Factors associated with influenza infection in a respiratory disease sentinel surveillance system in Zambia, 2017-2021

Respiratory Diseases
Health Information Systems (Including Surveillance)

Zambia conducts sentinel surveillance for influenza among outpatients with influenza like illness (ILI) and inpatients with severe acute respiratory illness (SARI) to detect pandemic influenza strains and surveillance for circulating seasonal strains. We sought to determine factors associated with influenza among patient with ILI or SARI in Zambia.
We analysed ILI/SARI surveillance data from 2017 through 2021. Patients were identified at ten sentinel sites and administered a case investigation form for each patient. Oropharyngeal or nasopharyngeal swabs were collected and tested for influenza and subtyped using Reverse Transcription-Polymerase Chain Reaction. Demographic, clinical and laboratory data were entered into influenza database. We used multivariable logistic regression to measure the odds of testing positive for influenza among patients with ILI/SARI.
A total of 15,119 patients were enrolled, with 6,322(41.8%) having SARI, 6,298(41.7%) ILI and 2,499(16.5%) unspecified. Half (7,556) were female, and the median age was 12.7 years (IQR 1.6-34.0) with 6,134(40.7%) aged <5 years. Overall, 1,245(8.2%) tested influenza positive, with 531(42.7%) being B, 363(29.1%) A/H3N2, and 276(22.2%) A/H1N1 Pandemic, and 76(6.1%) unsub-typable. Patients aged 26-65 years and ≥65 were at increase odds of testing positive for influenza compared with children under 5 years of age (OR 1.96 [95% confidence interval (CI): 1.69-2.29] and 1.79 [95% CI: 1.28-2.57], respectively), and patients with asthma had reduced odds of testing influenza positive (OR 0.57 [95% CI:0.37-0.94]). Additionally, patients with influenza had higher odds of presenting with dyspnoea (OR 1.27 [95% CI: 1.29-1.43]).
Although literature suggests that ages of <5 and >65, asthma increase influenza risk, our study shows that compared to children under-five years, adults aged >26 years had increased odds of testing positive to influenza, while patients with asthma had reduced odds. Considering the low influenza positivity rate, we recommend broadening testing capacity of ILI/SARI patients to include other pathogens.

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