Social mixing and time use throughout the year: Potential changes in disease transmission and age distribution of cases.

Susceptibility in children is a key driver of heterogeneity in the transmission of different respiratory viruses. For example, SARS-CoV-2 is associated with low susceptibility in children, while for the influenza and respiratory syncytial viruses it is thought that children have higher susceptibility, because adults will have built up natural immunity. We modelled seasonal changes in time use and social mixing based on age-stratified contact rates using historical nationally-representative surveys. We explored changes in the reproduction number and the age distribution of infections of respiratory diseases during the early phase of an epidemic, for different assumptions of susceptibility in children aged 0-15. Across ages, the estimated R0 and the age distribution of incidence fluctuated due to changes in time use. For the scenarios where adults have acquired natural immunity through past infection R0 fell and relative incidence decreased in children aged 0-15 but increased in other ages during holiday periods. If children were less susceptible than adults these changes were less pronounced. Our modelling findings suggest that changing contacts during the holiday periods may shift the age distribution of cases from children towards adults. Given that severity and deaths rise with age for many diseases, more intergenerational mixing risks the disease moving into the more vulnerable following the holidays even if the absolute number of infections did not increase.