Background: The World Health Organization classified coronavirus disease (COVID-19) as a pandemic by March 11, 2020. Children had a milder disease than adults, and many were asymptomatic. The pandemic could be seen as a natural experiment with several changes, including time spent at home. The relative influence of genes and environment on the variability of a trait can be studied using twin pairs. We aimed to investigate the occurrence of reported COVID-19 symptoms and to separate the relative influence of genes and environment on these traits by variance decomposition.
Methods: This was a population-based twin study conducted between November 2020 and March 2023. A survey was sent out to all parents of 9-year-old twins containing questions related to suspected COVID-19, symptoms of cough, fever and rhinorrhea, as well as concerns for their own and others' health. Twin modeling with variance decomposition was used to separate the relative influence of genes and environment.
Results: In total, 3094 individuals from 1547 complete twin pairs were included (618 monozygotic and 929 dizygotic twin pairs). Suspected COVID-19 increased over time and concerns for own and others' health decreased. We found variability to be attributed mainly to shared environment in all traits investigated ranging between 63.0% and 86.2%. The rest of the variance of each trait was attributed to genetic factors (11.6%-33.5%) and nonshared environment (2.1%-9.9%).
Conclusions: We found results of substantial shared environment in all traits analyzed among 9-year-old twins. Age and timing seem crucial when investigating the possible influence of genes and the environment when a new virus is introduced before herd immunity is reached.
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