Decision letter: Nationwide participation in FIT-based colorectal cancer screening in Denmark during the COVID-19 pandemic: An observational study

Abstract

Full text Figures and data Side by side Abstract Editor's evaluation Introduction Methods Results Discussion Data availability References Decision letter Author response Article and author information Metrics Abstract Background: Worldwide, most colorectal cancer screening programmes were paused at the start of the COVID-19 pandemic, while the Danish faecal immunochemical test (FIT)-based programme continued without pausing. We examined colorectal cancer screening participation and compliance with subsequent colonoscopy in Denmark throughout the pandemic. Methods: We used data from the Danish Colorectal Cancer Screening Database among individuals aged 50–74 years old invited to participate in colorectal cancer screening from 2018 to 2021 combined with population-wide registries. Using a generalised linear model, we estimated prevalence ratios (PRs) and 95% confidence intervals (CIs) of colorectal cancer screening participation within 90 days since invitation and compliance with colonoscopy within 60 days since a positive FIT test during the pandemic in comparison with the previous years adjusting for age, month and year of invitation. Results: Altogether, 3,133,947 invitations were sent out to 1,928,725 individuals and there were 94,373 positive FIT tests (in 92,848 individuals) during the study period. Before the pandemic, 60.7% participated in screening within 90 days. A minor reduction in participation was observed at the start of the pandemic (PR = 0.95; 95% CI: 0.94–0.96 in pre-lockdown and PR = 0.85; 95% CI: 0.85–0.86 in first lockdown) corresponding to a participation rate of 54.9% during pre-lockdown and 53.0% during first lockdown. This was followed by a 5–10% increased participation in screening corresponding to a participation rate of up to 64.9%. The largest increase in participation was observed among 55–59 years old and among immigrants. The compliance with colonoscopy within 60 days was 89.9% before the pandemic. A slight reduction was observed during first lockdown (PR = 0.96; 95% CI: 0.93–0.98), where after it resumed to normal levels. Conclusions: Participation in the Danish FIT-based colorectal cancer screening programme and subsequent compliance to colonoscopy after a positive FIT result was only slightly affected by the COVID-19 pandemic. Funding: The study was funded by the Danish Cancer Society Scientific Committee (Grant number R321-A17417) and the Danish regions. Editor's evaluation The authors convincingly demonstrate that, in the absence of any shutdowns, the Danish colorectal cancer screening program experienced only minor decreases in program participation during the COVID-19 pandemic period. This likely ensured ongoing program effectiveness in detecting early colorectal cancers and precancerous polyps. The evidence is solid and may serve as guidance for other countries when facing similar public health threats in the future. https://doi.org/10.7554/eLife.81808.sa0 Decision letter Reviews on Sciety eLife's review process Introduction The COVID-19 pandemic has impacted the society and the healthcare systems worldwide considerably. In efforts to mitigate the impact of the COVID-19 pandemic on the healthcare system and to minimise the spread of the infection, population-wide restrictions (lockdown) were imposed worldwide. Large parts of the society were closed down and, within the healthcare system, elective procedures were cancelled or postponed and resources were reallocated to take care of patients in need of hospitalisation because of COVID-19. As a result of the re-organisations within the healthcare systems, the cancer screening programmes were, in most countries, paused at the start of the pandemic (Morris et al., 2021; Dinmohamed et al., 2020; Kortlever et al., 2021; Vives et al., 2022; Walker et al., 2021). In Denmark, however, the cancer screening programmes including the faecal immunochemical test (FIT)-based colorectal cancer screening programme using faecal samples obtained at home continued throughout the pandemic. Results from other European countries using FIT-based screening programmes have shown that alterations to the colorectal cancer screening programme at the start of the pandemic led to large reductions in the number of people referred, diagnosed and treated for colorectal cancer at the start of the pandemic (Morris et al., 2021; Dinmohamed et al., 2020) and to reduced participation in screening and screening-derived colonoscopy (Kortlever et al., 2021) and longer time interval from a positive screening test to colonoscopy (Vives et al., 2022). A study from Canada also found marked reductions in the colorectal cancer faecal test volumes at the start of the pandemic (Walker et al., 2021) resulting from a suspension of the FIT-based screening programme. Moreover, it is estimated that the disruptions to the FIT-based colorectal cancer screening programme would result in additional colorectal cancer diagnoses (de Jonge et al., 2021). The participation in colorectal cancer screening in Denmark throughout the pandemic has not yet been described–however, one study has shown a 24% reduction in the number of colon cancers diagnosed at the start of the pandemic in Denmark (Skovlund et al., 2022) indicating that either the general health-seeking behaviour or the participation in colorectal cancer screening may have changed at the start of the pandemic. It is well known that social inequities exist across the entire colorectal cancer screening pathway. For example, studies have shown that younger individuals, immigrants, individuals living alone and individuals with a lower income are less likely to participate in colorectal cancer screening (Larsen et al., 2017; Pallesen et al., 2021). Furthermore, the compliance with colonoscopy is lower among older patients and among patients with underlying disease (Thomsen et al., 2018), among immigrants and among individuals living alone (Pallesen et al., 2021). A concern is that these social inequities in colorectal cancer screening participation may have been exacerbated during the pandemic. We examined the colorectal cancer screening participation and compliance with subsequent colonoscopy during the COVID-19 pandemic in Denmark compared with the previous years. Furthermore, we examined whether the participation in colorectal cancer screening and compliance with screen-derived colonoscopy during the COVID-19 pandemic differed across population sub-groups. Methods Setting The study was conducted in Denmark, which has a population of approximately 5.8 million inhabitants (Statistics Denmark, 2021). All residents in Denmark are eligible for tax-supported health care provided by the Danish government. Nationwide population-based registries in Denmark record extensive administrative and medical data of the whole population, which can be linked using the unique personal identifier, that is assigned to all residents at birth or immigration (Schmidt et al., 2014–Schmidt et al., 2019). The colorectal cancer screening programme In Denmark, screening for colorectal cancer was implemented in 2014 and is offered free-of-charge every 2 years to all individuals aged 50–74 years old living in Denmark. The test is a home-based test, which is mailed directly together with an invitation letter to all invitees. The screening procedure is based on a single-sample FIT (OC Sensor; Eiken Chemical Company, Tokyo, Japan), which can detect invisible amounts of blood in stool samples, which may be associated with bleeding lesions from precancerous adenomas or colorectal cancer at early stages of the disease (Hewitson et al., 2008; Garborg et al., 2013). Non-participants to screening receive a reminder after 6 weeks. All individuals with a positive FIT test (≥100 µg haemoglobin/L faeces) receive an invitation for a colonoscopy with a pre-booked time for appointment within 14 days after the positive screening result. Non-participants to colonoscopy are contacted by the administrative regions. The COVID-19 pandemic in Denmark In Denmark, three main waves of the COVID-19 pandemic have occurred; in the spring of 2020, in the winter of 2020/2021 and again in the winter of 2021/2022 (Statens Serum Institut, 2021a). The pandemic response included population-wide restrictions (lockdowns), COVID-19 testing and COVID-19 vaccination. During the lockdowns large parts of the society were closed down and people were advised to stay at home if possible. Large-scale COVID-19 testing was provided free-of-charge to all inhabitants since May 2020 (Pottegård et al., 2020). COVID-19 vaccination began in December 2020 and by March 2022, approximately 81% of the population had received two doses and more than 61% had received three doses of the vaccine (Statens Serum Institut, 2021b). The vaccination strategy comprised vaccinating individuals living in nursing homes first, thereafter individuals ≥85 years, then healthcare personnel, thereafter individuals with underlying health conditions and their relatives and finally, individuals were offered the COVID-19 vaccination by decreasing age (75–79, 65–74, 60–64 years, etc.) (Sundhedsstyrelsen, 2021). Study population The study population comprised all invitations in individuals aged 50–74 years old invited to participate in colorectal cancer screening from 1 January 2018 to 30 September 2021, as registered in the Danish Colorectal Cancer Screening Database (Thomsen et al., 2017), which contain information on all individuals in Denmark invited to participate in colorectal cancer screening. To examine participation in colorectal cancer screening, we excluded invitations in individuals who emigrated within 1 year since invitation (N=11,832), invitations in individuals residing in the Faroe Islands or Greenland (N=540), invitations in individuals with an unknown postal address (N=2621) and registrations of stool samples received before an invitation or reminder was sent out (Figure 1). Figure 1 Download asset Open asset Flow-chart of the study population (participation in colorectal cancer screening). To examine compliance with colonoscopy, we included all positive FIT tests from colorectal cancer screening among individuals aged 50–74 years old from 1 January 2018 to 30 September 2021. In all, 146 positive FIT tests were excluded due to low counts across time periods (Figure 2). Figure 2 Download asset Open asset Flow-chart of the study population (compliance with colonoscopy). Exposure of interest The COVID-19 pandemic is the exposure of interest. We defined the different phases of the pandemic in Denmark in accordance with the governmental responses to the COVID-19 pandemic in Denmark, as follows: Pre-pandemic period: 1 January 2018 to 31 January 2020 Pre-lockdown period: 1 February to 10 March 2020 1st lockdown: 11 March to 15 April 2020 1st re-opening: 16 April to 15 December 2020 2nd lockdown: 16 December 2020 to 27 February 2021 2nd re-opening: 28 February 2021 to 30 September 2021 (end of inclusion period) Pre-lockdown and first lockdown was termed ‘the start of the pandemic’ in this study. The above-mentioned time periods refer to the time of invitation for colorectal cancer screening and the time of a positive FIT result for each of the outcomes of interest. Outcome of interests The two main outcomes of interests were colorectal cancer screening participation within 90 days since invitation and compliance with colonoscopy within 60 days since a positive FIT result. Further, we evaluated participation within 180 and 365 days since invitation and compliance with colonoscopy within 365 days since a positive FIT result. We thus calculated the proportion of individuals participating in colorectal cancer screening within 90, 180 and 365 days since invitation and the compliance with colonoscopy within 60 and 365 days since a positive FIT result. Explanatory variables The following variables were examined independently: age, sex, ethnicity, cohabitation status, educational level, disposable income and healthcare usage. Age was defined at the date of invitation, as registered in the Danish Colorectal Cancer Screening Database (Thomsen et al., 2017). From Statistics Denmark, 2021, we obtained information on ethnicity, cohabitation status, educational level and level of income. Ethnicity was categorised as Danish descent, Western immigrant, Non-western immigrant and descendants of immigrants. Cohabitation status was categorised as living alone, cohabiting/co-living, and married (i.e., married or registered partnership) in accordance with Statistics Denmark, 2021. Educational level was defined in accordance with the International Standard Classification of Education (ISCED) of the United Nations Education, Scientific and Cultural Organization (UNESCO). We thus categorised level of education into short (ISCED levels 1–2: primary education to upper secondary education), medium (ISCED levels 3–5: vocational education and training to vocational bachelors educations) and long (ISCED levels 6–8: bachelors programmes to PhD programmes) (Statistics Denmark, 2021). Income was defined as official disposable income depreciated to 2015 level and categorised into five quintiles. To indicate the level of healthcare use by each patient, we counted the total number of contacts (comprising face-to-face, telephone and e-mail consultations) to general practitioners, private practising medical specialists, physiotherapists and chiropractors in the year for invitation as registered in the Danish National Health Service Register (Andersen et al., 2011), which contain information on visits to primary healthcare (e.g., general practitioners and medical specialists) in Denmark since 1990. We categorised healthcare usage into five quintiles of the data as rare (0–3 visits per year), low (4–6 visits per year), average (7–11 visits per year), high (12–18 visits per year) and frequent (≥19 visits per year). Information on cohabitation status was only available from Statistics Denmark until the end of February 2021, whereas all other socioeconomic variables were available until end of the study period. Statistical analyses We examined characteristics of persons invited to participate in colorectal cancer screening and characteristics of persons with a positive FIT test during the study period. Thereafter, we examined the participation in colorectal cancer screening within 90, 180 and 365 days since invitation overall and stratifying by the explanatory variables per month and during the pandemic phases. Similarly, we examined compliance with colonoscopy within 60 and 365 days since a positive FIT test overall and stratifying by the explanatory variables per month and during the pandemic phases. We also examined the median number of days and interdecentile interval (IDI) from invitation to participation overall and during the different phases of the pandemic, among individuals eventually participating in the screening programme. Using a generalised linear model (GLM) with log link for the Poisson family with robust standard errors (SEs), we estimated prevalence ratios (PRs) and 95% confidence intervals (CIs) of participation in colorectal cancer screening within 90, 180 and 365 days since invitation among persons invited to participate in colorectal cancer screening and compliance with colonoscopy within 60 and 365 days since a positive FIT test during the different phases of the pandemic overall and stratifying by the explanatory variables. First, we calculated unadjusted analyses. Thereafter, the analyses were adjusted for month of invitation to allow for seasonality and year of invitation to take into account the annual change in colorectal cancer screening participation. Finally, the analyses were adjusted for age to take into account the effect of age on the other explanatory variables. We performed a sensitivity analysis to take into account an IT-error that occurred in the spring of 2020 resulting in a reduction in the number of invitations sent out in weeks 11–14 2020 (Central Denmark Region in weeks 11–14 2020, Northern Denmark Region in weeks 12–14 2020 and the rest of Denmark in weeks 13–14 2020). The error meant, that only individuals entering or leaving the screening programme were invited. Thus, during the period with the IT-error only 50 years olds entering the programme, individuals entering the country from abroad, and 73–74 years old leaving the programme were invited. We re-ran the analyses for this by introducing a dummy variable expressing the IT-error in the GLM model. All analyses were conducted using STATA version 17.0. Results Altogether, 3,133,947 invitations were sent out to 1,928,725 individuals during the study period. Among those 50.5% were women and the median age was 60 years (IQI = 54–67), the majority were of Danish descent (91.4%), most were married (59.4%) and 56% had a short educational level. The distribution of the descriptive characteristics was similar across the pandemic phases (Table 1). Table 1 Baseline characteristics of people invited to participate in colorectal cancer screening in Denmark from 2018 to 2021. Pre-pandemic (1 January 2018 to 31 January 2020)Pre-lockdown (1 February 2020 to 10 March 2020)1st lockdown (11 March 2020 to 15 April 2020)1st re-opening (16 April 2020 to 15 December 2020)2nd lockdown (16 December 2020 to 27 February 2021)2nd re-opening (28 February 2021 to 30 September 2021)TotalN (%)N (%)N (%)N (%)N (%)N (%)N (%)Total1,804,770 (57.6)104,892 (3.3)40,146 (1.3)562,124 (17.9)176,469 (5.6)445,546 (14.2)3,133,947 (100.0)SexMen893,009 (49.5)52,523 (50.1)19,285 (48.0)276,731 (49.2)87,667 (49.7)218,713 (49.1)1,547,928 (49.4)Women911,761 (50.5)52,369 (49.9)20,861 (52.0)285,393 (50.8)88,802 (50.3)226,833 (50.9)1,586,019 (50.6)Age at invitation50–54 years478,804 (26.5)14,982 (14.3)7322 (18.2)127,306 (22.6)49,217 (27.9)138,988 (31.2)816,619 (26.1)55–59 years375,309 (20.8)38,988 (37.2)5739 (14.3)150,615 (26.8)32,217 (18.3)61,318 (13.8)664,186 (21.2)60–64 years332,766 (18.4)18,089 (17.2)2929 (7.3)101,925 (18.1)33,503 (19.0)86,806 (19.5)576,018 (18.4)65–69 years308,725 (17.1)16,334 (15.6)2859 (7.1)92,026 (16.4)30,398 (17.2)79,821 (17.9)530,163 (16.9)70–74 years309,166 (17.1)16,499 (15.7)21,297 (53.0)90,252 (16.1)31,134 (17.6)78,613 (17.6)546,961 (17.5)Median (IQI)60 (54–67)59 (55–66)72 (56–75)60 (55–67)60 (54–67)61 (54–67)60 (54–67)EthnicityDanish descent1,651,658 (91.6)95,490 (91.1)36,665 (91.4)512,355 (91.2)150,194 (91.0)366,141 (91.2)2,812,503 (91.4)Descendant of immigrant2932 (0.2)169 (0.2)65 (0.2)987 (0.2)284 (0.2)617 (0.2)5054 (0.2)Western immigrant48,061 (2.7)2871 (2.7)1164 (2.9)15,877 (2.8)4642 (2.8)10,902 (2.7)83,517 (2.7)Non-western immigrant99,730 (5.5)6334 (6.0)2238 (5.6)32,755 (5.8)9939 (6.0)23,620 (5.9)174,616 (5.7)Cohabitation statusLiving alone567,436 (31.5)34,149 (32.6)14,487 (36.1)177,512 (31.6)51,320 (31.1)120,737 (30.1)965,641 (31.4)Cohabiting/co-living163,186 (9.1)9679 (9.2)2758 (6.9)53,372 (9.5)15,775 (9.6)37,404 (9.3)282,174 (9.2)Married/registered partner1,071,632 (59.5)61,036 (58.2)22,887 (57.0)331,090 (58.9)97,964 (59.4)243,139 (60.6)1,827,748 (59.4)Educational level (ISCED)ISCED15 levels 1–2976,876 (55.1)58,698 (57.0)18,575 (47.2)316,639 (57.3)98,365 (56.8)250,198 (57.2)1,719,351 (55.9)ISCED15 levels 3–5597,535 (33.7)33,332 (32.3)15,297 (38.8)177,249 (32.1)55,821 (32.2)139,785 (32.0)1,019,019 (33.1)ISCED15 levels 6–8198,531 (11.2)11,011 (10.7)5513 (14.0)58,592 (10.6)18,970 (11.0)47,211 (10.8)339,828 (11.0)Disposable incomeLowest quintile353,612 (19.6)18,722 (17.9)9939 (24.8)97,358 (17.4)29,966 (17.0)74,581 (16.8)584,178 (18.7)Second quintile358,955 (19.9)20,579 (19.7)9583 (23.9)105,809 (18.9)33,736 (19.2)83,982 (18.9)612,644 (19.6)Third quintile366,888 (20.3)21,023 (20.1)7332 (18.3)114,154 (20.4)33,347 (18.9)82,529 (18.6)625,273 (20.0)Fourth quintile366,219 (20.3)21,767 (20.8)6614 (16.5)119,544 (21.3)36,705 (20.8)94,378 (21.2)645,227 (20.6)Highest quintile358,608 (19.9)22,588 (21.6)6644 (16.6)123,676 (22.1)42,412 (24.1)108,662 (24.5)662,590 (21.2)Healthcare usageRare386,510 (21.4)23,944 (22.8)7293 (18.2)125,799 (22.4)37,849 (21.4)95,024 (21.3)676,419 (21.6)Low338,179 (18.7)18,944 (18.1)6054 (15.1)103,813 (18.5)32,147 (18.2)80,134 (18.0)579,271 (18.5)Average401,358 (22.2)23,271 (22.2)8569 (21.3)125,600 (22.3)39,176 (22.2)98,833 (22.2)696,807 (22.2)High333,529 (18.5)19,230 (18.3)8343 (20.8)104,646 (18.6)33,618 (19.1)85,706 (19.2)585,072 (18.7)Frequent345,194 (19.1)19,503 (18.6)9887 (24.6)102,266 (18.2)33,679 (19.1)85,849 (19.3)596,378 (19.0)Time from invitation to participation, median (IDI)28 (16–72)25 (15–78)32 (16–64)28 (16–66)24 (16–63)25 (16–59)28 (16–70) IQI = interquartile interval.; IDI = interdecentile interval.; ISCED = International Standard Classification of Education. Participation during the COVID-19 pandemic Before the pandemic, 60.7% participated in colorectal cancer screening within 90 days since invitation (Figure 3 and Supplementary file 1). The results were similar extending the length of follow-up time to 180 and 365 days (data not shown). Figure 3 Download asset Open asset Participation in colorectal cancer screening (%) in Denmark within 90, 180 and 365 days since invitation from 2018 to 2021. A reduction in screening participation within 90 days occurred during February and March 2020 (Figure 1) reflected in a PR of 0.95 (95% CI: 0.94–0.96) during pre-lockdown and a PR of 0.85 (95% CI: 0.85–0.86) during first lockdown (Table 2). This reduction corresponded to an overall participation rate of 54.9% during pre-lockdown and 53.3% during first lockdown (Supplementary file 1). Subsequently, an increase in screening participation was observed (Figure 1) reflected in overall PRs of 1.04 (95% CI: 1.04–1.05), 1.09 (95% CI: 1.09–1.10) and 1.11 (95% CI: 1.10–1.12) during first re-opening, second lockdown and second re-opening, respectively (Table 2). These increases corresponded to participation rates of 62.4% during first re-opening, 63.0% during second lockdown and 64.9% during second re-opening (Supplementary file 1). These estimates were similar when extending the length of follow-up time to 180 and 365 days (data not shown). Table 2 Prevalence ratios (PRs) and 95% confidence intervals (CIs) of participation in colorectal cancer screening within 90 days since invitation in Denmark 2018–2021*. NPre-pandemic (1 January 2018 to 31 January 2020)Pre-lockdown (1 February 2020 to 10 March 2020)1st lockdown (11 March 2020 to 15 April 2020)1st re-opening (16 April 2020 to 15 December 2020)2nd lockdown (16 December 2020 to 27 February 2021)2nd re-opening (28 February 2021 to 30 September 2021)N=180,477N=104,892N=40,146N=562,124N=176,469N=445,546PR[95% CI]PR[95% CI]PR[95% CI]PR[95% CI]PR[95% CI]PR[95% CI]Overall3,133,9471.00–0.95[0.94–0.96]0.85[0.85–0.86]1.04[1.04–1.05]1.09[1.09–1.10]1.11[1.10–1.12]SexMen1,547,9281.00–0.88[0.88–0.89]0.91[0.89–0.92]1.03[1.02–1.03]1.04[1.04–1.05]1.07[1.07–1.08]Women1,586,0191.00–0.92[0.92–0.93]0.84[0.83–0.85]1.03[1.02–1.03]1.03[1.03–1.04]1.06[1.06–1.07]Age at invitation50–54 years816,6191.00–0.89[0.87–0.91]1.14[1.12–1.17]1.00[0.99–1.01]1.04[1.03–1.06]1.16[1.14–1.18]55–59 years664,1861.00–0.93[0.91–0.94]1.09[1.06–1.11]1.11[1.10–1.12]1.24[1.22–1.25]1.18[1.16–1.19]60–64 years576,0181.00–0.94[0.93–0.96]1.03[1.00–1.06]1.03[1.02–1.04]1.08[1.07–1.10]1.09[1.07–1.10]65–69 years530,1631.00–0.96[0.95–0.97]0.99[0.96–1.02]1.02[1.01–1.03]1.07[1.06–1.09]1.07[1.06–1.08]70–74 years546,9611.00–1.01[1.00–1.03]0.77[0.76–0.79]1.04[1.03–1.05]1.09[1.07–1.10]1.10[1.08–1.11]EthnicityDanish descent2,812,5031.00–0.95[0.94–0.95]0.85[0.84–0.86]1.04[1.04–1.05]1.09[1.08–1.10]1.11[1.10–1.12]Descendant of immigrant50541.00–0.95[0.79–1.14]0.83[0.62–1.11]1.04[0.93–1.17]1.08[0.91–1.28]1.12[0.94–1.32]Western Immigrant83,5171.00–0.95[0.91–1.00]0.88[0.82–0.94]1.08[1.05–1.11]1.16[1.11–1.21]1.20[1.15–1.25]Non-western immigrant174,6161.00–0.97[0.94–1.00]1.00[0.95–1.05]1.11[1.09–1.13]1.18[1.15–1.22]1.24[1.20–1.28]Cohabitation statusLiving alone965,6411.00–0.94[0.93–0.95]0.85[0.83–0.87]1.06[1.05–1.07]1.12[1.11–1.14]1.16[1.14–1.17]Cohabiting/co-living282,1741.00–0.95[0.92–0.97]0.97[0.93–1.00]1.05[1.04–1.07]1.12[1.09–1.14]1.14[1.12–1.17]Married/registered partner1,827,7481.00–0.95[0.95–0.96]0.86[0.85–0.87]1.04[1.03–1.04]1.08[1.07–1.09]1.09[1.09–1.10]Educational level (ISCED)ISCED15 levels 1–21,719,3511.00–0.95[0.94–0.95]0.91[0.90–0.92]1.05[1.04–1.05]1.09[1.08–1.10]1.12[1.11–1.13]ISCED15 levels 3–51,019,0191.00–0.96[0.94–0.97]0.80[0.79–0.82]1.04[1.03–1.04]1.09[1.08–1.10]1.10[1.09–1.11]ISCED15 levels 6–8339,8281.00–0.95[0.94–0.97]0.83[0.81–0.85]1.05[1.04–1.06]1.11[1.09–1.13]1.12[1.10–1.13]Disposable incomeLowest quintile584,1781.00–0.95[0.94–0.97]0.73[0.71–0.75]1.04[1.03–1.05]1.08[1.06–1.10]1.11[1.09–1.12]Second quintile612,6441.00–0.96[0.95–0.98]0.78[0.76–0.79]1.05[1.04–1.06]1.10[1.08–1.11]1.13[1.11–1.14]Third quintile625,2731.00–0.95[0.94–0.97]0.86[0.84–0.88]1.05[1.04–1.06]1.08[1.07–1.10]1.11[1.10–1.12]Fourth quintile645,2271.00–0.95[0.94–0.96]0.94[0.92–0.96]1.04[1.04–1.05]1.08[1.07–1.10]1.10[1.09–1.11]Highest quintile662,5901.00–0.93[0.92–0.94]0.98[0.96–1.00]1.04[1.03–1.05]1.10[1.09–1.11]1.10[1.09–1.11]Healthcare usageRare676,4191.00–0.92[0.90–0.93]0.93[0.90–0.95]1.04[1.03–1.05]1.10[1.08–1.12]1.11[1.09–1.12]Low579,2711.00–0.95[0.94–0.97]0.90[0.88–0.92]1.05[1.04–1.06]1.10[1.08–1.11]1.11[1.10–1.13]Average696,8071.00–0.95[0.94–0.97]0.86[0.85–0.88]1.05[1.04–1.06]1.09[1.08–1.10]1.11[1.10–1.12]High585,0721.00–0.97[0.95–0.98]0.84[0.82–0.86]1.05[1.04–1.05]1.09[1.08–1.11]1.12[1.10–1.13]Frequent596,3781.00–0.97[0.95–0.98]0.81[0.79–0.83]1.04[1.03–1.05]1.09[1.08–1.10]1.11[1.10–1.12] * Adjusted for year, month and age at invitation; PR = prevalence ratio; CI = confidence interval; ISCED = International Standard Classification of Education. Participation during the COVID-19 pandemic according to socio-economic variables Throughout the study period, the participation in colorectal cancer screening was lowest among the youngest age group, among men, among immigrants, among individuals living alone or cohabiting, among individuals with a low educational level, a low income and among individuals who rarely use the healthcare system (Supplementary file 1). During first lockdown, women, 70–74 years old and individuals with a low income had the lowest participation in screening. From first re-opening and onwards, the largest relative increases in participation was observed among 55–59 years old and among immigrants (Table 2). Participation among first-time invitees Altogether, 8.8% (N=276,495) of the study population were first-time invitees. The median age of first-time invitees was 50 years old (IQI: 50–50), 15% were immigrants and 33% rarely used the primary healthcare system (Supplementary file 2). Before the pandemic, 53% of first-time invitees participated in screening within 90 days, 54% within 180 days and 55% within 365 days (data not shown). A slight reduction in participation within 90 days was observed during pre-lockdown (PR = 0.95; 95% CI: 0.93–0.98), an increase in participation was found during first lockdown (PR = 1.06; 95% CI: 1.03–1.09), whereas the participation was similar to the previous years for the remaining part of the study period (Supplementary file 3). Table 3 Baseline characteristics of people with a positive FIT test from colorectal cancer screening in Denmark from 2018 to 2021. Pre-pandemic (1 January 2018 to 31 January 2020)Pre-lockdown (1 February 2020 to 10 March 2020)1st lockdown (11 March 2020 to 15 April 2020)1st re-opening (16 April 2020 to 15 December 2020)2nd lockdown (16 December 2020 to 27 Febraury 2021)2nd re-opening (28 February 2021 to 30 September 2021)TotalN (%)N (%)N (%)N (%)N (%)N (%)N (%)Total54,886 (58.2)2942 (3.1)1319 (1.4)16,628 (17.6)5383 (5.7)13,215 (14.0)94,373 (100.0)SexMen29,880 (54.4)1532 (52.1)704 (53.4)8796 (52.9)2835 (52.7)6918 (52.3)50,665 (53.7)Women25,006 (45.6)1410 (47.9)615 (46.6)7832 (47.1)2548 (47.3)6297 (47.7)43,708 (46.3)Age at invitation50–54 years9481 (17.3)291 (9.9)185 (14.0)2615 (15.7)987 (18.3)2876 (21.8)16,435 (17.4)55–59 years8606 (15.7)838 (28.5)157 (11.9)3536 (21.3)813 (15.1)1479 (11.2)15,429 (16.3)60–64 years10,343 (18.8)524 (17.8)78 (5.9)2944 (17.7)1000 (18.6)2433 (18.4)17,322 (18.4)65–69 years12,126 (22.1)559 (19.0)111 (8.4)3526 (21.2)1165 (21.6)2888 (21.9)20,375 (21.6)70–74 years14,330 (26.1)730 (24.8)788 (59.7)4007 (24.1)1418 (26.3)3539 (26.8)24,812 (26.3)Median (IQI)64 (57–70)63 (56–69)74 (59–75)63 (56–69)64 (57–70)64 (57–70)64 (57–70)EthnicityDanish descent51,287 (93.6)2735 (93.0)1221 (92.6)15,484 (93.1)4716 (92.4)11,344 (92.5)86,787 (93.3)Western immigrant1306 (2.4)66 (2.2)37 (2.8)388 (2.3)130 (2.5)302 (2.5)2229 (2.4)Non-western immigrant2205 (4.0)140 (4.8)61 (4.6)756 (4.5)257 (5.0)620 (5.1)4039 (4.3)Cohabitation statusLiving alone16,451 (30.0)939 (31.9)463 (35.1)5046 (30.3)1578 (30.9)3575 (29.1)28,052 (30.1)Cohabiting/co-living4273 (7.8)231 (7.9)88 (6.7)1377 (8.3)431 (8.4)1037 (8.5)7437 (8.0)Married/registered partner34,074 (62.2)1771 (60.2)768 (58.2)10,205 (61.4)3094 (60.6)7654 (62.4)57,566 (61.9)Educational level (ISCED)ISCED15 levels 1–227,676 (51.3)1577 (54.6)605 (46.6)8988 (54.9)2823 (53.6)7064 (54.4)48,733 (52.5)ISCED15 levels 3–520,618 (38.2)1048 (36.3)521 (40.1)5741 (35.1)1907 (36.2)4682 (36.1)34,517 (37.2)ISCED15 levels 6–85666 (10.5)264 (9.1)172 (13.3)1632 (10.0)540 (10.2)1231 (9.5)9505 (10.2)Disposable incomeLowest quintile11,851 (21.6)556 (18.9)321 (24.3)3124 (18.8)962 (17.9)2481 (18.8)19,295 (20.5)Second quintile12,530 (22.8)690 (23.5)351 (26.6)3555 (21.4)1211 (22.5)2954 (22.4)21,291 (22.6)Third quintile11,077 (20.2)608 (20.7