According to a recent report from the Centers for Disease Control and Prevention (CDC), mid-season data from the Southern Hemisphere has shown promising results regarding the effectiveness of this year's trivalent and quadrivalent inactivated flu vaccines in preventing severe outcomes.
The study found that less than a quarter of individuals who were hospitalised and met the study's criteria had received the flu vaccine prior to falling ill. This trend is attributed to reduced vaccination rates worldwide, a phenomenon influenced by the ongoing COVID-19 pandemic.
To assess the efficacy of the current influenza vaccines for the season, researchers employed a test-negative case-control design, analysing 2,780 hospitalizations linked to severe acute respiratory infections (SARIs) in the REVELAC-i network (Network for the Evaluation of Vaccine Effectiveness in Latin America and the Caribbean–influenza) from March 27 to July 9 of the current year. The cases were drawn from multiple countries, with Chile (41.7%), Brazil (33%), Uruguay (16.1%), Paraguay (6%), and Argentina (3.2%) represented.
The evaluation of vaccine effectiveness commenced two weeks after each country's vaccination campaign launch. The analysis focused on three specific groups: young children (45.4% of cases), older adults (40.6%), and individuals with preexisting medical conditions (14%).
SARI cases were defined as those with acute respiratory infections accompanied by fever and cough, resulting in hospitalisation. Positive flu cases were confirmed through RT-PCR testing, and vaccination status was cross-referenced with national immunisation records.
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Influenza was detected in 32.4% of the cases, but the prevalence varied significantly among different groups: 48.4% among older adults, 35.8% among those with preexisting health conditions, and 17% among young children (P<0.001).
Among the 900 positive specimens, the majority were influenza type A (90.6%) or type B (9.4%) viruses. Notably, 99.3% of the influenza A viruses were subtype A(H1N1)pdm09.
Overall, 23.9% of the patients included in the analysis had received the 2023 influenza vaccine, with 15.3% of case patients and 28% of controls being vaccinated. The vaccine's effectiveness in preventing flu-related hospitalizations ranged from 37.6% in older adults to 70.2% in children. However, the vaccine's effectiveness against hospitalisation due to influenza B was not statistically significant (46.2%, 95% CI -7.9 to 73.2).
It's important to note that the study had certain limitations, such as the exclusion of nearly 25% of initially screened patients who were not tested for the flu using RT-PCR. Additionally, the researchers were unable to determine whether children had received one or two doses of the vaccine.
