Germs in the Family: The Short- and Long-Term Consequences of Intra-Household Disease Spread

Abstract

While regular exposure to infectious disease is inevitable for most preschool-aged children, their sickness might exert important externalities on more vulnerable family members, such as their infant siblings. We use Danish population-level administrative data on 35 birth cohorts of children to document a striking difference in the likelihood of severe respiratory illness by birth order: younger siblings have two to three times higher rates of hospitalization for respiratory conditions before age one than older siblings at the same age. The hospitalization gap is larger if the younger sibling is born during seasons of high respiratory disease spread and for siblings with shorter birth spacing, who are prone to close contact. These patterns suggest that the family unit is central in virus transmission, with older children “bringing home” viruses to their younger siblings. We then combine the birth order variation with within-municipality variation in respiratory disease prevalence among preschool-aged children to identify differential long-term impacts of early-life respiratory illness between younger and older siblings. We find that moving from the 25th to the 75th percentile in the local disease prevalence distribution is associated with a 30.9 percent differential increase in the number of respiratory illness hospitalizations in the first year of life for younger compared to older siblings. In the long term, for younger relative to older siblings, we find a 0.4 percent differential reduction in the likelihoods of high school and college graduation, and a 1.2 percent additional reduction in age-30 earnings.