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Most adults hospitalized with influenza are , putting them at for respiratory failure, sepsis, ischemic coronary events, and death.

Amidst the current global SARS-CoV-2 pandemic, annual influenza vaccination is more important than ever, according to experts. A flu shot may not provide complete protection but it's still the best way to prevent potentially deadly flu-related illness, particularly in seniors.

Findings from a recent study of influenza vaccine effectiveness during the 2019-2020 flu season provides more evidence that vaccination can significantly reduce the risk of severe influenza illness requiring hospitalization. Analysis of data from 533 patients enrolled in the Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN) showed that the overall adjusted vaccine effectiveness for reducing hospitalization risk was 41%.

Notably, the highest vaccine effectiveness was observed in patients 65 years and older, according to Mark W. Tenforde, MD, PhD, of the CDC's National Center for Immunization and Respiratory Disease in Atlanta, and colleagues. Effectiveness of the 2019-2020 flu vaccine against influenza A(H1N1)pdm09 and B viruses was 40% and 33%, respectively, the study authors reported in

"Based on , this decrease in hospitalization likely led to commensurate reductions in economic burden and resource utilization because of the high burden of complicated influenza-related hospitalizations in these complex patient groups," Tenforde and colleagues wrote. "With cocirculation of SARS-CoV-2 and potential strain on hospital resources, prevention of influenza-associated hospitalizations through increased vaccine uptake, along with other preventive measures, pharmacologic and non-pharmacologic, will be important this influenza season."

In the following interview, Tenforde discussed the importance of annual influenza vaccination and the key role that clinicians play.

How severe was the 2019-2020 influenza season?

In spite of minimal A(H3N2) activity, which is known to , the 2019-2020 influenza season in the U.S. had the second highest estimated number of cases compared with the previous decade, associated with an estimated 410,000-740,000 hospitalizations and 24,000-62,000 deaths.

How did the efficacy of the 2019-2020 influenza vaccine compare with vaccine effectiveness in previous seasons?

Tenforde: In recent U.S. seasons, vaccination reduced the risk of influenza-related hospitalization by about half, which makes 2019-2020 estimates generally similar. Although genetic drift in influenza A(H3N2) viruses is often associated with reduced vaccine effectiveness, we saw new subgroups emerge in predominant circulating influenza A(H1N1) and B viruses in 2019-2020. This resulted in a net overall vaccine effectiveness. Influenza viruses are constantly changing, and multiple different influenza viruses may circulate in any given season. However, these results were reassuring and showed flu vaccination provided overall protection against severe hospitalized influenza.

Were there any unexpected findings?

Tenforde: A lower and nonsignificant vaccine effectiveness was observed in adults 18 to 49 years of age. Many younger influenza patients reported highly immunocompromising conditions such as solid organ, bone marrow, or stem cell transplant, which may have contributed to the reduced vaccine effectiveness we observed in this age group. Also, 65% of vaccinated adults 65 years of age and older received high-dose vaccines compared with 1% of younger patients.

What were the characteristics of the hospitalized patients in your study?

Tenforde: The patients enrolled in HAIVEN during the 2019-2020 season were primarily older, with a median age of 63. A large proportion were chronically ill, with almost 60% reporting one or more hospitalizations in the previous year, almost one-third reporting home oxygen use, and almost 40% reporting immunocompromising conditions. Because and vaccination has been less effective, an approximate 40% reduction in risk of influenza hospitalizations in association with vaccination is encouraging, especially during a season with two drifted viruses circulating.

Did any other factors contribute to vaccine effectiveness in older individuals or, conversely, limit your findings?

Tenforde: Our analysis showed that 79% of elderly patients with influenza were infected with A(H1N1)pdm09 viruses with 5 additional non-subtyped A viruses. from initial exposure to A(H1N1) viruses and immunologic imprinting may have contributed to higher vaccine effectiveness observed among elderly patients in this study, as described previously. Second, most of these hospitalized patients had complex underlying conditions so unmeasured confounding or selection bias may also have differentially affected vaccine effectiveness estimates by age.

Did you observe any racial disparities in vaccination rates?

Tenforde: We found that non-Hispanic Black populations, which have been heavily impacted by SARS-CoV-2 infections in the U.S., had lower influenza vaccination uptake compared with non-Hispanic white populations, even after adjusting for age. More effective public health strategies are needed to reduce these racial disparities in annual influenza vaccination rates.

What are the implications of your findings for the 2020-2021 influenza season?

Tenforde: The that were associated with a lack of vaccine effectiveness predominated in the U.S. toward the end of the [2019-2020] influenza season and were not included in the . Ongoing surveillance, including genetic sequencing data, is needed to monitor circulating strains in the U.S. during the 2020-2021 season.

The CDC recommends an annual flu vaccination beginning at the age of 6 months. Why is is this so important?

Tenforde: Since immune protection from vaccination declines over time, annual vaccination is needed to achieve optimal protection against influenza. Additionally, influenza viruses are constantly changing so vaccine composition is reviewed each year and updated as needed, based on the most virulent strains.

What is your take-home message to physicians?

Tenforde: A flu vaccine can make the difference between a mild infection and severe flu that ends in hospitalization or even death. A strong recommendation from a healthcare provider is a critical factor that affects whether a patient gets a flu vaccine. Most adults believe vaccines are important but some may need a reminder to get vaccinated. For patients at high risk of developing serious flu complications, such as older adults and those with certain medical conditions, flu vaccination is especially important. Healthcare providers play a key role in driving healthy actions like flu vaccination.

What impact might increased compliance with recommendations for annual influenza vaccination have on response to the global pandemic?

Tenforde: During the COVID-19 pandemic, getting influenza and other routine vaccines is vital. We've seen tremendous strain on our hospital resources due to COVID-19, so influenza vaccination along with other preventive measures, like mask use and social distancing, may prevent further strain on healthcare resources, such as hospital beds and ventilators.

What's next for your research?

Tenforde: We're continuing to look at respiratory virus infections in hospitals, including evaluating SARS-CoV-2 vaccine effectiveness. The 2019-2020 flu season highlighted that it's important to evaluate vaccine effectiveness against different genetic subgroups of influenza viruses. We're planning to apply similar methods in evaluating SARS-COV-2 vaccine effectiveness should different SARS-CoV-2 variant viruses circulate in the U.S.

You can read the study abstract here.