Southern Hemisphere Seasonal Influenza Vaccine Shown to Diminish Hospitalization Risk by 52%


This new Morbidity and Mortality Weekly Report on the CDC site indicates the importance of influenza vaccination, especially for at-risk populations.

The 2023 Southern Hemisphere influenza vaccine is effective in reducing influenza-associated hospitalizations, according to new findings, though vaccination coverage against influenza continues to be low despite its benefit.1

These findings resulted from an interim assessment of the 2023 Southern Hemisphere influenza vaccine formulation executed through the use of data drawn from 5 countries in the Southern Hemisphere: Argentina, Brazil, Chile, Paraguay, and Uruguay.

Latin American as well as Caribbean nations have reportedly often dealt with influenza's impact, and the countries have reported significant numbers of respiratory hospitalizations and fatalities linked to the virus annually.2

This new research into the vaccinations effects at reducing hospitalization was led by Ashley L. Fowlkes, ScD, from the Influenza Division at the CDC’s National Center for Immunization and Respiratory Diseases. The research was seen as invaluable given that while the region has a history of high coverage, declines in vaccinations since 2019 have been witnessed.

Background and Findings

The effectiveness of vaccines in prevention of severe influenza-associated hospitalization has been a major focus of several nations in the Southern Hemisphere, particularly during the influenza season. The Network for the Evaluation of Vaccine Effectiveness in Latin America and the Caribbean–influenza (REVELAC–i) is known to be highly invaluable as a means to estimate and monitor effectiveness of vaccines.

The data was evaluated in a period spanning from March to July of 2023. The investigators’ chosen methodology used a test-negative case-control which was made to assess vaccine effectiveness against influenza-linked hospitalizations.

Specifically, the team’s research focused on severe acute respiratory infection (SARI) patients, known to experience acute respiratory infection as well as fever or higher temperature and cough resulting in hospitalization. Testing was rigorous, and the team adjusted for elements such as week of onset, age, and the presence of preexisting conditions in patients.

The investigators looked at data from 486 sentinel hospitals in the aforementioned countries, and they conducted a comprehensive assessment of participants prioritized for influenza vaccination based upon each country’s policies. The participants ended up being in 3 distinct target groups: young children, people with preexisting conditions, and older adults.

The research team reported that the adjusted effectiveness against severe acute respiratory infection (SARI)–associated hospitalizations at the time of the 2023 influenza season in the region was a notable 51.9%, with 55.2% effectiveness observed against the prevalent A(H1N1)pdm09 strain. The team’s interim estimates show the significant benefits of vaccination for the condition, specifically in reducing the risk of severe cases.

The investigators noted that, overall, vaccine effectiveness varied among the target groups, with young children seeing the most benefits with a rate of 70.2%. Older adults, on the other hand, were found to have an effectiveness rate 37.6%.

Despite such findings, the research team pointed to a major gap in coverage for vaccinations in the countries looked at, with less than 30% of eligible people having reported being given the influenza vaccine prior to falling ill.

The investigators concluded that this underlines the value of understanding and addressing the elements contributing to reduced rates following the COVID-19 pandemic, and an ultimate goal of optimizing coverage in the near future.


  1. Fowlkes AL, Nogareda F, Regan A, et al. Interim Effectiveness Estimates of 2023 Southern Hemisphere Influenza Vaccines in Preventing Influenza-Associated Hospitalizations — REVELAC–i Network, March–July 2023. MMWR Morb Mortal Wkly Rep 2023;72:1010–1015. DOI:
  2. Palekar RS, Rolfes MA, Arriola CS, et al. Burden of influenza-associated respiratory hospitalizations in the Americas, 2010–2015. PLoS One 2019;14:e0221479.
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