Swedish data comes from a historical cohort of 424,386 clients of public dental clinics aged 23 and older in the Stockholm region with inception between October 2015 and January 2020, with follow-up from February 2020 to December 2020. In Sweden, the public dental clinics (Folktandvården, FTV) provide routine preventive visits (oral check-ups) to all residents who choose to receive care in these clinics. At each health check-up smoking and snus use are ascertained as past use, current use, and amount of current use. The national personal numbers assigned to every resident in Sweden at birth or at immigration will be used to obtain information on diagnoses of COVID-19 and of other diseases through record-linkage with regional health care registers. Demographic information will be extracted through record-linkage with the register of the total population of the Stockholm region held by Statistics Sweden.

The Finnish data will come from three pooled cross-sectional national health surveys in Finland (FinSote 2018-2020) of 44,199 participants aged 20 and older. The study samples included permanent residents in Finland from the FinSote surveys 2018, 2019, and 2020. The unique personal identifier assigned to all Finnish residents will be linked to the Communicable Diseases Registry to obtain information on diagnoses of COVID-19, to the Care Register for Health Care (HILMO) to obtain information on hospital admissions due to COVID-19, and to Statistics Finland Mortality Data to obtain information on deaths. Data on some sociodemographic characteristics will be also obtained from the Digital and Population Data Services Agency.

The Norwegian data will be based on the Norwegian Mother, Father and Child Cohort Study (MoBa) (Magnus et al., 2016), and the Norwegian Influenza Pregnancy Cohort (NorFlu) (Laake, 2018), with linkages to the Norwegian Surveillance System for Communicable Diseases (MSIS), the Norwegian Immunisation Registry (SYSVAK), and the Norwegian Population Registry. MoBa is a nation-wide population-based cohort consisting of 280 000 participants, where parents were recruited during pregnancy from 1999 to 2008, while NorFlu is a pregnancy cohort consisting of 9 000 participants recruited in Oslo and Bergen during the swine flu pandemic in 2009-2010. Demographic information is extracted from the registries via linkage to the existing cohort databases. For the purpose of this study, all subjects who died before the onset of the pandemic (February 2020) in the three countries will be excluded from the analysis.

Sweden: at least a positive polymerase chain reaction test (PCR) reported by the laboratories to Sweden's national electronic surveillance system for communicable diseases (SmiNet).

Finland: cases with a positive SARS-CoV-2 reverse transcription PCR (RT-PCR), either informed by a laboratory or by a physician as a record of an International Classification of Diseases, Tenth Revision (ICD-10) code U07.1 (which requires a positive SARS-CoV-2 RT-PCR).

Norway: a positive test for SARS-CoV-2 based on PCR obtained from The Norwegian Surveillance System for Communicable Diseases (MSIS) or the presence of antibodies for SARS-CoV-2.

Out of these definitions a categorical variable will be created (No recorded COVID-19 diagnosis= 1, registered COVID-19 diagnosis= 2)

Any hospital admission with a diagnosis of COVID-19 (ICD-10 codes U071 and U072). The diagnosis could be registered either as a main or as a concomitant diagnosis.

A. A categorical variable for hospital admission with any diagnosis of COVID-19 (either main or secondary diagnosis) will be created (No admissions= 1, any admission= 2).

B. A categorical variable for hospital admission with COVID-19 as the main diagnosis only (No admissions= 1, any admission= 2).

Admission to an intensive care unit (ICU) because of a diagnosis of COVID-19 (ICD-10 codes as above). A categorical variable for intensive unit care because of a diagnosis of COVID-19 (No/Yes) will be created (No ICU care= 1, any ICU care= 2).

Death due to COVID-19 will be established using the Swedish, Finnish, and Norwegian Cause of Death Registries. All deaths occurring during the follow-up period with COVID-19 registered as the main cause will be included.

As a secondary analysis, we will use a negative binomial count model (Hilbe, 2011, 2014) to assess inpatient length of stay measured as the cumulative number of days of hospitalization, as an indicator of disease severity among tobacco users compared to non-tobacco users.