COVID-19 infection and severity among childhood cancer survivors in Denmark and Sweden: a register-based cohort study with matched population and sibling comparisons

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Background: During the COVID-19 pandemic, vulnerable groups faced a higher risk of severe COVID-19 outcomes. The effect of the pandemic on adult childhood cancer survivors is a significant public health concern but not sufficiently understood. We aimed to assess whether adult childhood cancer survivors had a higher risk of severe COVID-19 and registered COVID-19 infections compared to the general population. Methods: This Nordic register-based cohort study included 5-year childhood cancer survivors diagnosed before age 20 years in Denmark and Sweden, two countries with very different pandemic responses. As comparisons, we randomly selected individuals from the general population, matched on year of birth, sex, and country, and identified all siblings of the survivors. All individuals at least 20 years old and alive on January 1, 2020, were followed until December 31, 2022. We plotted the cumulative hazard rates of severe COVID-19 and registered COVID-19 infection and fitted Cox proportional hazards models estimating adjusted hazard ratios (aHR) with 95% confidence intervals (95% CI). Findings: The cohort included 13,659 cancer survivors, 58,803 matched comparisons, and 17,531 siblings. Childhood cancer survivors had a lower risk of registered COVID-19 infection (aHR = 0·91; 95% CI = 0·89-0·94) compared to their comparisons but a higher risk of severe COVID-19 (aHR = 1·58; 95% CI = 1·25-1·98). The latter was particularly evident during periods of widespread viral transmission, as reflected in differences between Denmark and Sweden. Interpretation: These findings underscore the heightened vulnerability of childhood cancer survivors, even many years after their cancer diagnosis, and emphasize the need for closer monitoring and tailored interventions to safeguard this population during future health crises. Funding: Danish and Swedish Childhood Cancer Foundation, Danish National Research Centre for Childhood Cancer, Swedish Research Council, and NordForsk.

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