Estimating vaccine effectiveness against COVID-19 using cause-specific sick leave as an indicator: a nationwide population-based cohort study, Norway, July 2021 - December 2022

• Published in: BMC public health Vol. 24; no. 1; pp. 1861 - 8
• Main Authors: Meijerink, Hinta, Veneti, Lamprini, Kristoffersen, Anja Bråthen, Danielsen, Anders Skyrud, Stecher, Melanie, Starrfelt, Jostein
• Format: Journal Article
• Language: English
• Published: England BioMed Central Ltd 11-07-2024; BioMed Central; BMC
• Subjects: Adult; Analysis; Cohort Studies; Collating; Confidence intervals; COVID-19; COVID-19 - epidemiology; COVID-19 - prevention & control; COVID-19 vaccines; COVID-19 Vaccines - administration & dosage; Effectiveness; Emergency management; Emergency medical services; Emergency preparedness; Employee benefits; Employees; Evaluation; Female; Health aspects; Humans; Immunization; Infection; Laws, regulations and rules; Male; Medical personnel; Medical research; Medicine, Experimental; Middle Aged; Norway - epidemiology; Norway Real-world evidence; Population studies; Public health administration; Respiratory tract infection; SARS-CoV-2; SARS-CoV-2 - immunology; Severe acute respiratory syndrome coronavirus 2; Sick leave; Sick Leave - statistics & numerical data; Statistical models; Surveillance; Vaccination; Vaccine effectiveness; Vaccine efficacy; Vaccine Efficacy - statistics & numerical data; Vaccines; Norway; COVID-19; COVID-19 vaccines; SARS-CoV-2; Norway Real-world evidence; Surveillance; Cohort studies; Vaccine effectiveness; Population registers; Sick leave
• ISSN: 1471-2458; 1471-2458
• DOI: 10.1186/s12889-024-19374-0

Due to changes in testing policy and increased use of rapid tests, other indicators for SARS-CoV-2 infections are needed to monitor vaccine effectiveness (VE). We aimed to estimate VE against COVID-19 sick leave (> 3 days, certified by a medical professional) among employed individuals (25-64-years-old) in Norway.
We performed a nationwide cohort study by collating data from the Emergency preparedness register for COVID-19. We used adjusted Cox proportional hazard models with vaccine status as a time-varying covariate and presented results as adjusted hazard ratios (aHRs) with corresponding 95% confidence intervals. Separate models were run against sick leave and against SARS-CoV-2 infections during the Delta period (June-December 2021), and against sick leave during the Omicron period (January-December 2022) when SARS-CoV-2 PCR-testing was replaced by rapid self-tests and infections were underreported.
We included 2,236,419 individuals during the Delta period, of whom 73,776 (3.3%) had a reported infection and 54,334 (2.4%) were registered with sick leave. Of the 2,206,952 included individuals in the Omicron period, 300,140 (13.6%) were registered with sick leave. During the Delta period, 55% (26,611) of individuals who had registered sick leave also had a positive test, compared to 32% (96,445) during the Omicron period. The VE against sick leave during the Delta period followed a similar waning pattern to that against SARS-CoV-2 infections. After the second and third dose, the lowest aHRs were estimated for 2-7 days after vaccination for both sick leave (0.25; 95%CI 0.24-0.26 and 0.26; 95% CI 0.24-0.29) and infection ( 0.16; 95% CI 0.15-0.17 and 0.18; 95% CI 0.16-0.19) respectively. During the Omicron period, aHRs for sick leave were higher than during the Delta period, but the lowest aHRs were still found in 2-7 weeks after receiving the second (0.61; 95% CI 0.59-0.64) or third dose (0.63; 95% CI 0.62-0.64).
Our results showed that sick leave could be a relevant indicator for VE in the surveillance of COVID-19 and a finding that may be important in the surveillance of other respiratory infection.