Influenza With and Without Fever: Clinical Predictors and Impact on Outcomes in Patients Requiring Hospitalization

Background: The Infectious Diseases Society of America influenza guidelines no longer require fever as part of their influenza case definition in patients requiring hospitalization. However, the impact of fever or lack of fever on clinical decision-making and patient outcomes has not been studied.

Methods: We conducted a retrospective review of adult patients admitted to our tertiary health service between April 2016 and June 2019 with laboratory-confirmed influenza, with and without fever (≥37.8ºC). Patient demographics, presenting features, and outcomes were analyzed using Pearson's chi-square test, the Wilcoxon rank-sum test, and logistic regression.

Results: Of 578 influenza inpatients, 219 (37.9%) had no fever at presentation. Fever was less likely in individuals with a nonrespiratory syndrome (adjusted odds ratio [aOR], 0.44; 95% CI, 0.26-0.77), symptoms for ≥3 days (aOR, 0.53; 95% CI, 0.36-0.78), influenza B infection (aOR, 0.45; 95% CI, 0.29-0.70), chronic lung disease (aOR, 0.55; 95% CI, 0.37-0.81), age ≥65 (aOR, 0.36; 95% CI, 0.23-0.54), and female sex (aOR, 0.69; 95% CI, 0.48-0.99). Patients without fever had lower rates of testing for influenza in the emergency department (64.8% vs 77.2%; P = .002) and longer inpatient stays (median, 2.4 vs 1.9 days; P = .015). These patients were less likely to receive antiviral treatment (55.7% vs 65.6%; P = .024) and more likely die in the hospital (3.2% vs 0.6%; P = .031), and these differences persisted after adjustment for potential confounders.

Conclusions: Absence of fever in influenza is associated with delayed diagnosis, longer length of stay, and higher mortality.