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Mid-season data from the Southern Hemisphere suggest the current season's trivalent and quadrivalent inactivated flu vaccines protect against the most serious outcomes, according to a CDC report.

The findings from five South American nations showed an overall vaccine of 51.9% (95% CI 39.2-62.0) against flu-related hospitalization, and that protection reached 55.2% (95% CI 41.8-65.5) when looking at the predominant circulating strain, A(H1N1)pdm09, reported researchers led by Ashley Fowlkes, ScD, of CDC's National Center for Immunization and Respiratory Diseases in Atlanta.

Whether A(H1N1)pdm09 will continue as the dominant strain for the Northern Hemisphere's flu season remains to be seen, the researchers said, but most cases in the U.S. so far have involved either that strain or B/Victoria viruses.

"These early, interim estimates, provided before the expected end of seasonal influenza virus circulation, suggest that vaccination substantially reduced the risk for severe influenza illnesses, underscoring the benefits of influenza vaccination," the group wrote in .

But they found that less than a fourth of the hospitalized individuals who met criteria for study inclusion had been vaccinated before illness onset, noting that reduced vaccination coverage has been seen globally since the start of the COVID-19 pandemic.

"Health authorities worldwide should encourage influenza vaccination for persons at increased risk for severe disease, including young children, persons with preexisting health conditions, and older adults, as well as those at increased risk for exposure to or transmission of influenza virus, such as health care personnel," the researchers wrote.

The current study focused on those particular high-risk groups, but the CDC also lists pregnancy as a high-risk situation; regardless, the agency recommends that all people in the U.S. ages 6 months and older receive a flu shot, preferably in September or October.

A higher percentage of positive flu specimens in the new dataset were detected in earlier surveillance weeks compared with the average from 2011 to 2019, though the intensity and timing across the two hemispheres don't always mirror each other, said Fowlkes and co-authors.

To evaluate the effectiveness of the current season's vaccines, the researchers employed a test-negative case-control design and looked at 2,780 hospitalizations associated with severe acute respiratory infections (SARIs) in REVELAC-i (Network for the Evaluation of Vaccine Effectiveness in Latin America and the Caribbean–influenza) from March 27 to July 9 of this year. This included cases from Chile (41.7%), Brazil (33%), Uruguay (16.1%), Paraguay (6%), and Argentina (3.2%).

Evaluation for vaccine effectiveness started 2 weeks after the launch of each country's vaccination campaign. Researchers looked at three target groups for their analysis: young children (45.4% of cases), older adults (40.6%), and people with preexisting medical conditions (14%).

SARI cases involved those with an acute respiratory infection plus a fever and cough that led to hospitalization, with positive flu cases identified by RT-PCR and vaccination status by national immunization records.

Influenza was detected in 32.4% of cases, though proportions varied significantly by group: 48.4% of the older adult cases, 35.8% of the group with preexisting health conditions, and 17% of young kids (P<0.001).

Most of the 900 positive specimens were influenza type A (90.6%) or type B (9.4%) viruses, and 99.3% of the influenza A viruses subtyped were A(H1N1)pdm09.

Overall, 23.9% of the included patients had received a 2023 influenza vaccine, 15.3% of the case patients and 28% of controls. Vaccine effectiveness against flu-related hospitalizations ranged from 37.6% in older adults to 70.2% in children. Effectiveness against flu-related hospitalization from influenza B was not significant (46.2%, 95% CI -7.9 to 73.2).

Limitations included that nearly 25% of the patients initially screened for the analysis weren't tested for the flu with RT-PCR. Researchers were also unable to assess whether children had received one or two doses of vaccine.

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