Turning point in COVID-19 severity and fatality during the pandemic: a national cohort study in Qatar

Abstract

Objective: To assess the evolution of COVID-19 severity and fatality in a unique setting that consistently applied, throughout the pandemic, rigorous and standardised criteria for defining severe COVID-19 outcomes.

Methods and analysis: We conducted a national cohort study on 312 109 Qatari citizens to investigate incidence of severe, critical or fatal COVID-19 classified according to the WHO criteria between 28 February 2020 and 21 April 2023. Incidence rates for severe, critical or fatal COVID-19 were estimated during the pre-omicron phase, first omicron wave, combined phases and throughout the pandemic.

Results: Cumulative incidence of severe, critical or fatal COVID-19 after 3.14 years of follow-up was 0.45% (95% CI 0.43% to 0.47%). Incidence rate for severe, critical or fatal COVID-19 throughout the pandemic was 1.43 (95% CI 1.35 to 1.50) per 1000 person years. In the pre-omicron phase, first omicron wave, and combined phases, it was 2.01 (95% CI 1.90 to 2.13), 3.70 (95% CI 3.25 to 4.22) and 2.18 (95% CI 2.07 to 2.30) per 1000 person years, respectively. The post-first omicron phase saw a drastic drop to 0.10 (95% CI 0.08 to 0.14) per 1000 person years, a 95.4% reduction. Among all severe, critical and fatal cases, 99.5% occurred during the primary infection. Cumulative incidence of fatal COVID-19 was 0.042% (95% CI 0.036% to 0.050%), with an incidence rate of 0.13 (95% CI 0.11 to 0.16) per 1000 person years. In the post-first omicron phase, incidence rate of fatal COVID-19 decreased by 90.0% compared with earlier stages. Both severity and fatality exhibited an exponential increase with age and a linear increase with the number of coexisting conditions.

Conclusion: The conclusion of the first omicron wave was a turning point in the severity of the pandemic. While vaccination and enhanced case management reduced severity gradually, the rapid accumulation of natural immunity during the first omicron wave appears to have played a critical role in driving this shift in severity.

Keywords: COVID-19; SARS-CoV-2; epidemiology.

Figure 1. COVID-19 acute-care and intensive care unit (ICU) bed hospitalisations and COVID-19 severity and criticality among Qataris throughout the pandemic. (A) Comparison of the number of new COVID-19 admissions into acute-care hospital beds and the number of new severe COVID-19 cases according to the WHO definition for COVID-19 severity. (B) Comparison of the number of new COVID-19 admissions into ICU hospital beds and the number of new critical COVID-19 cases according to the WHO definition for COVID-19 criticality.

Figure 2. All-cause and COVID-19 deaths among Qataris throughout the pandemic. (A) Number of all-cause deaths and of COVID-19 deaths. (B) Proportion of COVID-19 deaths out of all-cause deaths.

Figure 3. Temporal patterns of infections and severe and fatal COVID-19 during the waves and phases of the pandemic. (A) Daily count of newly diagnosed SARS-CoV-2 infections. (B) Incidence rate of severe, critical or fatal COVID-19. (C) Incidence rate of fatal COVID-19.

Figure 4. Cumulative incidence of (A) severe, critical or fatal COVID-19 and (B) fatal COVID-19 since the onset of the pandemic.

Figure 5. Incidence rates of (A) severe, critical or fatal COVID-19 and (B) fatal COVID-19 during the pre-omicron phase, first omicron wave and post-first omicron phase.

Figure 6. Adjusted HRs for severe, critical or fatal COVID-19 and for only fatal COVID-19 across sex (A and E), age (B and F), number of coexisting conditions (C and G), and vaccination dose status (D and H), respectively.