The number of patients admitted to Bærum Hospital with COVID-19 followed the same trend as in society as a whole, in three waves. In all three waves, but in the first in particular, Bærum Hospital had a high number of admissions in relation to the number of inhabitants and the number of hospitalised patients in Norway as a whole
(2). According to the report from the Norwegian Pandemic Registry, mortality among patients admitted to Norwegian hospitals amounted to 7.7 %, 7.2 % and 4.8 % in the three waves respectively, while the average age was 59.8, 58.8 and 53.7 years (8). We believe that the significantly higher mortality in Bærum Hospital during the first wave can largely be attributed to the higher average age. Most of the patients who died in our hospital during the first wave were vulnerable or frail, and the majority were considered too frail to tolerate ventilation treatment (4). Local infection outbreaks in nursing homes were among the reasons why so many elderly, frail patients were admitted to Bærum Hospital during the first wave. The outcomes from Bærum Hospital during the second and third waves are more likely to be representative of patient cohorts in Norwegian hospitals, since both average age and mortality were more comparable to the national average and published data from hospital cohorts in Norway in the first wave (5, 6, 8). Among 70 COVID-19 patients included in a study from Østfold Hospital, mortality amounted to 10 % (7 out of 70), and in Oslo University Hospital 13 deaths occurred among 169 (7 %) patients included in a study conducted during the first wave of the outbreak (6).
The prevalence of risk factors such as hypertension and diabetes mellitus was relatively constant during the three waves, which is consistent with findings made by the Norwegian Pandemic Registry
(8). Body mass index appears to have been higher during the second and third waves. This is also consistent with data from the Norwegian Pandemic Registry, where the average body mass index was 27.7 and 29.3 kg/m 2 in the first and third waves respectively (8). A number of studies have shown that overweight predisposes patients to a serious course of COVID-19, most likely because of low-grade respiratory failure caused by excessive abdominal volume (15). Furthermore, it is known from other patient groups that overweight is associated with inflammation, which can have a bearing on COVID-19 (16).
The treatment of severe respiratory failure has changed since the start of the pandemic. No consensus has yet been reached on the best treatment method, and practices vary. Treatment with dexamethasone was frequently used for respiratory failure during the second and third waves, on the basis of temporary evidence from the RECOVERY trial, published in July 2021
(17). Since more prolonged treatment with steroids is associated with an increased risk of adverse effects (18), such treatment beyond ten days was assessed on an individual basis for patients with severe respiratory failure. In the intensive care unit, patients were systematically placed in the prone position, and to a lesser extent also patients with less severe respiratory failure. At the start of the pandemic, non-invasive ventilation support and high-flow nasal cannulas were little used for reasons of infection control, but such treatment was somewhat more widely used during the second and third waves. Whether the timing of intubation affects mortality in patients with COVID-19 and severe respiratory failure is a contentious issue (19, 20). We assessed the timing of intubation for each patient individually in light of the degree of respiratory failure, response to non-invasive ventilation support, amount of respiratory distress in the patient and how long the patient had been ill, as well as other factors such as the effect on circulation and lactate level.
Better knowledge and more experience may have helped improve the treatment of respiratory failure and prevent serious complications as the pandemic progressed. At an early stage venous thromboembolism was identified as a frequent complication of COVID-19
(4, 21), and eight out of ten patients were provided with thrombosis prophylaxis in the first wave. In the second and third waves, this proportion exceeded 90 %.
Even though changes in treatment methods may have helped reduce mortality, other factors, such as the spread of different virus variants in society and vaccination status, may also have had an impact. The B.1.1.7 virus variant (the British variant) became dominant during the second wave
(2) and was reported to be associated with higher mortality than the original virus (22). The B.1.617.2 variant (Delta) was detected in Norway in mid-April 2021 and rapidly became dominant in Viken county and the city of Oslo (23). However, the number of hospitalised and deceased patients with a confirmed Delta infection remained low until 15 August 2021 (24). Unfortunately, we had no access to complete data on virus variants for our study, but new variants do not appear to have caused a rise in mortality among the hospitalised patients included in the study. In Norway, vaccination against COVID-19 started on 27 December 2020, and elderly people and those with risk factors were vaccinated first. It is difficult to estimate how the gradually increasing proportion of vaccinated persons in the population during the second and third waves affected the mortality of hospitalised patients. However, since elderly, frail persons are at the highest risk of serious illness and death from COVID-19, there is reason to assume that the lower mortality during the third wave is largely attributable to the fact that this group had been vaccinated, and that the vaccine was effective in preventing serious illness and death.
Especially during the first wave, the lack of clarity on the infection situation in society and rapid increase in the number of patients hospitalised with COVID-19 led to a considerable burden on hospitals as well as staff. In April 2020, 25 days after admission of the first COVID-19 patient to the hospital, a maximum number of 24 patients were hospitalised at the same time. Until May 2020, the response time for COVID-19 tests was up to 24 hours, and there were hence many patients with undetermined infection status. In periods with many admissions, the capacity was increased in both the intensive care and the infection units by measures such as reducing the number of elective surgeries, converting a post-operative unit into an intensive care unit and opening a new ward.
Two out of three patients admitted to Bærum Hospital because of COVID-19 were treated for respiratory failure. Changes in both patient characteristics and treatment methods, e.g. use of dexamethasone and non-invasive ventilation support, may have contributed to the apparently lower mortality during the second and third waves of infection. Factors that are not registered in this study, such as information on vaccination status, may also have had an impact on mortality.