During the 2015-2016  influenza  season in the United States, vaccination halved the risk of influenza-associated hospitalization among adults, shows data  published online  December 14 in the  Journal of Infectious Diseases .

"Vaccine protection was similar across influenza types and comparable to that observed for influenza-associated hospitalization among European adults," write Jill M. Ferdinands, PhD, from the US Centers for Disease Control and Prevention (CDC), Atlanta, Georgia, and colleagues.

Vaccination against influenza virus is considered important to improve protection against severe influenza outcomes by reducing disease severity after infection. However, studies have failed to consistently confirm this protection, and evidence demonstrating the effectiveness of influenza vaccination for prevention of severe illness remains limited.

Influenza Vaccine Performance

To further improve understanding of influenza vaccine performance in preventing the most severe outcomes, CDC launched the prospective US Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN) in 2015.

The case-control study enrolled adults aged 18 years and older who were admitted to one of eight US hospitals with acute respiratory infection. Patients who tested positive for influenza by polymerase chain reaction test were cases and those who tested negative were controls.

During the first year, Ferdinands and colleagues analyzed data from 1467 patients (influenza cases, n = 236; controls, n = 1231).   Among the influenza cases, 192 patients had influenza A, and of these, 180 patients had influenza A (H1N1) pdm09, the predominant strain circulating at the time. The remaining 44 patients had influenza B.

Most of the patients were at risk of serious influenza because of comorbidity or age, the authors note. The mean age of influenza case patients and control patients was 57 years and 58 years, respectively. And 34% of case patients and 38% of controls were aged 65 years or older.

High risk medical condition

"Ninety-three percent of study participants had one or more high risk medical condition," the authors add, "with heart disease (53%), diabetes (36%), and renal disorders (36%) the most common."

According to the authors, 50% of influenza case patients and 70% of controls were vaccinated. In terms of preventing hospitalization because of influenza, the study showed vaccination was 51% (95% CI, 29-65) effective for influenza A (H1N1) pdm09 and 53% (95% CI, 11-76) effective for influenza B.

Vaccination was also protective for all age groups (18-49 years, 50-64 years, and 65 years and older). However, Ferdinands and colleagues emphasize that data from a single season may not be generalizable to other seasons in which influenza A H3N2 predominates.

"The US Flu VE network detected issues regarding the reduction and / or loss of effectiveness of live, attenuated influenza vaccines in recent years," they write, adding that VE against influenza A H3N2 has been low to absent.

Public health benefits

Nevertheless, Keitel and Atmar stress that clinicians should not be discouraged from recommending immunization because even modest reductions in risk of developing influenza among vaccinated persons offer significant public health benefits.

For example, despite the relatively low estimated 40% influenza VE for the 2016-2017 season, "an estimated 5.29 million illnesses, 2.64 medical visits, and 84,700 hospitalizations were averted," they write. "Results from studies conducted by HAIVEN are expected to contribute to our understanding of the value of influenza vaccination," the editorialists concludes.