1644. Performance of Symptom-Based Case Definitions to Identify Influenza Virus Infection among Pregnant Women in Middle-Income Countries: Findings from the Pregnancy and Influenza Multinational Epidemiologic (PRIME) Study

Abstract

BackgroundThe World Health Organization (WHO) recommends case definitions for influenza surveillance that are also used in public health research, though their performance has not been assessed in many risk groups, including pregnant women in whom influenza may manifest differently. Â We evaluated the performance of symptom-based case definitions to detect influenza in a cohort of pregnant women in India, Peru, and Thailand.MethodsIn 2017, we contacted 4774 pregnant women twice a week during the influenza season to identify illnesses with new or worsened cough, runny nose, sore throat, difficulty breathing or myalgia, and collected data on other symptoms and nasal swabs for influenza rRT–PCR testing. To identify symptom predictors of influenza, we used multivariable logistic regression with forward selection of symptoms significant in univariate analysis after controlling for country, chronic conditions, influenza vaccination, and time from symptom onset to swab collection. We calculated sensitivity and specificity of each symptom, WHO respiratory illness case definitions and a case definition based on significant predictors from the multivariable model.ResultsOf 2431 eligible illness episodes among 1,716 participants, 142 (5.8%) were positive for influenza. Among individual symptoms, runny nose was most sensitive and measured fever ≥ 38° Celsius was most specific (Figure 1). In a multivariable model, measured fever ≥ 38° Celsius [adjusted odds ratio = 3.8, 95% confidence interval [CI] = 2.0–7.2], cough [2.7, CI 1.6–4.7], chills [2.2, CI 1.2–3.8], and myalgia [1.2, CI 2.2, 5.3] were independently associated with influenza illness. A case definition based on these four (measured fever, cough, chills or myalgia), was 91%-sensitive and 37% specific. Sensitivity and specificity of case definitions varied (Figure 2).ConclusionWhile a case definition based on one or more of fever, chills, cough or myalgia is highly-sensitive and moderately specific among pregnant women, case definitions requiring measured or subjective fever may miss many influenza cases making them sub-optimal for studies of burden or vaccine efficacy. The intended use of case definitions should be considered when evaluating the tradeoff between sensitivity and specificity. Disclosures All authors: No reported disclosures.