Research Highlights Featured Chart

July 22, 2026

The long shadow of infant illness

Danish data reveal how viruses brought home by older siblings influence lifetime income.

Tyler Smith

Source: famveldman

Respiratory syncytial virus (RSV) is the leading cause of infant hospitalization in the United States, with most children getting the infection by the age of two. For typical preschoolers, the virus produces little more than cold-like symptoms, but in an infant it can progress to bronchiolitis or pneumonia and require intensive care.

In a paper in the American Economic Review, authors N. Meltem Daysal, Hui Ding, Maya Rossin-Slater, and Hannes Schwandt show that when older children carry common respiratory viruses back home to their infant siblings, the economic consequences can persist for decades.

Drawing on Danish administrative data covering over one million children, the authors first documented a notable pattern: younger siblings are hospitalized for acute respiratory conditions at two to three times the rate of their older siblings during the first year of life, and the gap is largest when measured in the first three months of life. 

The researchers explain the findings by noting that older children encounter viruses such as RSV and influenza in group childcare and then transmit them to infant siblings, whose lungs, brains, and immune systems are still developing.

To move from correlation to causation, the researchers combined birth-order variation with local disease waves and compared younger versus older siblings within the same area. This design helped to isolate the effect of infancy disease exposure from other factors tied to birth order and season.

Figure 7 illustrates the long-run consequences of that early exposure.

Figure 7 from Daysal et al. (2026)

The chart plots the effects of younger siblings’ greater disease exposure on their position in the adult income distribution relative to their older siblings, measured over ages 25 to 32. The horizontal axes divide the Danish income distribution into six percentile bins, from the bottom of the income distribution to the top. The vertical axes show the estimated relative differences between younger and older siblings in the likelihood of landing in each bin, with Panel A based on disease exposures in the first year of a child's life and Panel B based on the first six months. 

The pattern is a clear downward shift. Coefficients are negative for the highest three income bins and positive for the lowest three, indicating that exposed younger siblings become more likely to occupy the bottom of the distribution and less likely to reach the top. The effect is more pronounced when exposure is measured over the first six months of life, as shown in Panel B.

The authors also uncovered long-term detrimental effects on educational attainment, suggesting that a reduction in human capital is likely a key driver of the effects on income in adulthood. They point to breastfeeding and newly approved RSV vaccines as potential means for mitigating these negative effects and note that policy responses should prioritize the first six months of life.