The influenza vaccine has been less effective in the southern hemisphere this year, a recent report from the Center for Disease Control and Prevention,(CDC) revealed.

The flu shot reduced the risk of hospitalization among high-risk groups by 35% in five South American countries, Argentina, Brazil, Chile, Paraguay, and Uruguay, according to the interim vaccine effectiveness report by the CDC. High-risk individuals include young children, older adults, and those with pre-existing conditions.

Although the vaccine effectiveness rate is still significant, it is a notable drop from last year's 51.9% in these countries. The CDC warns that similar results are expected in the U.S. this winter if A(H3N2) viruses dominate the 2024-25 flu season. Every year, influenza results in 716,000–829,000 hospitalizations and 41,007–71,710 deaths in the Americas.

However, the current vaccine effectiveness estimate highlights the need to boost vaccination rates, as getting the flu shot still remains crucial for protecting high-risk groups from serious complications, according to the CDC.

"These interim vaccine effectiveness estimates suggest that although the proportion of hospitalized patients who were vaccinated was modest, vaccination with the Southern Hemisphere influenza vaccine significantly lowered the risk of hospitalization. Northern Hemisphere countries should, therefore, anticipate the need for robust influenza vaccination campaigns and early antiviral treatment to achieve optimal protection against influenza-associated complications," the CDC report said.

"This evaluation suggests that while only one in five SARI patients had received the 2024 influenza vaccine, those who were vaccinated were at significantly lower risk for hospitalization from any influenza virus infection, including the predominant influenza A(H3N2) and influenza A(H1N1)pdm09 subtypes," the report stated.

This year, the flu shots are targeted to protect against widely circulating strains of H1N1, H3N2, and influenza B virus. The CDC strongly advises everyone aged 6 months and older who is eligible to get the seasonal influenza vaccine in the U.S.

According to a CDC spokesperson, the effectiveness of the vaccine against the H1N1 strain is still "within the range" of previous seasons.

"Overall, flu vaccination reduced the risk of hospitalization by about one-third, which would have a significant impact on disease burden. Receiving a flu vaccine can offer significant protection against severe outcomes," David Daigle, the CDC spokesperson told CBS News.

Apart from vaccination, for high-risk groups, the CDC further recommends using antiviral treatments to reduce the severity of infections and prevent complications. Meanwhile, the public is also urged to use nonpharmaceutical measures, such as hand washing and masks to reduce the risk of contracting the virus.

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