Influenza vaccine effectiveness against hospitalizations in children and older adults—Data from South America, 2013–2017. A test negative design

• Published in: Vaccine: X Vol. 3; p. 100047
• Main Authors: Sofia Arriola, Carmen, El Omeiri, Nathalie, Azziz-Baumgartner, Eduardo, Thompson, Mark G., Sotomayor-Proschle, Viviana, Fasce, Rodrigo A., Von Horoch, Martha, Enrique Carrizo Olalla, José, Aparecida Ferreira de Almeida, Walquíria, Palacios, Jacqueline, Palekar, Rakhee, Couto, Paula, Descalzo, Miguel, María Ropero-Álvarez, Alba, Gonzalez, Cecilia, Loayza, Sergio, Vergara, Natalia, Bustos, Patricia, Andrade, Winston, Magda S. Domingues, Carla, Issac Montenegro Renoiner, Ernesto, Tatiane da Silva, Érica, Lima Palmeira, Swamy, Araujo da Silva, Daiana, Carolina de Lacerda Sousa, Ana, Mendonça Siqueira, Marilda, Vazquez, Cynthia, Battaglia, Silvia, Vizzotti, Carla, Baumeister, Elsa, Giovacchini, Carlos, Katz, Nathalia, Pacheco, Oscar, Barbosa, Juliana, Malo, Diana, Pulido, Paola, Garcia, Diego, Pinzón, Consuelo
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 10-12-2019; Elsevier
• Subjects: Adults; Children; Hospitalizations; Influenza vaccine effectiveness; Latin America; Regular paper; Severe acute respiratory infections; Southern hemisphere; Test-negative case-control design; Test-negative case-control design; Severe acute respiratory infections; Hospitalizations; Adults; Influenza vaccine effectiveness; Children; Southern hemisphere; Latin America
• ISSN: 2590-1362; 2590-1362
• DOI: 10.1016/j.jvacx.2019.100047

In 2013, the Pan American Health Organization established a multi-site, multi-country network to evaluate influenza vaccine effectiveness (VE). We pooled data from five consecutive seasons in five countries to conduct an analysis of southern hemisphere VE against laboratory-confirmed influenza hospitalizations in young children and older adults.
We used a test-negative design to estimate VE against laboratory-confirmed influenza in hospitalized young children (aged 6─24 months) and older adults (aged ≥60 years) in Argentina, Brazil, Chile, Colombia, and Paraguay. Following country-specific influenza surveillance protocol, hospitalized persons with severe acute respiratory infections (SARI) at 48 sentinel hospitals (March 2013–December 2017) were tested for influenza virus infection by rRT-PCR. VE was estimated for young children and older adults using logistic random effects models accounting for cluster (country), adjusting for sex, age (months for children, and age-in-year categories for adults), calendar year, country, preexisting conditions, month of illness onset and prior vaccination as an effect modifier for the analysis in adults.
We included 8426 SARI cases (2389 children and 6037 adults) in the VE analyses. Among young children, VE against SARI hospitalization associated with any influenza virus was 43% (95%CI: 33%, 51%) for children who received two doses, but was 20% (95%CI: −16%, 45%) and not statistically significant for those who received one dose in a given season. Among older adults, overall VE against SARI hospitalization associated with any influenza virus was 41% (95%CI: 28%, 52%), 45% (95%CI: 34%, 53%) against A(H3N2), 40% (95%CI: 18%, 56%) against A(H1N1)pdm09, and 20% (95%CI: −40%, 54%) against influenza B viruses.
Our results suggest that over the five-year study period, influenza vaccination programs in five South American countries prevented more than one-third of laboratory confirmed influenza-associated hospitalizations in young children receiving the recommended two doses and vaccinated older adults.