Covid-19 and post-acute sick leave: a hybrid register and questionnaire study in the adult Danish population (preprint)

Long covid follows 10-20% of first-time SARS-CoV-2 infections, but the societal burden of long covid and risk factors for the condition are not well-understood. Here, we report findings about self-reported sick leave and risk factors thereof from a hybrid survey and register study, which included 37,482 RT-PCR confirmed SARS-CoV-2 cases and 51,336 test-negative controls who were tested during the index and alpha waves. An additional 33 individuals per 1000 took substantial sick leave following acute infection compared to persons with no known history of infection, where substantial sick leave was defined as >1 month of sick leave within the period 1-9 months after the RT-PCR test date. Being female, [≥]50 years, and having certain pre-existing conditions such as fibromyalgia increased risks for taking substantial sick leave. Further research exploring this heterogeneity is urgently needed and may provide important evidence for more targeted preventative strategies.

Patterns of testing in the extensive Danish national SARS-CoV-2 test set-up (preprint)

Background: The Danish national SARS-CoV-2 mass test system was among the most ambitious worldwide. We describe its set-up and analyse differences in patterns of testing per demography and time period in relation to the three waves of SARS-CoV-2 transmission in Denmark. Methods: We included all reported PCR- and rapid antigen-tests performed between 27 February 2020 and 10 March 2022 among all residents aged 2 years or above. Descriptive statistics and Poisson regression models were used to analyse characteristics of individuals tested for SARS-CoV-2 using a national cohort study design. Results: A total of 63.7 million PCR-tests and 60.0 million antigen-tests were performed in the study period, testing 91.1% and 79.2% of the Danish population at least once by PCR or antigen, respectively. Female sex, younger age, Danish heritage and living in the capital area were all factors positively associated with the frequency of PCR-testing. The association between COVID-19 vaccination and PCR-testing changed from negative to positive over time. Conclusion: We provide details of the widely available, free-of-charge, national SARS-CoV-2 test system, which served to identify infected individuals, assist isolation of infectious individuals and contact tracing, and thereby mitigating the spread of SARS-CoV-2 in the Danish population. The test system was utilized by nearly the entire population at least once, and widely accepted across different demographic groups. However, demographic differences in the test uptake did exist and should be considered in order not to cause biases in studies related to SARS-CoV-2, e.g., studies of transmission and vaccine effectiveness.

Post-acute symptoms four months after SARS-CoV-2 infection during the Omicron period: a nationwide Danish questionnaire study (preprint)

ObjectivesTo evaluate the effects of the Omicron variant on the post-acute symptoms, four months after infection with SARS-CoV-2. DesignA nationwide questionnaire study. SettingDenmark. Participants44,004 individuals aged 15 years or older with either a SARS-CoV-2 RT-PCR positive test result from the period of Delta dominance (July to November 2021), or a positive or negative RT-PCR test result from the period of Omicron dominance (December 2021 to January 2022). MethodsA questionnaire based cohort study with outcomes on post-acute physical, fatigue, cognitive, mental health symptoms, and new-onset general health problems, four months after testing. Risk differences (RDs) were estimated by comparing cases and controls during the Omicron period, cases during the Delta and Omicron periods, and vaccinated cases with two and three doses during the Omicron period, adjusted for age, sex, BMI, self-reported chronic diseases, Charlson comorbidity index, healthcare occupation and vaccination status. ResultsFour months after testing for SARS-CoV-2 during the Omicron period, cases experienced higher risk of 18 out of 26 post-acute symptoms and five out of five new-onset general health problems, compared to controls. Cases during the Omicron period experienced lower risks of 8 of the 18 symptoms and of all five new-onset general health problems, compared to Delta cases. The most prominent RDs estimated when comparing Omicron cases to controls were: memory issues (RD=5.4%, 95% CI 4.8 to 6.1), post-exertional malaise (RD=5.3%, 95% CI 3.1 to 7.7), fatigue/exhaustion (RD=5.2%, 95% CI 3.7 to 6.9), substantial fatigue (RD=5.0%, 95% CI 2.7 to 7.5), and dyspnea (RD=4.8%, 95% CI 3.8 to 5.9). Compared to cases from the Delta period, Omicron cases reported reduced risks of post-acute altered/reduced sense of smell (dysosmia) (RD=-15.1%, 95% CI -17.0 to -12.9) and -taste (dysgeusia) (RD=-11.6%, 95% CI -13.6 to -9.7). Cases vaccinated with three doses prior to Omicron infection reported reduced risk of 13 of the 26 post-acute symptoms and of three of the five new-onset general health problems, compared to those vaccinated with two doses. ConclusionsA considerable amount of cases infected during the Omicron period experienced post-acute symptoms and new-onset health problems, four months after testing, although milder compared to Delta cases. During the Omicron period, a booster vaccination dose was associated with fewer post-acute symptoms and new-onset health problems, four months after infection, compared to two doses of COVID-19 vaccine.

Societal COVID-19 epidemic counter measures and activities associated with SARS-CoV-2 infection in an adult unvaccinated population - a case-control study in Denmark, June 2021 (preprint)

Measures to restrict physical inter-personal contact in the community have been widely implemented during the COVID-19 pandemic. We studied determinants for infection with SARS-CoV-2 with the aim of testing the efficiency of such measures. We conducted a national matched case-control study among unvaccinated persons aged 18-49 years. Cases were selected among those testing positive for SARS-CoV-2 by RT-PCR over a five-day period in June 2021. Controls were selected from the national population register and were individually matched on age, sex and municipality of residence and had not previously tested positive. Cases and controls were interviewed via telephone about contact with other persons and exposures in the community. We included 500 cases and 529 controls and determined odds ratios (ORs) and 95% confidence intervals (95%CIs) by conditional logistical regression with adjustment for household size and immigration status. We found having had contact with another individual with a known infection as the main determinant for SARS-CoV-2 infection. Reporting close contact with an infected person who either had or did not have symptoms resulted in ORs of 20 (95%CI:9.8-39) and 8.5 (95%CI 4.5-16) respectively. In contrast, community exposures were generally not associated with disease; several exposures were negatively associated. Exceptions were: attending fitness centers, OR=1.4 (95%CI:1.0-2.0) and consumption of alcohol in restaurants or cafes, OR=2.3 (95%CI:1.3-4.2). For reference, we provide a timeline of non-pharmaceutical interventions in place in Denmark from February 2020 to March 2022. Fitness centers and alcohol consumption were mildly associated with infection, in agreement with findings of our similar study conducted six month earlier (Epidemiology & Infection 2021;150:e9.). Transmission of disease through involvement in community activities appeared to occur only rarely, suggesting that community restrictions in place were efficient. Instead, transmission appeared to primarily take place in a confined space via contact to known persons.

Vaccine effectiveness against SARS-CoV-2 infection and COVID-19-related hospitalization with the Alpha, Delta and Omicron SARS-CoV-2 variants: a nationwide Danish cohort study (preprint)

Background The continued occurrence of more contagious SARS-CoV-2 variants and waning immunity over time require ongoing re-evaluation of the vaccine effectiveness (VE). This study aimed to estimate the effectiveness in two age groups (12-59 and 60 years or above) of two and three vaccine doses (BNT162b2 mRNA or mRNA-1273 vaccine) by time since vaccination against SARS-CoV-2 infection and COVID-19-related hospitalization in an Alpha, Delta and Omicron dominated period. Methods A Danish nationwide cohort study design was used to estimate VE against SARS-CoV-2 infection and COVID-19-related hospitalization with the Alpha, Delta and Omicron variants. Information was obtained from nationwide registries and linked using a unique personal identification number. The study included all residents in Denmark aged 12 years or above (18 years or above for the analysis of three doses) in the Alpha (February 20 to June 15, 2021), Delta (July 4 to November 20, 2021) and Omicron (December 21, 2021 to January 31, 2022) dominated periods. VE estimates including 95% confidence intervals (CIs) were calculated using Cox proportional hazard regression models with adjustments for age, sex and geographical region. Vaccination status was included as a time-varying exposure. Findings In the oldest age group, VE against infection after two doses was 91.0% (95% CI: 88.5; 92.9) for the Alpha variant, 82.2% (95% CI: 75.3; 87.1) for the Delta variant and 39.9% (95% CI: 26.4; 50.9) for the Omicron variant 14-30 days since vaccination. The VE waned over time and was 71.5% (95% CI: 54.7; 82.8), 49.8% (95% CI: 46.5; 52.8) and 4.7% (95% CI: 0.2; 8.9) >120 days since vaccination against the three variants, respectively. Higher estimates were observed after the third dose with VE estimates against infection of 86.0% (Delta, 95% CI: 83.3; 88.3) and 57.6% (Omicron, 95% CI: 55.8; 59.4) 14-30 days since vaccination. Among both age groups, VE against COVID-19-related hospitalization 14-30 days since vaccination with two or three doses was 94.8% or above for the Alpha and Delta variants, whereas among the youngest age group, VE estimates against the Omicron variant after two and three doses were 62.4% (95% CI: 46.3; 73.6) and 89.8% (95% CI: 87.9; 91.3), respectively. Conclusions Two vaccine doses provided high protection against SARS-CoV-2 infection and COVID-19-related hospitalization with the Alpha and Delta variants with protection waning over time. Two vaccine doses provided only limited protection against SARS-CoV-2 infection and COVID-19-related hospitalization with the Omicron variant. The third vaccine dose substantially increased the protection against Delta and Omicron.

Post-acute symptoms, new onset diagnoses and health problems 6 to 12 months after SARS-CoV-2 infection: a nationwide questionnaire study in the adult Danish population (preprint)

Background. A considerable number of individuals infected with SARS-CoV-2 continue to experience symptoms after the acute phase. More information on duration and prevalence of these symptoms in non-hospitalized populations is needed. Methods. We conducted a nationwide cross-sectional study including 152 880 individuals aged 15-years or older, consisting of RT-PCR confirmed SARS-CoV-2 cases between September 2020-April 2021 (N=61 002) and a corresponding test-negative control group (N=91 878). Data were collected 6, 9 or 12 months after the test using web-based questionnaires. The questionnaire covered acute and post-acute symptoms, selected diagnoses, sick leave and general health, together with demographics and life style at baseline. Risk differences (RDs) between test-positives and -negatives were reported, adjusted for age, sex, single comorbidities, Charlson comorbidity score, obesity and healthcare-occupation. Findings. Six to twelve months after the test date, the risks of 18 out of 21 physical symptoms were elevated among test-positives and one third (29.6%) of the test-positives experienced at least one physical post-acute symptom. The largest risk differences were observed for dysosmia (RD = 10.92%, 95%CI 10.68-11.21%), dysgeusia (RD=8.68%, 95%CI 8.43-8.93%), fatigue/exhaustion (RD=8.43%, 95%CI 8.14-8.74%), dyspnea (RD=4.87%, 95%CI 4.65-5.09%) and reduced strength in arms/legs (RD=4.68%, 95%CI 4.45-4.89%). More than half (53.1%) of test-positives reported at least one of the following conditions: concentration difficulties (RD=28.34%, 95%CI 27.34-28.78%), memory issues (RD=27.25%, 95%CI 26.80-27.71%), sleep problems (RD=17.27%, 95%CI 16.81-17.73%), mental (RD=32.58%, 95%CI 32.11-33.09%) or physical exhaustion (RD=40.45%, 95%CI 33.99-40.97%), compared to 11.5% of test-negatives. New diagnoses of anxiety (RD=1.15%, 95%CI 0.95-1.34%) or depression (RD=1.00%, 95%CI 0.81-1.19%) were also more common among test-positives. Interpretation. At the population-level, where the majority of test-positives (96.0%) were not hospitalized during acute infection, a considerable proportion experience post-acute symptoms and sequelae 6-12 months after infection. Funding. None

Occurrence and significance of Omicron BA.1 infection followed by BA.2 reinfection (preprint)

The newly found Omicron SARS-CoV-2 variant of concern has rapidly spread worldwide. Omicron carries numerous mutations in key regions and is associated with increased transmissibility and immune escape. The variant has recently been divided into four subvariants with substantial genomic differences, in particular between Omicron BA.1 and BA.2. With the surge of Omicron subvariants BA.1 and BA.2, a large number of reinfections from earlier cases has been observed, raising the question of whether BA.2 specifically can escape the natural immunity acquired shortly after a BA.1 infection. To investigate this, we selected a subset of samples from more than 1,8 million cases of infections in the period from November 22, 2021, until February 11, 2022. Here, individuals with two positive samples, more than 20 and less than 60 days apart, were selected. From a total of 187 reinfection cases, we identified 47 instances of BA.2 reinfections shortly after a BA.1 infection, mostly in young unvaccinated individuals with mild disease not resulting in hospitalization or death. In conclusion, we provide evidence that Omicron BA.2 reinfections do occur shortly after BA.1 infections but are rare.

Adverse SARS-CoV-2 Associated Outcomes Among People Experiencing Homelessness, Imprisonment, Supported Psychiatric Housing, Mental Disorders, Substance Abuse or Chronic Medical Disorders: A Population-Based Cohort Study Among 4.4 Million People (preprint)

Background: Marginalised or deprived groups may be at high risk of a serious COVID-19 outcome. We examined adverse outcomes associated with SARS-CoV-2 infection among vulnerable segments of society. Methods: Using health and administrative registers, we performed a population-based cohort study of 4.38 million Danes, 27 February 2020­ to 6 April 2021. Main predictors were experiences of 1) homelessness, 2) imprisonment, 3) supported psychiatric housing, 4) mental disorder, 5) substance abuse, and 6) chronic medical condition. COVID-19-related outcomes were: 1) hospitalisation, 2) intensive care, 3) 30-day mortality, and 4) overall mortality. PCR-confirmed SARS-CoV-2 infection and PCR-testing were also studied. Vulnerable groups were compared with the general population (using adjusted incidence and mortality rate ratios: IRRs, MRRs). Findings: Among individuals with a positive PCR-test experiencing homelessness and supported psychiatric housing, 8∙6% (95% CI 6∙9-10∙4) and 11% (7∙8-14∙7), respectively, were admitted to hospital within two weeks and 1∙8% (1∙1-2∙7) and 2∙9% (1∙4-5∙1), respectively, had died within 30 days. The probability of hospitalisation was higher for all predictors compared with the general population (p=<0∙0001). After adjustments, vulnerable housing situations, i.e. homelessness, imprisonment, and supported psychiatric housing, increased the risk of adverse outcomes 1∙7 to 3∙2-times; highest 30-day MRR after COVID-19 was for homelessness (3∙2, 95% CI 2∙0-5∙1) and supported psychiatric housing: (2∙7, 1∙4-5∙2). Mental disorder, substance abuse, and chronic medical conditions were associated with 1∙1 (mental disorder and intensive care, p=0∙37) to 2∙0-times increased risk of adverse outcomes (30-day MRR for substance abuse: 2∙0, 1∙8-2∙3). Overall mortality during the study period was increased for all predictors and highest for homelessness combined with a PCR-confirmed SARS-CoV-2 infection (MRR 22∙1, 15∙2-32∙2). Interpretation: This study highlights that pandemic preparedness should address inequalities in health, including infection prevention and vaccination of vulnerable groups. Higher awareness of people in vulnerable living situations is needed. Funding Information: This study was funded by a grant from the Novo Nordisk Foundation to MN (grant number NFF20SA0063142).Declaration of Interests: We declare no competing interests.Ethics Approval Statement: Permission was obtained from Statistics Denmark and the Danish Health Data Authority. Ethical permission was not required for this study according to Danish regulations.

Testing Denmark: A Danish nationwide surveillance study of COVID-19 (preprint)

BackgroundNational data on the spread of SARS-CoV-2 infection and knowledge on associated risk factors are important for understanding the course of the pandemic. "Testing Denmark" is a national large-scale epidemiological surveillance study of SARS-CoV-2 in the Danish population. MethodsBetween September and October 2020, approximately 1.3 million of 5.8 million Danish citizens (age > 15 years) were randomly invited to fill in an electronic questionnaire covering COVID-19 exposures and symptoms. The prevalence of SARS-CoV-2 antibodies was determined by Point-of Care rapid Test (POCT) distributed to participants home addresses. FindingsIn total 318,552 participants (24.5% invitees) completed the questionnaire and provided the result of the POCT. Of these, 2,519 (0.79%) were seropositive (median age 55 years) and women were more often seropositive than men, interquartile range (IQR) 42-64, 40.2% males. Of participants with a prior positive Polymerase Chain Reaction (PCR) test (n=1,828), 29.1% were seropositive in the POCT. Seropositivity increased with age irrespective of sex. Elderly participants (>61 years) reported less symptoms and had less frequently been tested for SARS-CoV-2 compared to younger participants. Seropositivity was associated with physical contact with SARS-CoV-2 infected individuals (Risk ratio (RR) 7.43, 95% CI: 6.57-8.41) and in particular household members (RR 17.70, 95% CI: 15.60-20.10). Home care workers had a higher risk of seropositivity (RR 2.09 (95% CI: 1.58-2.78) as compared to office workers. Geographic population density was not associated to seropositivity. A high degree of compliance with national preventive recommendations was reported (e.g., > 80% use of face masks), but no difference was found between seropositive and seronegative participants. InterpretationThis study provides insight into the immunity of the Danish population seven to eight months after the first COVID-19 case in Denmark. The seroprevalence was lower than expected probably due to a low sensitivity of the POCT used or due to challenges relating to the reading of test results. Occupation or exposure in local communities were major routes of infection. As elderly participants were more often seropositive despite fewer symptoms and less PCR tests performed, more emphasis should be placed on testing this age group.

Seroprevalence of SARS-CoV-2 antibodies in social housing areas in Denmark (preprint)

Background COVID-19 is suggested to be more prevalent among ethnic minorities and individuals with low socioeconomic status. We aimed to investigate the prevalence of SARS-CoV-2 antibodies during the COVID-19 pandemic among citizens 15 years or older in Denmark living in social housing (SH) areas . Methods As part of Testing Denmark, a nationwide sero-epidemiological surveillance survey, we conducted a study between January 8th and January 31st, 2021 with recruitment in 13 selected SH areas in Denmark. Participants were offered a point-of-care rapid SARS-CoV-2 IgM and IgG antibody test and a questionnaire concerning previous testing (viral throat- and nasopharyngeal swab or antibody test), test results for COVID-19, demographics, household characteristics, employment, risk factors for SARS-CoV-2 infection and history of symptoms associated with COVID-19. Data on seroprevalence from Danish blood donors in same period using a total Ig ELISA assay were used as a proxy for the general Danish population. Findings Of the 13,279 included participants, 2,296 (17.3%) were seropositive (mean age 46.6 (SD 16.4) years, 54.2% female), which was 3 times higher than in the general Danish population (mean age 41.7 (SD 14.1) years, 48.5% female) in the same period (5.8%, risk ratios (RR) 2.96, 95% CI 2.78-3.16, p>0.001). Seropositivity was higher among males than females (RR 1.1, 95% CI 1.05-1.22%, p=0.001) and increased with age, with an OR seropositivity of 1.03 for each 10-year increase in age (95% CI 1.00-1.06, p=0.031). Close contact with COVID-19-infected individuals was associated with a higher risk of infection, especially among members of the same households (OR 5.0, 95% CI 4.1-6.2 p<0,001). Adjusted for age, gender and region living at least 4 people in a household significantly increased the OR of seropositivity (OR 1.3, 95% CI 1.1-1.6, p=0.02) as did living in a multi-generational household (OR 1.3 per generation, 95% CI 1.1-1.5, p=0.007). Only 1.6% of participants reported not following any of the national COVID-19 recommendations. Anosmia (RR 3.2 95% CI 2.8-3.7, p<0.001) and ageusia (RR 3.3, 95% CI 2.9-3.8, p<0.001) were strongest associated with seropositivity. Interpretation Danish citizens living in SH areas of low socioeconomic status had a three times higher SARS-CoV-2 seroprevalence compared to the general Danish population. The seroprevalence was significantly higher in males and increased with age. Living in multiple generations or more than four persons in a household was an independent risk factor for being seropositive. Results of this study can be used for future consideration of the need for preventive measures in the populations living in SH areas.

SARS-CoV-2 antibody prevalence among homeless people, sex workers and shelter workers in Denmark: a nationwide cross-sectional study (preprint)

Background People experiencing homelessness (PEH) and associated shelter workers may be at higher risk of infection with 'Severe acute respiratory syndrome coronavirus 2' (SARS-CoV-2). The aim of this study was to determine the prevalence of SARS-CoV-2 among PEH and shelter workers in Denmark. Design and methods In November 2020, we conducted a nationwide cross-sectional seroprevalence study among PEH and shelter workers at 21 recruitment sites in Denmark. The assessment included a point-of-care test for antibodies against SARS-CoV-2, followed by a questionnaire. The seroprevalence was compared to that of geographically matched blood donors considered as a proxy for the background population, tested using a total Ig ELISA assay. Results We included 827 participants in the study, of whom 819 provided their SARS-CoV-2 antibody results. Of those, 628 were PEH (median age 50.8 (IQR 40.9-59.1) years, 35.5% female) and 191 were shelter workers (median age 46.6 (IQR 36.1-55.0) years and 74.5% female). The overall seroprevalence was 6.7% and was similar among PEH and shelter workers (6.8% vs 6.3%, p=0.87); and 12% among all participants who engaged in sex work. The overall participant seroprevalence was significantly higher than that of the background population (2,9%, p <0.001). Participants engaging in sex work had a significantly increased risk of seropositivity compared to other participants (95% CI: 0.86-2.36, p=0.02). Seropositive and seronegative participants reported a similar presence of at least one SARS-CoV-2 associated symptom (49% and 54%, respectively). Interpretations The prevalence of SARS-CoV-2 antibodies was more than twice as high among PEH and associated shelter workers, compared to the background population. The subset of the study participants who were also sex workers were at particularly high risk of COVID-19 infection. Funding Tryg and HelseFonden.

Prevalence of SARS-CoV-2 antibodies in Denmark: Nationwide, population-based seroepidemiological surveys (preprint)

Background. Seroprevalence studies have proven an important tool to monitor the progression of the coronavirus disease 2019 (COVID-19) epidemic. We present results of consecutive population-based seroprevalence surveys performed in Denmark in 2020. Methods. Invitation letters including a questionnaire covering symptoms were sent to representatively drawn samples of the population in spring, late summer and autumn/winter of 2020. Blood samples from participants taken at public test-centers were analyzed for total Ig and seroprevalence estimates per population segment calculated and compared to other surveillance parameters. Results. From 34,081 participating individuals (response rate 33%), we obtained seroprevalence estimates increasing from 1.1% (95%CI: 0.7%-1.7) in May to 4.0 % (95%CI: 3.4%-4.7%) in December 2020. By December 2020, 1.5% of the population 12 years and older had tested positive by PCR. Seroprevalence estimates were roughly 3 times higher in those aged 12-29 compared to 65+ and higher in metropolitan municipalities. Among seropositives, loss of taste/smell were the more specific symptoms, 32%-56% did not report any symptoms. In half of seroconverted families, we did not see evidence of transmission between generations. Infected individuals in older age groups were hospitalized several fold more often than in younger. Conclusions. Seroprevalence increased during 2020; younger age groups were primarily infected in the autumn/winter surge. Approximately half were asymptomatically infected. Denmark has a high per capita test rate; roughly two undiagnosed infections of COVID-19 were estimated to occur for each diagnosed case. The epidemic appears to have progressed relatively modestly during 2020 in Denmark.

Increased Risk of Hospitalisation Associated with Infection with SARS-CoV-2 Lineage B.1.1.7 in Denmark (preprint)

Background: The more infectious SARS-CoV-2 virus lineage B.1.1.7, rapidly spread in Europe after December 2020, and a concern of B.1.1.7 causing more severe disease has been raised. Denmark has one of Europe´s highest capacities per capita of SARS-CoV-2 reverse transcription polymerase chain reaction (RT PCR) test and whole genome sequencing (WGS). We used national health register-data to explore whether B.1.1.7 increases the risk of COVID-19 hospitalisation. Methods and Findings: In an observational cohort study we included all SARS-CoV-2 RT PCR test-positive individuals in Denmark sampled between the 1st January and until the 9th February, 2021, identified in the national COVID-19 surveillance system. The surveillance system includes national individual RT PCR test results and viral WGS analyses and data from national health registers including COVID-19 related hospital admissions defined as first admission within 14 days of the test-positive swab. The odds ratio (OR) of admission according to infection with B.1.1.7, vs other co-existing lineages, was calculated in a logistic regression model adjusted for sex, age, period, follow-up time less than 14 days, region, and comorbidities. A total of 35,887 test-positive individuals were identified, 23,057 (64%) had WGS performed, of whom 18,499 (80%) resulted in a viral genome and a total of 2,155 of these were lineage B.1.1.7. The proportion of individuals with B.1.1.7 increased from 4% in early January to 45% in early February. Among the individuals with viral genome data, B.1.1.7 was associated with a crude OR of admission of 0.87 (95%CI, 0.72-1.05) and an adjusted OR of 1.64 (95%CI, 1.32-2.04) based on 128 admissions after B.1.1.7 infection and 1,107 admissions after infection with other lineages. The adjusted OR was increased in all strata of age and calendar time - the two most important confounders of the crude OR. Conclusions: Infection with lineage B.1.1.7 was associated with an increased risk of hospitalisation compared with other lineages. This finding may have serious public health impact in countries with spread of B.1.1.7 and can support hospital preparedness and modelling of projected impact of the epidemic.Funding Statement: The authors received no specific funding for this work.Declaration of Interests: The authors declare that they have no competing interest.Ethics Approval Statement: This study was conducted on administrative register data. According to Danish law, ethics approval is not needed for such research.