A Little Pain for Birth Weight Gain: Influenza Vaccines and Neonatal Health

Influenza places a substantial burden on health in the U.S. each year, resulting in 9.2 to 60.8 million illnesses, 140,000 to 710,000 hospitalizations, and 12,000 to 56,000 deaths annually since 2010 according to the CDC. Not only does pregnancy increase a woman's risk for influenza-related morbidity, mortality, and other complications, but contracting the flu while pregnant can also negatively impact the fetus.

Research studying short and long-run impacts of influenza are predominantly focused on flu pandemics, when the negative implications are most stark. Much less is known about the impact of the regular flu season, which we experience in most years. Given that influenza-related mortality has declined substantially over the 20th century as medical technologies and hygiene practices have advanced, the impact of modern day seasonal influenza may differ substantially from what it once was in the early part of the 20th century when much previous research was conducted.

In this paper, we examine the impact of the typical seasonal flu activity on birth outcomes and investigate the protective qualities of the influenza vaccine in the case of pregnant women. Health outcomes in a given flu season depend on a number of factors including the amount and type of circulating flu viruses, atmospheric conditions (such as absolute humidity), vaccine quality, as well as the vaccination rate of the population. The observed severity of a flu season is endogenously determined by both the vaccine quality and the vaccination rate of the population.

To address these endogeneity problems, we exploit three sources of exogenous variation. First, we take advantage of the strong exogenous relationship between weather and influenza activity by using absolute humidity as an instrument for the observed flu season severity. Second, we exploit an exogenous supply shortage in vaccines to instrument for the population vaccination rate. Third, we rely on the exogenously determined match rate between the flu vaccine and circulating viruses as a measure of vaccine quality. The match rate signifies the ability of the flu vaccine to produce an immune response against the circulating flu viruses and depends on the pre-season selection of virus strains used in the vaccine.

Our results suggest that even in years when flu severity is much lower than pandemic levels, flu activity has a significant negative impact on neonatal health. A one-standard-deviation increase in severity leads to a 3 percent and 7 percent increase in the probability of low birth weight and preterm birth, respectively. However, higher vaccine effectiveness can mitigate poor birth outcomes from seasonal influenza. A one-standard-deviation increase in the vaccine match rate or the vaccination rate is associated with a 4 percent or 2 percent decrease in the probability of low birth weight and a 9 percent or 4 percent decrease in the probability of preterm birth, respectively.