Morbidity of late-season influenza during pregnancy

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 of late-season influenza could result in an increased risk of severe infection.

This study aimed to assess the incidence and morbidity associated with late-season influenza in pregnancy, compared with influenza in other seasons.

This was a retrospective cohort study using the 2007-2018 National Inpatient Sample. Pregnant patients with discharge diagnosis codes consistent with influenza infection were compared on the basis of hospital admission quarter (quarter 1: October to December; quarter 2: January to March; quarter 3: April to June; quarter 4: July to September), with quarter 3 defined as "late-season." The primary outcome was the severe maternal morbidity composite defined by the Centers for Disease Control and Prevention. The 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.

Of 7355 hospitalizations, corresponding to a weighted national estimate of 36,042, 2266 (30.8%) occurred in quarter 1, 4051 (55.0%) in quarter 2, 633 (8.6%) in quarter 3, and 405 (5.5%) in quarter 4. A nonsignificant trend toward higher rates of severe maternal morbidity was seen in the "late-season" compared with other quarters (13.9% [quarter 3] vs 10.5% [quarter 1] vs 12.1% [quarter 2] vs 13.6% [quarter 4]; P=.07). Moreover, sepsis was more common in patients with late-season influenza (8.0% [quarter 3] vs 4.8% [quarter 1] vs 5.8% [quarter 2] vs 5.9% [quarter 4]; P=.03). In the adjusted analyses, patients with late-season influenza had a 1.34 (95% confidence interval, 1.01-1.78) higher risk of severe maternal morbidity and 1.57 (95% confidence interval, 1.06-2.32) higher risk of sepsis than patients with influenza in quarter 1.

Influenza infection between April and June, that is, late-season influenza, is associated with a higher risk of severe maternal morbidity and sepsis in pregnant patients. Obstetrical providers must continue to have awareness and suspicion for influenza infection during these months.