Fertility and early-life mortality. Evidence from smallpox vaccination in Sweden
What is the causal effect of early-life mortality on fertility? Recent research by Philipp Ager, Casper W. Hansen and Peter S. Jensen sheds new light on this question by testing how the introduction of vaccination in Sweden at the end of 1801 affected early life mortality and fertility at the parish level.
Edward Jenner (1749-1823), father of vaccination uses cowpox vaccine to inoculate children against smallpox.
During the 18th century smallpox was a severe disease in Sweden that wiped out around 10 percent of the population; among them many infants. When vaccination reached Sweden, smallpox mortality dropped remarkably (see Figure 1), in particular, infants, who had the highest mortalities before vaccination, experienced the largest decline (see Table 1).
In line with the historical narrative, Ager, Hansen and Jensen first show that parishes in counties with higher levels of smallpox mortality prior to the introduction of vaccination indeed experienced a greater decline in infant mortality afterwards. Based on this finding, the authors argue that they can identify the causal effect of early-life mortality on fertility. They construct an instrumental variable for early-life mortality where they exploit cross-sectional differences in pre-vaccination smallpox mortality (before 1801) along with the time variation arising from the introduction of the smallpox vaccine. Their instrumental variable estimate reveals that the vaccination-induced decline in early-life mortality lowered the birth rate, while the number of surviving children and population growth remained largely unaffected. Ager, Hansen and Jensen’s conclusion is that the decline in early-life mortality cannot be considered as a major determinant of the onset of Sweden’s fertility decline during the 19th century.