Morbidity of Late-Season Influenza During Pregnancy.

STRUCTURED ABSTRACT Background In the northern hemisphere, influenza season typically starts in December and lasts through March. Pregnant people are at increased risk for influenza-related morbidity and mortality. Potentially, new viral strains or reduced provider suspicion leading to delayed diagnosis in late influenza season could result in increased risk for severe infection. Objective The objective of this study is to assess the incidence and morbidity associated with late-season influenza in pregnancy, as compared to influenza in other seasons. Study Design Retrospective cohort using the 2007-2018 National Inpatient Sample. Pregnant patients with discharge diagnosis codes consistent with influenza infection were compared based on hospital admission quarter (Quarter 1: October-December; Quarter 2: January-March; Quarter 3: April-June; Quarter 4: July-September), with Quarter 3 defined as “late season”. Primary outcome was the severe maternal morbidity composite defined by the Centers for Disease Control and Prevention. Secondary outcomes included sepsis, shock, acute renal failure, acute heart failure, temporary tracheostomy and invasive mechanical ventilation. Associations between outcomes and quarter of infection were adjusted for age, hospitalization type (antepartum, delivery, or postpartum), and comorbid conditions using relative risk regression, weighted to reflect the National Inpatient Sample design. Results Of 7,355 hospitalizations included, corresponding to a weighted national estimate of 36,042, there were 2,266 (30.8%) hospitalizations which occured in Quarter 1, 4,051 (55.0%) in Quarter 2, 633 (8.6%) in Quarter 3, and 405 (5.5%) in Quarter 4. A non-significant trend toward higher rates of severe maternal morbidity was seen in the ‘late season’ compared to other quarters (13.9% (Quarter 3) versus 10.5% (Quarter 1) versus 12.1% (Quarter 2) versus 13.6% (Quarter 4), p=0.07). Sepsis was also more common in patients with late season influenza (8.0% (Quarter 3) versus 4.8% (Quarter 1) versus 5.8% (Quarter 2) versus 5.9% (Quarter 4), p=0.03). In adjusted analyses, patients with late-season influenza had 1.34 (95% CI 1.01, 1.78) higher risk of severe maternal morbidity, and 1.57 (95% CI 1.06, 2.32) higher risk of sepsis compared to Quarter 1. Conclusion Influenza infection between April-June, i.e. late-season influenza, is associated with higher risk of severe maternal morbidity and sepsis in pregnant patients. Obstetric providers must continue to have awareness and suspicion for influenza infection during these months.