Comparative Analysis of Influenza Epidemiology Before and After the COVID-19 Pandemic in Argentina (2018-2019 vs. 2022-2023)

Abstract

Introduction: The COVID-19 pandemic altered the epidemiology of respiratory viruses other than SARS-CoV-2. This study investigated the clinical-epidemiological pattern of hospitalized pediatric patients with acute lower respiratory tract infections (ALRI) and influenza in Argentina, comparing prepandemic and postpandemic periods.

Materials and methods: This multicenter, cross-sectional study included patients under 18 years old admitted for ALRIs in five tertiary centers of Argentina before (2018 and 2019) and after (2022 and 2023) COVID-19. Changes in viral detection rates, seasonality, and case fatality rate (CFR), along with epidemiological and clinical characteristics, were analyzed. Indirect immunofluorescence assay (IFA) or RT-PCR was used for virological diagnosis pre-pandemic, and only RT-PCR in post-pandemic. Epi Info 7 and SPSS 15.0 was used for data analysis.

Results: A total of 5838 cases of ALRI were included (mean age: 9.5 months; IQR: 4-22 months); 96.6% were tested for viral detection, and 66.4% were positive (3877 cases). Respiratory syncytial virus (RSV) was the most prevalent. Influenza showed typical winter seasonality in 2018, 2019, and 2023. However, 2022 exhibited a bimodal pattern: late summer and spring, with co-circulation of influenza A and B in the second peak. CFR varied by viral diagnosis; influenza showed the highest CFR, all deaths related to influenza A. Among 354 influenza cases, 81% were < 5 years old, 53% were male, 63% had comorbidities, and 14.1% required intensive care. Mean of influenza vaccine coverage (6-24 months) was 21.4%. In both periods, patients with influenza were more likely to have pneumonia. Additionally, in the postpandemic period, malnourishment or being 3 years of age or older was also associated with a higher likelihood of influenza infection compared with infection with other respiratory viruses.

Conclusions: Influenza primarily affected children under 5 years old. Postpandemic cases involved older individuals, and increased circulation of influenza A H3N2 was observed. Vaccination coverage was notably low. Influenza returned to its usual seasonal pattern in 2023.

Keywords: ALRI; case fatality rate; influenza; influenza vaccine coverage.

FIGURE 1

Epidemic curve of hospitalized cases due to acute lower respiratory tract infection (ALRI) caused by RSV, influenza, parainfluenza, adenovirus, and human metapneumovirus. Years 2018–2023. References: RSV: respiratory syncytial virus; Flu A: influenza A; Flu B: influenza B; PIV: parainfluenza virus; ADV: adenovirus; hMNV: human metapneumovirus.

FIGURE 2

Epidemic curve of hospitalized cases with acute lower respiratory tract infection (ALRI) positive for influenza by types and subtypes per epidemiological week. Years 2018–2023. Note: “not subtyped” refers to samples with a Ct value too high for subtype determination, indicating that the viral load was insufficient for successful subtyping rather than the absence of a subtype.