Hospitalizations With Respiratory Illness Among Pregnant Women During Influenza Season.

Abstract


OBJECTIVE:

To examine hospitalizations with respiratory illness among pregnant women in the United States during periods of influenza activity.

METHODS:

Data were obtained from the Healthcare Cost and Utilization Project National Inpatient Sample (NIS), the largest publicly available all-payer hospital discharge database. Hospitalizations for respiratory illness and pregnancy were classified with International Classification of Diseases, 9th Revision, Clinical Modification codes. Analyses were stratified by delivery status. Discharge characteristics, length of stay, and complications of delivery among hospitalized pregnant women with and those without respiratory illness were compared.

RESULTS:

During the 1998–2002 influenza seasons, 3.4 per 1,000 hospitalizations of pregnant women included diagnoses of respiratory illness. Characteristics of pregnancy hospitalizations associated with higher odds of respiratory illness were presence of a high-risk condition for which influenza vaccination is recommended, Medicaid/Medicare as primary expected payer of care, and hospitalization in a rural area. During influenza season, hospitalized pregnant women with respiratory illness had significantly longer lengths of stay and higher odds of delivery complications than hospitalized pregnant women without respiratory illness.

CONCLUSION:

Hospitalizations with respiratory illness among pregnant women during influenza season are associated with increased burden for patients and the health care system. Intervention efforts to decrease influenza-related respiratory morbidity among pregnant women should be encouraged.

Introduction

A population of concern for increased influenza-related morbidity is women who will be in their second or third trimester of pregnancy during influenza season. Influenza-attributable risk of acute cardiopulmonary hospitalization increases with stage of pregnancy. Women in their third trimester of pregnancy were 3–4 times more likely than their postpartum counterparts to be hospitalized for an acute cardiopulmonary illness during influenza season, placing them at risk equal to or higher than persons with high-risk medical conditions for whom the annual influenza vaccine is traditionally recommended.1 The Advisory Committee of Immunization Practices of the Centers for Disease Control and Prevention (CDC) and the American College of Obstetricians and Gynecologists (ACOG) recommend influenza vaccination for all women who will be pregnant during influenza season but, in practice, vaccination rates among pregnant women are low.

This report describes hospitalizations with respiratory illness among pregnant women during influenza season using the Healthcare Cost and Utilization Project (HCUP) National Inpatient Database (NIS), a nationally representative sample of inpatient care. Long durations of stay and complications of delivery represent measures of increased burden to health-care systems and pregnant women. We compared characteristics of hospitalizations among pregnant women (pregnancy hospitalizations) with and without a diagnosis of pneumonia or influenza, described the duration of pregnancy hospitalizations with respiratory illness, and noted frequent complications among hospitalized pregnant women who delivered a liveborn infant (delivery hospitalizations) in those with respiratory diagnoses during influenza season.

DISCUSSION

During influenza season, 3.4 of every 1,000 pregnancy hospitalizations also included a diagnosis of respiratory illness. This is substantially higher than the proportion of pregnancy hospitalizations with respiratory illness (1.8 per 1,000) during the rest of the year. The majority of hospitalizations with respiratory illness among pregnant women during influenza season did not include a delivery. This contrasts with overall hospitalizations among pregnant women, of which 88% are for delivery. Excluding hospitalizations during which a delivery occurred, the proportion of pregnancy hospitalizations with respiratory illness is 23.2 per 1,000 during influenza season and 11.7 per 1,000 during the rest of the year. Influenza seasons in which the A (H3N2) viral strain predominates are often the most severe in terms of morbidity and mortality. Although we found an increased risk of hospitalization during these years, length of stay did not differ by calendar year and did not reflect season severity.

Pregnancy hospitalizations that included a diagnosis of respiratory illness had longer lengths of stay than other pregnancy hospitalizations during influenza season. The difference in hospital stay is particularly striking among delivery hospitalizations and may be related to complications of delivery. A previous report using a state-based Medicaid cohort found no difference in hospital length of stay, preterm labor, or mode of delivery comparing pregnancy outcomes of women who experienced an acute cardiopulmonary hospitalization during influenza season with women who did not experience such an event, controlling for the woman's age, race, education, marital status, smoking status, trimester of pregnancy, presence of a high-risk condition, and history of hospitalization in previous 6 months. This analysis reflects estimates adjusted for some risk factors associated with delivery complications, including age, presence of high-risk conditions, primary intended payer of care, hospital location, geographic region, and calendar year of influenza season.

The majority of pregnancy hospitalizations with respiratory illness do not include a delivery; these pregnant women may later deliver without increased risk of complication. Discharged pregnant women who had respiratory illness and delivered a liveborn infant during their hospitalization had higher odds of codiagnosis of preterm labor with delivery, fetal distress, and cesarean delivery. This association may be due to other unmeasured factors associated with both complications of delivery and respiratory illness, and the association may be strongly mediated by gestational age. Although we can make no inferences about causality, it is important to describe the experience of women who deliver while concurrently diagnosed with respiratory illness as compared with women who deliver without these conditions.

Discharge records for 23% of hospitalizations with respiratory illness among pregnant women listed another condition for which CDC guidelines specifically recommend influenza vaccination. The presence of a comorbidity increased the odds of having respiratory illness 3- to 6-fold among hospitalized pregnant women, depending on delivery status. In addition, the presence of a high-risk condition was associated with significantly longer lengths of hospital stay. Rates of serious illness are higher among persons with high-risk medical conditions. Still, current data suggest that vaccination rates among persons with high-risk medical conditions are low.

Hospitalizations in rural locations and with intended payer of public origin (Medicaid/Medicare) were associated with increased odds of respiratory hospitalization during influenza season. Nonwhites, rural residents, and lower-income groups are less likely to be immunized than comparison groups, and persons who received the influenza vaccine are most likely to receive it at a private medical clinic. Reasons commonly cited for not receiving influenza vaccination include not believing vaccination is necessary, not thinking about vaccination, and believing vaccination would cause illness. Programs that aim to decrease morbidity from influenza-associated respiratory illness among pregnant women should propose interventions to increase influenza vaccination rates in this population by addressing erroneous beliefs and reducing disparities in vaccination uptake.

This analysis has limitations. Detailed medical histories, especially gestational age of pregnancy, influenza vaccination coverage, and predisposing medical factors not coded on the discharge record, were not available for a more thorough analysis of pregnancy hospitalizations with respiratory illness. We were not able to look at race as a covariate in this analysis.

Race/ethnicity data in the National Inpatient Database are incomplete because of differences in state procedures for collecting and reporting this variable. Misclassification due to ICD-9-CM coding preferences may bias the results. International Classification of Diseases, 9th Revision, Clinical Modification codes are primarily designed for insurance billing purposes and not research. Changes in health care funding, policies, and insurance reimbursement may influence the use of codes used to identify high-risk conditions, pregnancy, or respiratory illness. Because the unit of analysis is individual discharge records, individuals hospitalized repeatedly may be counted more than once. Pregnant women hospitalized with respiratory illness did not have serologically confirmed influenza exposure. During influenza season, excess respiratory hospitalizations are primarily attributed to pneumonia and influenza, but many studies of influenza-associated hospitalizations use all acute cardiopulmonary hospitalizations during influenza season to measure fully the impact of influenza morbidity. We chose to be more conservative in looking at discharge records with a listing of pneumonia or influenza. As population-based surveillance systems for laboratory-confirmed influenza-associated hospitalizations are developed, descriptions of influenza-related morbidity will become more specific.

The Healthcare Cost and Utilization Program provides a sample population that is representative of inpatient care in U.S. community hospitals. This paper provides a nationally representative overview of hospitalizations with respiratory illness among pregnant women during influenza season. It is meant to provide a health services perspective that describes the overall hospitalizations and clinical comorbidities that occur with them that may be associated with respiratory illness, but these issues must be explored further in clinical, rather than administrative, data. Researchers, health care providers, and policy makers can use these data to develop strategies for reducing influenza-associated morbidity in this high-risk population, specifically by promoting influenza immunization during pregnancy.