Interim Effectiveness Estimates of 2023 Southern Hemisphere Influenza Vaccines in Preventing Influenza-Associated Hospitalizations — REVELAC–i Network, March–July 2023

Abstract

Evaluations of vaccine effectiveness during the March- September Southern Hemisphere influenza season can provide valuable information for countries currently experiencing the influenza season and preceding the October-May Northern Hemisphere influenza season. Since 2013, multiple countries have participated in the Network for the Evaluation of Vaccine Effectiveness in Latin America and the Caribbean-influenza (la Red para la Evaluacion de Vacunas en Latino America y el Caribe-influenza [REVELAC-i]) to estimate and monitor vaccine effectiveness (VE) in preventing severe acute respira-tory infection (SARI)-associated hospitalization. Based on data contributed by Argentina, Brazil, Chile, Paraguay, and Uruguay on 2,780 SARI patients hospitalized during March 27-July 9, 2023, the adjusted VE against SARI hospitalization associated with any influenza virus during the 2023 Southern Hemisphere season was 51.9% (95% Confidence Interval [CI] 39.2%- 62.0%), including 55.2% (95% CI: 41.8%-65.5%) against the predominating A(H1N1)pdm09. These early, interim estimates, provided before the expected end of seasonal influ-enza virus circulation, suggest that vaccination substantially reduced the risk for severe influenza illnesses, underscoring the benefits of influenza vaccination. In anticipation of Northern Hemisphere influenza virus circulation, the World Health Organization and CDC recommend that health authorities encourage health care providers to administer annual influenza vaccination to all eligible persons, particularly emphasizing the importance of vaccination for persons at increased risk for severe outcomes (e.g., very young children, persons with preexisting health conditions [including pregnant women], and older adults).