Protective effect of influenza vaccine in Brazilian patients hospitalised for severe acute respiratory infection, 2023-2025

Every year, the air changes in Brazil, bringing with it a wave of respiratory viruses that can turn a simple cough into a life-threatening emergency. Among these, influenza is a familiar but dangerous guest, capable of causing severe illness that requires hospitalization. When a person's breathing becomes so difficult that they need intensive care, doctors look to vaccines as a primary shield. These shots are designed to train the body to recognize and fight the virus before it takes hold, preventing the worst outcomes like death or long stays in the hospital. However, the virus is not static; it changes, and the patterns of infection shift, especially after major global health events like the recent pandemic. Understanding how well these vaccines work in the real world, particularly during the most intense weeks of the flu season, is vital for protecting the most vulnerable members of society.

In a recent analysis covering the years 2023 through 2025, researchers in Brazil examined the records of thousands of patients who were hospitalized with severe respiratory infections. Their goal was to see if receiving the flu vaccine actually made a difference for those who were already sick enough to need a hospital bed. They focused on a specific type of vaccine, known as trivalent, which protects against three different strains of the virus. By looking at data from hospitals across the country, the team compared the outcomes of patients who had received no shots, those who had received one dose, and those who had received two doses within the same calendar year. They tracked whether these patients survived, how long they stayed in the hospital, and whether they needed to be moved to an intensive care unit.

The story the data told was one of a shifting landscape. In 2023, about 70 percent of the patients in the study had been vaccinated, but this number dropped significantly in the following years, falling to a 40-50% level in 2024 and 2025. This decline happened even as the number of severe cases and deaths began to climb sharply, particularly among older adults. The researchers found that the vaccine was still working, even when coverage was lower. Patients who had received just a single dose of the vaccine saw their risk of dying in the hospital, being admitted to intensive care, or staying there for a long time drop by roughly 10 to 13 percent compared to those who had not been vaccinated at all.

The protection became even stronger for those who received two doses in a year. For these patients, the risk of dying in the hospital was cut by 32 percent. The benefit was even more dramatic for intensive care; the likelihood of needing a bed in the intensive care unit fell by 52 percent, and the time spent there was reduced by 28 percent. The study highlighted that age played a major role in the severity of the illness, with patients over 50 facing much higher risks than younger groups. While the current vaccination priority in Brazil focuses on those aged 60 and older, the researchers suggested that expanding this priority group to include people aged 50 to 59 could save more lives and reduce the burden on hospitals.

Despite the clear benefits, the study noted that vaccine uptake was not keeping pace with the rising threat. The researchers observed that hesitation and delays in getting vaccinated allowed the virus to cause more harm than necessary. They pointed out that while the vaccine is available for free in public health centers, many people did not take advantage of it until the toll of the disease became undeniable in the middle of 2025. The findings serve as a reminder that even when a vaccine is not perfect, receiving the full recommended course of shots provides a significant layer of defense, turning a potentially fatal hospital stay into a survivable illness for many.