UCLA Fielding School of Public Health-led research demonstrates the importance of influenza vaccination globally

LOS ANGELES (Oct. 5, 2022) – An international team of researchers has demonstrated that among patients hospitalized for influenza, those who were vaccinated had less severe infections, including reducing the odds for children requiring admittance to an intensive care unit by almost half.

In addition, the researchers found that deaths among hospitalized adults, 65 or older, who had been vaccinated were 38% lower compared to those who had not been vaccinated.

“A common complaint about influenza vaccine is that they are typically 40-60% effective against infection – or the ‘what’s the point?’ complaint. So it is important to note that although everyone in this study was hospitalized, vaccinated individuals were less likely to be severely ill or die, suggesting that you are likely to have far less severe consequences if vaccinated,” said Dr. Annette Regan, UCLA Fielding School of Public Health assistant professor of epidemiology and lead author of the peer-reviewed research, published this week in the October edition of The Lancet Infectious Diseases. “This is an important point, especially in light of the upcoming influenza season coupled with ongoing COVID-19 activity, both this season and into the future.”

Globally, influenza contributes to 9.5 million hospitalizations, 81.5 million hospital days, and 145,000 deaths each year. Vaccination offers the best method of preventing influenza illness, reducing illness in the general population by 40–60%, experts say.

Specifically, The Lancet analysis found that three groups routinely targeted for influenza vaccination experiences less severe illness. Children who had received only part of their first series of influenza vaccines had 36% lower chances of being admitted to an intensive care unit (ICU), and children who had fully completed their first series of influenza vaccines had 48% lower chances of admission to ICU compared to unvaccinated children, the researchers found.

The study – “Severity of influenza illness associated with seasonal influenza vaccination among hospitalized patients in four South American countries” – is the product of an international team of researchers from the United States, Argentina, Brazil, Chile, and Paraguay, and drew on data from all four South American countries over a period of seven years. Data were obtained through the Network for the Evaluation of Vaccine Effectiveness in Latin America and the Caribbean, influenza (REVELAC-i) which is coordinated by the Pan American Health Organization (PAHO).

“Although several studies have reported drops in influenza illness following influenza vaccination, the results have focused predominantly on adults in the United States, and this study aimed to evaluate the severity of influenza illness by vaccination status in a broad range of age groups, and across multiple South American countries,” said Dr. Marta Von Horoch, a co-author who serves as coordinator of the National Immunization Program in Paraguay. “We were very pleased to work with our partners in the U.S. and across the continent, and these findings demonstrate, quite clearly, the importance of influenza vaccination for children and adults, no matter where they live.”

The study – the first-ever on this scale in South America – examined influenza-related hospitalization rates and outcomes across all four countries from 2013-19. Specifically, the analysts reviewed the outcomes for some 2,747 patients hospitalized with confirmed influenza virus infection, in three age groups – children aged 6–24 months, adults aged 18–64 years, and adults aged 65 years or older.

Given the reality that vaccination rates have fallen, in the U.S. and globally during the COVID-19 pandemic, including among children, the findings should help make clear the benefits of timely, pro-active immunization campaigns to the public, the researchers said.

“With influenza season approaching this winter and influenza vaccines now available, these results highlight the importance of getting vaccinated for flu for anyone six months of age or older – as CDC recommends,” Regan said. “It is critical that healthcare providers and the public understand the risks of missing out on vaccinations – it is so much better to prevent a serious illness than to suffer through it, for the patient and everyone in their community.”

PRINCIPAL INVESTIGATOR: Regan, also a faculty affiliate with UCLA’s Bixby Center on Population and Reproductive Health, has taught at the university since 2019. She earned her PhD from the University of Western Australia in 2016. As part of her doctoral work, Regan established one of the largest population-based cohorts to investigate the safety of influenza vaccination in pregnancy and Australia’s first rapid surveillance system for monitoring the safety of vaccines given during pregnancy. She is also currently leading a large U.S. study on COVID-19 vaccination during pregnancy. Regan has served as an epidemiologist with the Western Australia Department of Health (2013-16) and Epidemiologist for the U.S. Centers for Disease Control and Prevention (2007-11). She is also on faculty at the University of San Francisco’s School of Nursing and Health Professions.

METHODS: Using surveillance data from four South American countries, the team examined indicators of severity of illness among influenza-associated hospitalizations, including length of hospital stay, admission to ICU, and death in hospital. Data collection conformed to a common protocol, reducing heterogeneity in measurements. As a result, the researchers were able to evaluate the health effects associated with influenza vaccination in a large sample of three priority groups, including children aged 6–24 months. The large sample size also enabled analyses by influenza virus subtype and by number of pre-existing health conditions. The results for each group was subjected to separate statistical analyses.

FUNDING: This work was supported by a grant from the U.S. Centers for Disease Control and Prevention (CDC) through cooperative agreements with the Pan American Health Organization and the World Health Organization.

CITATION: Regan A, Arriola CS, Couto P, et al. Severity of influenza illness associated with seasonal influenza vaccination among hospitalised patients in four South American countries, 2013–19: a surveillance-based cohort study. Lancet Infectious Diseases, S1473-3099(22)00493-5.


The UCLA Fielding School of Public Health, founded in 1961, is dedicated to enhancing the public’s health by conducting innovative research, training future leaders and health professionals from diverse backgrounds, translating research into policy and practice, and serving our local communities and the communities of the nation and the world. The school has 761 students from 26 nations engaged in carrying out the vision of building healthy futures in greater Los Angeles, California, the nation and the world.