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Background and Aims: Covid-19 is a multiorgan disease affecting the kidney. However, the risk of kidney injury after infection remains unknown. Therefore, this study aimed to determine the risk and time of first ever diagnosed kidney injury following Covid-19, and determine the impact of Covid-19 disease severity on this risk.

Research Questions:

• Is the risk of a first ever kidney injury increased following Covid-19 in a nationwide Swedish whole population setting?
• If so, which patients are at higher risk?

Method: The total Swedish adult population were included in the study from January 1, 2020, to July 26, 2022. Nationwide multi-registry data was used and cross-linked with Personal Identification Numbers. Data was retrieved from the Swedish Inpatient, Outpatient, Covid-19 (SmiNet), Vaccination, Intensive Care, Cancer and Cause of Death Registries. Covid-19 date (exposure) was defined as the first SARS-CoV-2 related date registered in SmiNet based on the date of symptom onset, sample date, diagnosis date or date of report. All ICD-codes (IDC9 and ICD10) used for kidney injury registered since 1987 and 2000 respectively were identified to exclude patients with previous kidney injury. Outcome was defined as all first-time events of kidney injury diagnosis (acute, non-specified and chronic) reported with ICD10 codes post Covid-19 date, during the study period. We used the Self-Controlled Case Series (SCCS) method to determine the incidence rate ratio (IRR) of kidney injury during risk periods up to 6 months after infection. In the SCCS analysis, all individuals with reported Covid-19 (exposure) and first time diagnosed kidney injury (outcome) were included. The adjusted SCCS model was used, which considers deaths in the model.

Results: Among the total 2 426 423 Covid-19 cases, there were 10 351 first kidney injury events diagnosed. In a preliminary analysis, the risk of developing kidney injury after Covid-19 was increased up to 90 days post infection with the highest IRR during the first week (day 1–7) of Covid-19 onset being 56.1 (95% CI 52.5–60). The highest risk was day 0 with nearly 200-fold increased risk, however, due to risk of selection bias (Fonseca-Rodriguez O., 2021) when including day 0, this data is excluded from the first week and should be considered with caution. The IRR was elevated for a longer time for men than for women. For women, the IRR was elevated up until 60 days post infection, compared to 90 days for men. Stratified by age groups, the IRRs were highest in the group aged 51–74 years, compared with those ≤50 years and >74 years. Individuals with higher Covid-19 disease severity were also at higher risk than those with mild infection.

Conclusion: Covid-19 is associated with an increased risk of kidney injury according to this nationwide preliminary analysis, including more than two million Covid-19 cases in Sweden. The risk of a first ever diagnosed kidney injury is elevated up to 90 days after Covid-19, which is later than previously shown. The risk was higher for men than for women and was highest for age group 51–74 years. These results highlight the importance of vaccination to prevent severe Covid-19, and if diseased by Covid-19, follow up to detect kidney injury post Covid-19.