Abstract
Background
The virulence of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) changed during the pandemic. In order to provide a rationale for treatment priorities of respiratory infections and the adaption of in-house infection control strategies, this study evaluated treatment on an intensive care unit (ICU), requirement for mechanical ventilation (MV), requirement for extracorporeal membrane oxygenation (ECMO) and death for inpatients infected with the influenza virus or SARS-CoV-2 during the wild-type, Alpha, Delta, Omicron BA.1/2 and Omicron BA.5 waves of the pandemic.
Design
Single-centre retrospective case–control study.
Setting
Tertiary hospital in Germany.
Participants
One thousand three hundred and sixteen adult inpatients infected with SARS-CoV-2 and 218 adult inpatients infected with influenza virus.
Methods
Demographic data, outcome parameters and underlying comorbidities of patients were obtained from the hospital information system. Multi-variate regression analysis was performed for the assessment of significant associations between risk factors and outcome variables.
Results
Compared with inpatients infected with influenza virus, patients infected with SARS-CoV-2 showed significantly higher rates for in-hospital mortality, admission to ICU and requirement for MV in the wild-type, Alpha and Delta waves, and a significantly higher rate for requirement for ECMO in the wild-type wave. In the Omicron BA.1/BA.2 and Omicron BA.5 waves, patients infected with SARS-CoV-2 did not show significantly higher risk of in-hospital mortality, admission to ICU, or requirement for MV or ECMO compared with patients infected with influenza virus. The length of hospital stay of patients infected with SARS-CoV-2 decreased from 10.8 to 6.2 days, which was less than that of patients infected with influenza virus (8.3 days).
Conclusions
Treatment capacities should be shared equally between SARS-CoV-2 and influenza virus infections. Similar levels of infection control could be applied, at least regarding the severity of infection.
Keywords: COVID-19, SARS-CoV-2, Variants, Alpha, Delta, Omicron, Influenza virus, Mortality, ICU, Ventilation, ECMO
Background
Influenza and coronavirus disease 2019 (COVID-19) are both contagious acute respiratory infections. Influenza is caused by influenza A and B viruses. Annual seasonal influenza epidemics result in three to five million cases of severe illness and up to half a million deaths per year. Symptoms of influenza infection may include abrupt onset of respiratory symptoms, myalgia and fever. In most cases, patients recover within 1 week, but some patients can experience severe complications such as bacterial pneumonia and acute respiratory distress syndrome. Different influenza vaccines are available for prophylaxis, and antiviral drugs targeting neuraminidase, a viral surface glycoprotein, can be used for treatment [[1], [2], [3]].
Common symptoms of COVID-19, caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), include fever, cough, chest discomfort, olfactory and gustatory disturbances, and, in severe cases, dyspnoea and bilateral lung infiltration. Vaccination against SARS-CoV-2 can provide partial protection against infection, whereby the protective effect depends on the vaccine, the number of vaccination doses, the time of the last vaccination dose, and the variant of SARS-CoV-2. Various intravenous and oral antiviral drugs are available for treatment of COVID-19. Furthermore, convalescent plasma may be an option for treatment, especially in patients with immunosuppression [[4], [5], [6], [7], [8], [9]].
According to the World Health Organization, more than 6.6 million people died of COVID-19 worldwide between 2020 and 2022, including more than 163,000 in Germany [10]. Retrospectively, the pandemic can be classified into phases dominated by specific SARS-CoV-2 variants. In Germany, the wild-type variant of SARS-CoV-2 was dominant until calendar week (CW) 08/2021. This was followed by the Alpha variant (from CW 09/2021 to CW 23/2021) and the Delta variant (from CW 31/21 to CW 51/2021) in the third and fourth waves. Subsequently, infections in Germany were mainly caused by various Omicron variants (fifth wave: BA.1/BA.2 dominated from CW 52/2021 to CW 21/2022; sixth wave: BA.5 dominated from CW 22/2022 onwards) [11].
After very low incidence of influenza in Germany in 2020 and 2021, a ‘twindemic’ situation was present in late 2022, and a large number of inpatients infected with influenza or COVID-19 had to be treated simultaneously in hospitals. As the simultaneous presence of both viruses is anticipated in the coming years, a head-to-head comparison of the outcome parameters of inpatients with COVID-19 and influenza was performed in order to better prioritize treatment capacities and the level of required infection control precautions for both diseases.
Methods
Setting
University Hospital Magdeburg is a tertiary hospital in central Germany. It has approximately 4,700 staff members and treats approximately 50,000 inpatients annually. In 2020 and 2021, SARS-CoV-2-positive patients were treated in dedicated COVID-19 isolation wards [normal units under the management of the Infectious Diseases Department, and intensive care units (ICUs) led by the Anaesthesiology Department]. In 2022, there was an increasing change in strategy towards treating SARS-CoV-2-positive patients in single bedrooms (with anteroom if possible) on general wards according to the underlying admission diagnosis. SARS-CoV-2 polymerase chain reaction (PCR) admission screening of all inpatients was established in May 2020 and is still in place.
Data collection
Data processing was based on pseudonymized patient data obtained retrospectively, and did not include any experiments involving human participants (including the use of tissue samples). The requirement for informed consent was waived by the Ethics Committee of University Hospital Magdeburg. Lists of all inpatients positive for SARS-COV-2 and influenza virus were available in the section infection control from routine hospital surveillance. Demographic data (age, gender), outcome parameters [ICU admission, requirement for mechanical ventilation (MV), requirement for extracorporeal membrane oxygenation (ECMO), death], underlying comorbidities, main diagnosis, and dates of admission and discharge were obtained from the Hospital Information System for each case. The assignment of a case to the corresponding SARS-CoV-2 wave was based on the admission date, according to Tolksdorf et al. [11].
Case-control study, and inclusion and exclusion criteria
Adult inpatients between January 2018 and December 2022 who had a positive SARS-CoV-2 or influenza PCR result before or within 5 days of admission were included in this study. Patients were excluded if they had been treated for >24 h at another acute hospital before admission to the study hospital, as these patients were a pre-selected group and may have been at increased risk of a severe course of disease. Characteristics of patients positive for SARS-CoV-2 and the influenza virus are shown in Table I .
Table I.
Characteristics and outcomes of all inpatients with severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) or influenza virus infection (all)/subgroup of patients with acute respiratory main diagnosis (RMD)
| SARS-CoV-2 waves |
Influenza virus | |||||
|---|---|---|---|---|---|---|
| Pre-Omicron |
Omicron |
|||||
| Wild-type | Alpha | Delta | BA.1/BA.2 | BA.5 | ||
| In-hospital stay | ||||||
| Number of cases (all/RMD) | 176/128 | 128/103 | 133/101 | 320/95 | 370/91 | 182/86 |
| Acute RMD (%) | 73% | 80% | 76% | 30% | 25% | 47% |
| Mean length of stay (days) (all/RMD) | 10.8/11.2 | 10.4/11.3 | 10.2/11.6 | 7.2/9.2 | 6.2/7.2 | 8.3/7.7 |
| Demographic data | ||||||
| Mean age (years) (all/RMD) | 67.5/67.6 | 62.3/64.7 | 63.4/67.0 | 60.7/68.6 | 64.4/76.0 | 66.0/69.4 |
| Male gender (%) (all/RMD) | 59.7/66.4 | 46.9/52.4 | 51.9/55.4 | 43.1/58.9 | 44.9/57.1 | 51.1/57.0 |
| Comorbidities | ||||||
| Diabetes mellitus (%) (all/RMD) | 31.8/28.1 | 27.3/29.1 | 23.3/25.7 | 23.8/27.4 | 22.2/33.0 | 28.0/36.0 |
| Arterial hypertension (%) (all/RMD) | 74.4/77.3 | 65.6/70.9 | 63.9/70.3 | 49.1/58.9 | 51.6/79.1 | 56.6/65.1 |
| Cancer (%) (all/RMD) | 4.5/3.1 | 0.8/1.0 | 3.8/3.0 | 6.6/1.1 | 5.4/3.3 | 3.3/2.3 |
| Chronic renal failure (%) (all/RMD) | 28.4/29.7 | 25.0/25.2 | 24.1/29.7 | 22.2/34.7 | 18.9/28.6 | 23.6/29.1 |
| Haematological disease (%) (all/RMD) | 1.7/2.3 | 3.1/3.9 | 4.5/5.9 | 2.2/4.2 | 4.3/4.4 | 4.4/5.8 |
| Neurological disorder (%) (all/RMD) | 24.4/23.4 | 17.2/16.5 | 15.8/14.9 | 22.5/23.2 | 18.6/28.6 | 19.2/17.4 |
| Respiratory disorder (%) (all/RMD) | 10.8/11.7 | 16.4/18.4 | 15.0/14.9 | 13.4/20.0 | 10.8/16.5 | 22.0/29.1 |
| Chronic heart disease (%) (all/RMD) | 32.4/33.6 | 20.3/24.3 | 21.8/20.8 | 18.8/25.3 | 22.4/37.4 | 34.1/32.6 |
| Outcomes | ||||||
| Death (%) (all/RMD) | 19.3/22.7 | 17.2/16.5 | 15.0/17.8 | 9.1/12.6 | 7.0/12.1 | 8.8/10.5 |
| ICU admission (%) (all/RMD) | 27.3/33.6 | 18.0/21.4 | 17.3/20.8 | 8.1/7.4 | 3.2/3.3 | 7.1/5.8 |
| Requirement for MV (%) (all/RMD) | 18.2/25.0 | 7.8/9.7 | 7.5/9.9 | 3.1/2.1 | 1.6/2.2 | 2.7/2.3 |
| Requirement for ECMO (%) (all/RMD) | 4.0/5.5 | 0.8/1.0 | 1.5/2.0 | 0.0/0.0 | 0.0/0.0 | 0.0/0.0 |
ICU, intensive care unit; MV, mechanical ventilation; ECMO, extracorporeal membrane oxygenation.
In individual cases, it was difficult to decide whether a patient died or required ICU treatment, MV or ECMO due to the course of the infection or because of an underlying disease. In the latter case, the infection was considered as a secondary diagnosis. For this reason, multi-variate regression analyses were performed for inpatients with a positive influenza virus or SARS-CoV-2 test, and for the subgroup of patients with acute respiratory tract infection as the main diagnosis (including International Classification of Diseases Version 10 codes J04, J06, J10, J11, J12, J15, J18, J22, J80, J96 and J98). Patients in this subgroup are henceforth referred to as patients with COVID-19- or influenza-related respiratory main diagnosis (RMD).
Statistics
Data analyses were performed using Excel 2016 (Microsoft Corp., Redmond, WA, USA). Multi-variate regression analysis was used for the assessment of significant associations between risk factors (predominant SARS-CoV-2 variant/influenza, age, gender, diabetes mellitus, arterial hypertension, cancer, renal failure, haematological malignancies, neurological disorders, chronic respiratory diseases, chronic heart disease) and outcome variable (ICU admission, requirement for MV, requirement for ECMO, death). P<0.05 was considered to indicate significance. Bonferroni's correction was not used because of the explorative character of the study and because death, ICU admission, requirement for MV and requirement for ECMO are not strictly independent (e.g. inpatients who require MV or ECMO are always in ICUs).
Results
Characteristics of patients with SARS-CoV-2 and influenza infection are shown in Table I. Seventy-four percent of all inpatients infected with influenza had an influenza-related RMD within the observation period. While at the beginning of the pandemic, most SARS-CoV-2-positive patients had COVID-19-related RMD (wild-type wave, 73%; Alpha wave, 80%; Delta wave, 76%), in 2022, SARS-CoV-2-positive patients were more often admitted with other main diagnoses, and COVID-19-related RMD decreased to 30% in the Omicron BA.1/2 wave and 25% in the Omicron BA.5 wave. During the pandemic, the mean length of hospital stay of patients with COVID-19-related RMD decreased from 11.2 days in the wild-type wave to 7.2 days in the Omicron BA.5 wave, which was less than the mean length of hospital stay of patients with influenza-related RMD (7.7 days). The mean age of inpatients with COVID-19-related RMD decreased from 67.6 years during the wild-type wave to 64.7 years during the Alpha wave, and subsequently increased steadily to 76.0 years in the Omicron BA.5 wave. The mean age of inpatients hospitalized with influenza-related RMD was 69.4 years over the period from 2018 to 2022.
In-house mortality of patients with COVID-19-related RMD decreased from 22.7% at the beginning of the pandemic to 12.1% in the Omicron BA.5 wave, which was close to the mortality of patients with influenza-related RMD (10.5%), especially when considering the higher age of the patients with COVID-19 in the last Omicron wave. The ICU admission rate decreased from 33.6% to 3.3%, and the MV rate decreased from 25.0% to 2.2% in inpatients with COVID-19-related RMD, which was lower than the ICU admission rate (5.8%) and the MV rate (2.3%) of inpatients with influenza-related RMD (Table I).
The results of the multi-variate regression analysis of risk factors for death, ICU admission, requirement for MV and requirement for ECMO are shown in Table II and Figure 1 . In summary, during the SARS-CoV-2 wild-type wave, rates of mortality (P<0.005), ICU admission (P<0.005), requirement for MV (P<0.005) and requirement for ECMO (P<0.005) were significantly higher in all patients with COVID-19 compared with patients with influenza. In the Alpha and Delta waves, significantly higher rates of mortality (Alpha wave, P<0.005; Delta wave, P<0.05), ICU admission (both P<0.005) and requirement for MV (both P<0.05) were found in patients with COVID-19 compared with patients with influenza, but no significant differences were found in requirement for ECMO. In the Omicron BA.1/2 and BA.5 waves, no significant differences in mortality, ICU admission, requirement for MV or requirement for ECMO were found between patients with COVID-19 and patients with influenza.
Table II.
Multi-variate regression analysis of risk factors for death, intensive care unit (ICU) admission, requirement for mechanical ventilation (MV) and requirement for extracorporeal membrane oxygenation (ECMO) for either all patients with coronavirus disease 2019 (COVID-19) or influenza and the subgroup of patients with acute respiratory main diagnosis (RMD) alone
| Factor | Outcome | RMD alone |
All patients with COVID-19 or influenza |
||||
|---|---|---|---|---|---|---|---|
| Coefficient | Standard error | P-value | Coefficient | Standard error | P-value | ||
| SARS-CoV-2 wild-type wave compared with influenza | Death | 0.143 | 0.050 | 0.004 | 0.112 | 0.032 | 0.001 |
| ICU | 0.269 | 0.050 | 0.000 | 0.193 | 0.032 | 0.000 | |
| MV | 0.225 | 0.040 | 0.000 | 0.151 | 0.024 | 0.000 | |
| ECMO | 0.053 | 0.018 | 0.004 | 0.040 | 0.009 | 0.000 | |
| SARS-CoV-2 Alpha wave compared with influenza | Death | 0.101 | 0.052 | 0.053 | 0.116 | 0.035 | 0.001 |
| ICU | 0.148 | 0.052 | 0.005 | 0.109 | 0.035 | 0.002 | |
| MV | 0.081 | 0.042 | 0.051 | 0.055 | 0.026 | 0.033 | |
| ECMO | 0.009 | 0.019 | 0.629 | 0.008 | 0.010 | 0.418 | |
| SARS-CoV-2 Delta wave compared with influenza | Death | 0.092 | 0.052 | 0.078 | 0.085 | 0.035 | 0.014 |
| ICU | 0.150 | 0.053 | 0.004 | 0.103 | 0.035 | 0.003 | |
| MV | 0.085 | 0.042 | 0.042 | 0.051 | 0.026 | 0.046 | |
| ECMO | 0.020 | 0.019 | 0.284 | 0.015 | 0.010 | 0.134 | |
| SARS-CoV-2 Omicron BA.1 and BA.2 wave compared with influenza | Death | 0.028 | 0.053 | 0.599 | 0.021 | 0.028 | 0.449 |
| ICU | 0.026 | 0.053 | 0.630 | 0.014 | 0.028 | 0.616 | |
| MV | 0.005 | 0.042 | 0.900 | 0.008 | 0.021 | 0.713 | |
| ECMO | 0.002 | 0.019 | 0.909 | 0.000 | 0.008 | 0.972 | |
| SARS-CoV-2 Omicron BA.5 wave compared with influenza | Death | -0.021 | 0.054 | 0.697 | -0.012 | 0.028 | 0.662 |
| ICU | -0.014 | 0.054 | 0.793 | -0.031 | 0.028 | 0.268 | |
| MV | -0.005 | 0.043 | 0.902 | -0.005 | 0.020 | 0.821 | |
| ECMO | 0.002 | 0.019 | 0.918 | 0.001 | 0.008 | 0.934 | |
| Age (life years) | Death | 0.007 | 0.001 | 0.000 | 0.004 | 0.001 | 0.000 |
| ICU | -0.003 | 0.001 | 0.003 | -0.001 | 0.001 | 0.166 | |
| MV | -0.001 | 0.001 | 0.525 | 0.000 | 0.000 | 0.893 | |
| ECMO | 0.000 | 0.000 | 0.258 | 0.000 | 0.000 | 0.295 | |
| Male gender | Death | 0.001 | 0.030 | 0.972 | -0.009 | 0.017 | 0.611 |
| ICU | 0.035 | 0.030 | 0.239 | 0.036 | 0.017 | 0.037 | |
| MV | 0.021 | 0.024 | 0.377 | 0.024 | 0.013 | 0.052 | |
| ECMO | 0.011 | 0.011 | 0.318 | 0.007 | 0.005 | 0.133 | |
| Diabetes mellitus | Death | -0.012 | 0.034 | 0.730 | 0.015 | 0.021 | 0.466 |
| ICU | 0.000 | 0.034 | 0.999 | -0.021 | 0.021 | 0.313 | |
| MV | 0.012 | 0.027 | 0.648 | -0.013 | 0.015 | 0.384 | |
| ECMO | -0.002 | 0.012 | 0.886 | -0.003 | 0.006 | 0.628 | |
| Arterial hypertonia | Death | -0.073 | 0.036 | 0.047 | -0.082 | 0.022 | 0.000 |
| ICU | 0.068 | 0.037 | 0.063 | 0.037 | 0.022 | 0.089 | |
| MV | 0.023 | 0.029 | 0.433 | 0.009 | 0.016 | 0.589 | |
| ECMO | 0.005 | 0.013 | 0.706 | 0.004 | 0.006 | 0.526 | |
| Cancer | Death | 0.191 | 0.096 | 0.047 | 0.116 | 0.040 | 0.004 |
| ICU | 0.188 | 0.096 | 0.051 | -0.023 | 0.040 | 0.572 | |
| MV | 0.176 | 0.077 | 0.022 | 0.026 | 0.029 | 0.371 | |
| ECMO | 0.052 | 0.035 | 0.136 | 0.011 | 0.011 | 0.354 | |
| Renal failure | Death | -0.011 | 0.036 | 0.758 | 0.022 | 0.023 | 0.338 |
| ICU | -0.050 | 0.036 | 0.164 | 0.009 | 0.023 | 0.681 | |
| MV | -0.022 | 0.028 | 0.446 | 0.022 | 0.017 | 0.182 | |
| ECMO | -0.016 | 0.013 | 0.226 | -0.007 | 0.006 | 0.265 | |
| Haematological malignancies | Death | 0.118 | 0.071 | 0.096 | 0.073 | 0.047 | 0.119 |
| ICU | 0.040 | 0.072 | 0.580 | 0.021 | 0.047 | 0.650 | |
| MV | 0.003 | 0.057 | 0.958 | -0.001 | 0.034 | 0.978 | |
| ECMO | -0.014 | 0.026 | 0.577 | -0.006 | 0.013 | 0.654 | |
| Neurological disease | Death | -0.007 | 0.036 | 0.847 | 0.030 | 0.021 | 0.158 |
| ICU | 0.055 | 0.036 | 0.128 | 0.086 | 0.021 | 0.000 | |
| MV | 0.066 | 0.029 | 0.022 | 0.049 | 0.016 | 0.002 | |
| ECMO | -0.001 | 0.013 | 0.937 | -0.001 | 0.006 | 0.925 | |
| Chronic respiratory disease | Death | -0.011 | 0.038 | 0.773 | -0.002 | 0.025 | 0.924 |
| ICU | 0.064 | 0.038 | 0.094 | 0.033 | 0.025 | 0.179 | |
| MV | 0.046 | 0.030 | 0.133 | 0.032 | 0.018 | 0.073 | |
| ECMO | 0.008 | 0.014 | 0.569 | 0.006 | 0.007 | 0.385 | |
| Chronic heart disease | Death | 0.027 | 0.035 | 0.450 | 0.037 | 0.022 | 0.091 |
| ICU | 0.006 | 0.035 | 0.855 | 0.014 | 0.022 | 0.527 | |
| MV | 0.026 | 0.028 | 0.352 | 0.010 | 0.016 | 0.536 | |
| ECMO | 0.007 | 0.013 | 0.600 | 0.003 | 0.006 | 0.683 | |
SARS-CoV-2, severe acute respiratory syndrome coronavirus-2.
Bold values denote statistical significance at the P < 0.05 level.
Figure 1.
Multi-variate linear regression analysis of risk factors for death, intensive care unit (ICU) admission, requirement for mechanical ventilation and requirement for extracorporeal membrane oxygenation (ECMO) in all patients infected with severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) and influenza virus (solid bars), and the subgroup of patients infected with SARS-CoV-2 and influenza virus with acute respiratory main diagnosis (open bars). Error bars indicate standard error from the mean. ∗P<0.05. ∗∗P<0.005.
Considering only the subgroups of patients with COVID-19-/influenza-related RMD, significantly higher rates of mortality, ICU admission, requirement for MV and requirement for ECMO (all P<0.005) were found in the patients with COVID-19 compared with the patients with influenza in the wild-type wave (Figure 1). Later in the pandemic, there was still a significantly higher rate of ICU admission (P<0.005) in the Alpha wave, and significantly higher rates of ICU admission (P<0.005) and requirement for MV (P<0.05) in the Delta wave in patients with COVID-19 compared with patients with influenza (Figure 1, Table I).
In addition, the outcome parameters were evaluated in different age groups (Figure 2 ). In the Omicron BA.5 wave, patients with COVID-19-related RMD in all three age groups (18–57 years, 58–77 years, 78–97 years) showed equal or lower rates of ICU admission, requirement for MV and requirement for ECMO compared with patients with influenza-related RMD. Considering mortality in the Omicron BA.5 wave, patients with COVID-19-related RMD showed higher mortality in the age group 58–77 years and lower mortality in the age group 78–97 years compared with patients with influenza-related RMD.
Figure 2.
Relative disease severity of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) variants compared with influenza in different age groups. (a) Intensive care unit (ICU) admission rate in SARS-CoV-2-positive patients with respiratory main diagnosis (RMD) compared with all influenza-virus-positive patients with RMD (as baseline) in three different age groups. (b) Requirement for mechanical ventilation in SARS-CoV-2-positive patients with RMD compared with all influenza-virus-positive patients with RMD (as baseline) in three different age groups. (c) Requirement for extracorporeal membrane oxygenation in SARS-CoV-2-positive patients with RMD compared with all influenza-virus-positive patients with RMD (as baseline) in three different age groups. (d) In-house mortality in SARS-CoV-2-positive patients with RMD compared with all influenza-virus-positive patients with RMD (as baseline) in three different age groups. White bars, 18–57 years; grey bars, 58–77 years; black bars, 78–97 years.
Discussion
During the wild-type, Alpha and Delta waves, the mortality and ICU admission rates were significantly higher in patients with COVID-19 compared with patients with influenza, while no significant differences in these outcome parameters were found during the Omicron BA.1/2 and Omicron BA.5 waves.
Significantly higher mortality rates of hospitalized patients with COVID-19 compared with hospitalized patients with influenza during the SARS-CoV-2 wild-type wave of the pandemic have been reported previously [[12], [13], [14], [15]]. In these previous studies from the USA, France, Germany and Belgium, in-hospital mortality rates for patients with wild-type SARS-CoV-2 infection ranged from 14.0% to 20.0%, while in-hospital mortality rates for patients with influenza virus infection ranged from 5.0% to 9.8%. In the present study, the mortality rates for patients with SARS-CoV-2 infection and influenza virus infection were 19.3% and 8.8%, respectively, and thus were in the previously reported range.
During the wild-type wave, the ICU admission rates in these studies ranged from 15.0% to 36.8% for patients with COVID-19 and from 10.8% to 24% for patients with influenza. MV rates ranged from 9.7% to 15.0% for patients with COVID-19 and from 4.0% to 9.0% for patients with influenza. While the rates for ICU admission (27.3%) and requirement for MV (18.2%) for the patients with COVID-19 in this study are in the previously reported range, the rates for the patients with influenza in this study were slightly lower (ICU admission rate, 7.1%; requirement for MV rate, 2.7%). This could be explained, at least in part, by the fact that the average age of patients with influenza who died without being transferred to an ICU was 83 years. Many of these patients were multi-morbid and had explicitly expressed their opposition to intensive care measures.
The mortality rate of patients with COVID-19 in this study decreased from 15.0% in the Delta wave to 7.0% in the Omicron BA.5 wave. This reduction of 53.3% is in line with the observations of other studies [[16], [17], [18], [19]], which described a reduction in mortality rate between 44.4% and 61.7% between the Delta and Omicron waves.
Compared with the Delta wave, the ICU admission rate of patients with COVID-19 decreased by 40% and 58.5% in the Omicron BA.1/2 and Omicron BA.5 waves, respectively. Bouzid et al. [17] described a 75% reduction in the ICU admission rate between the Delta and early Omicron waves in hospitals in France. Adjei et al. [18] reported a reduction of 14.4% between the Delta and early Omicron waves, and of 46.8% between the Delta and late Omicron waves in the USA.
Compared with the Delta wave, this study found a 58.7% and 78.7% reduction in the rate of requirement for MV for patients with COVID-19 in the Omicron BA.1/2 and Omicron BA.5 waves, respectively. Bouzid et al. [17] described a 52% reduction in the ICU admission rate between the Delta and early Omicron waves in France. Adjei et al. [18] reported a reduction of 22.4% between the Delta and early Omicron waves, and of 65% between the Delta and late Omicron waves in the USA.
Interestingly, in comparison with patients with influenza-virus-related RMD, patients with COVID-19-related RMD showed higher mortality in the age group 58–77 years and lower mortality in the age group 78–97 years in the Omicron BA.5 wave. However, due to the small sample size (five patients died in this age group during the Omicron BA.5 wave) and the severe pre-existing comorbidities of some of these patients, this might be a statistical outlier. Further studies are needed to corroborate this observation.
This study has some limitations. First, it is a retrospective study. While the patient demographics and outcome variables are fairly reliable, there is some uncertainty in the complete coverage of the listed comorbidities. Furthermore, in the first SARS-CoV-2 wave, a greater proportion of oligosymptomatic patients were probably hospitalized over a longer period, as there was no possibility to isolate these patients in care facilities, such as nursing homes, at the beginning of the pandemic. While PCR admission screening of all inpatients has been in place for SARS-CoV-2 since May 2020, testing for influenza was only performed on symptomatic patients during the observation period. As a result, asymptomatic inpatients with influenza are often not detected, which can lead to an overestimation of disease severity in patients with influenza. In order to consider this, a corresponding subgroup analysis was undertaken for inpatients with RMD, which corroborated the general analysis of all virus-positive inpatients. In addition, due to the retrospective nature of the study, the impact of vaccination status on patient outcome could not be examined more closely, as vaccination status was only recorded sporadically in the patient files.
These data show that, regarding the severity of infection, it would be justified to apply similar levels of infection control for the prevention of the spread of COVID-19 and influenza infections in hospitals. However, for the implementation of infection control measures in hospitals, transmissibility is important as well as disease severity. Thus, a disease that has a lower mortality rate but significantly higher transmissibility can ultimately lead to more nosocomial deaths. For this reason, these data on the relative severity of community acquired COVID-19 and influenza infections should not be used as a basis for uncritically dispensing SARS-CoV-2 infection control measures in hospitals. Due to the longer incubation period and the higher transmission rate, we believe that stricter hygiene measures should be implemented in outbreak events, such as mandatory masks for all staff during the outbreak, individual breaks, etc.
In conclusion, in comparison with inpatients with influenza, inpatients infected with SARS-CoV-2 BA.1/2 and BA.5 do not show significantly higher risk of in-hospital mortality, ICU admission, requirement for MV or requirement for ECMO. Against this background, healthcare system resources should be shared equally between the two diseases.
Conflict of interest statement
None declared.
Funding sources
None.
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