Influenza-attributed hospitalization rates among pregnant women in Canada 1994-2000

Although it is recommended that pregnant women at risk for influenza complications receive influenza vaccine, it is not clear if healthy pregnant women are at increased risk for adverse outcomes. We aimed to estimate the rate of hospitalization attributable to influenza for healthy pregnant women and for those with known co-morbidities.

Hospital admission records of women admitted from 1994 to 2000 with a respiratory condition during pregnancy were extracted from the hospitalization database (Canadian Institute of Health Information). Admissions for childbirth were excluded. Weekly admissions, stratified by the presence of co-morbid conditions, were modelled as a function of viral activity, seasonality, trend, and holiday effects using Poisson regression with proxies for influenza and other viral activity developed previously for similar age-specific models of influenza-attributed hospital admissions.

Approximately 300 hospitalizations of pregnant women per year were attributed to influenza, of which 140 were in women with co-morbidities. This hospitalization rate corresponds to 150 (95% CI 140-170) hospitalizations per 100,000 pregnant women per year. An estimated 1 in 1000 healthy pregnant women were hospitalized due to influenza per year. Asthma was the most important risk factor, accounting for an estimated 450 (95% CI 300-600) admissions per 100,000 pregnant women. Admission rates in pregnant women were relatively constant across multiple influenza seasons of varying severity among older adults. During the four weeks of peak influenza activity, 60% of respiratory-related admissions of otherwise healthy pregnant women could be attributed to influenza.

Healthy pregnant Canadian women have consistently higher rates of hospital admission attributable to influenza infection than their non-pregnant peers. The admission rate for healthy pregnant women corresponds to the rate for men and women aged 65 to 69 years, which suggests that this population may benefit from annual influenza immunization.