the cohort of 7 655 502 beneficiaries of the Lombardy Region Health Service for at least 2 years, who on 21 February 2020 were alive, aged between 18 and 79 years and did not reside in a nursing home. During the first epidemic wave (until June 2020), this cohort experienced 9160 severe (intensive care unit admitted and mechanically ventilated via intubation) and/or fatal outcomes. Validation sets (right box) were based on: (1) 7 575 924 resident in Lombardy whom observation started on 1 July 2020 and who experienced 2822 severe/fatal outcomes within 31 December 2020; (2) 92 267, 1 110 570, 3 012 754 and 3 649 518 beneficiaries of Valle d’Aosta, Marche, Puglia and Sicily regional health services, whom observation started on 1 March 2020 and who, respectively, experienced 173, 542, 1953 and 1541 severe/fatal outcomes within 31 December 2020.

Figure 3

Calibration plot of observed (X-axis) versus predicted (Y-axis) risk of severe/fatal outcomes. The analysis was based on the pooled validation sets of 15 441 033 residents from Lombardy, Valle d’Aosta, Marche, Puglia and Sicily who experienced 7031 severe/fatal outcomes from starting (1 July 2020 in Lombardy, or 1 March 2020 in the other regions) until 31 December 2020.