Surveillance of Severe Acute Respiratory Infection and Influenza Vaccine Effectiveness among Hospitalized Italian Adults, 2021/22 Season

Abstract

Following an extremely low incidence of influenza during the first waves of the ongoing COVID-19 pandemic, the 2021/22 Northern Hemisphere winter season saw a resurgence of influenza virus circulation. The aim of this study was to describe epidemiology of severe acute respiratory infections (SARIs) among Italian adults and estimate the 2021/22 season influenza vaccine effectiveness. For this purpose, a test-negative case-control study was conducted in a geographically representative sample of Italian hospitals. Of 753 SARI patients analyzed, 2.5% (N = 19) tested positive for influenza, most of which belonged to the A(H3N2) subtype. Phylogenetic analysis showed that these belonged to the subclade 3C.2a1b.2a.2, which was antigenically different from the 2021/22 A(H3N2) vaccine component. Most (89.5%) cases were registered among non-vaccinated individuals, suggesting a protective effect of influenza vaccination. Due to a limited number of cases, vaccine effectiveness estimated through the Firth’s penalized logistic regression was highly imprecise, being 83.4% (95% CI: 25.8–97.4%) and 83.1% (95% CI: 22.2–97.3%) against any influenza type A and A(H3N2), respectively. Exclusion of SARS-CoV-2-positive controls from the model did not significantly change the base-case estimates. Within the study limitations, influenza vaccination appeared to be effective against laboratory-confirmed SARI.

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“…Furthermore, complications like pneumonia or congestive heart failure, as well as long-term hospital admissions that could lead to hospital-acquired infection, can exacerbate morbidity and raise costs [14,15]. These significant, clinical and economic repercussions, combined with widespread access to vaccines, establish vaccination as the most effective preventive measure [13,[16][17][18][19][20][21][22][23][24][25]. In Italy, influenza vaccination is recommended for healthcare professionals, the hospitalized elderly, those over 60, and young children aged between 6 months and 6 years old [26].…”

“…The impact on healthcare systems extends beyond clinical aspects and includes the economic burden of the disease. This element has resulted in expenses associated with hospitalization and lost workdays [ 10 , 11 , 12 , 13 ]. Furthermore, complications like pneumonia or congestive heart failure, as well as long-term hospital admissions that could lead to hospital-acquired infection, can exacerbate morbidity and raise costs [ 14 , 15 ].…”

“…These significant, clinical and economic repercussions, combined with widespread access to vaccines, establish vaccination as the most effective preventive measure [ 13 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ]. In Italy, influenza vaccination is recommended for healthcare professionals, the hospitalized elderly, those over 60, and young children aged between 6 months and 6 years old [ 26 ].…”

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“…To reduce severe influenza and COVID-19 disease, the only protection is vaccination [19,20]. In Italy, the administration of 'booster' doses started on 27 September 2021 as part of the SARS-CoV-2/COVID-19 vaccination campaign [21].…”