Birth Outcomes After Pertussis and Influenza Diagnosed in Pregnancy: A Retrospective, Population-Based Study

Objective: Adverse birth outcomes and the maternal severity of influenza in pregnancy are well documented but information on pertussis is limited.

Design: Population-based linkage data were collected during 2001-2016.

Setting: New South Wales, Australia.

Population or sample: A total of 1 453 037 singleton births.

Methods: Cox regression was used to estimate the associations between pertussis or influenza during pregnancy and birth outcomes with adjustment of covariates.

Main outcome measures: Adverse birth outcomes (preterm birth and low birth weight).

Results: Among 1 453 037 singleton births over 16 years, we identified pertussis in 925 (49; 5.3% hospitalised) and influenza in 2850 (1092; 38.3% hospitalised) women during pregnancy. Cases of pertussis were similarly distributed by trimester (32% 3rd) whereas 46% of influenza cases were in the 3rd trimester. Younger age, previous birth, and being overseas-born were associated with both pertussis and influenza, whereas identifying as Aboriginal or Torres Strait Islander, hypertension or diabetes before and during pregnancy, and a number of other factors were only associated with influenza. Both pertussis and influenza in pregnancy were associated with increased risk of preterm birth (pertussis: aHR = 1.30, 95% CI 1.01-1.68; influenza: aHR = 1.56, 95% CI 1.36-1.79) and these increased risks were greater when infections in the period within 2 weeks of birth were considered (pertussis: aHR = 2.36, 95% CI 1.26-4.41; influenza: aHR = 2.29, 95% CI 1.78-2.96).

Conclusions: Maternal pertussis and influenza infections close to the time of birth were associated with adverse birth outcomes. These findings highlight the benefits of vaccination during pregnancy.