Older adults who received more than one influenza vaccine over recent seasons had a lower risk for developing severe influenza than those who received only one vaccine, according to data published today in the Canadian Medical Association Journal.
Itziar Casado, MD, researcher at the Public Health Institute of Navarra in Spain, and colleagues said the findings “reinforce recommendations for annual vaccination for influenza in older adults.”
According to the researchers, data on the effectiveness of repeated influenza vaccinations are conflicting. Although some research has shown that influenza vaccination during previous seasons may provide protection against influenza during subsequent seasons, other studies have demonstrated a negative effect. However, no trials have examined the impact of repeated vaccination on severe disease, the researchers noted.
To better clarify the association, Casado and colleagues conducted a case-control study in 20 hospitals in Spain during the 2013-2014 and 2014-2015 seasons. Their analysis included 728 adults aged 65 years or older admitted to the hospital with severe (n = 130) or nonsevere influenza (n = 598). The researchers matched these patients with 333 and 1,493 controls, respectively, who were admitted to the same hospital for other reasons.
The dominant influenza virus circulating in Spain during the 2013-2014 season was A(H1N1) and A(H3N2) during the 2014-2015 season. These viruses matched strains used in the influenza vaccines for both seasons, according to the researchers. Overall, 325 patients with influenza had A(H1N1), 256 had A(H3N2), 106 had a nonsubtyped A virus, and 39 had influenza B. Information on virus type was lacking in two patients.
The data showed that influenza vaccination during the current season and any three previous seasons was 31% (95% CI, 13%-46%) effective in preventing hospital admissions for nonsevere influenza, 74% (95% CI, 42%-88%) effective in preventing admission to the ICU for influenza and 70% (95% CI, 34%-87%) effective in preventing mortality compared with no vaccination. There was no association between vaccination during the current season only and protection against severe disease (–1%; 95% CI, –147 to 59).
Further analysis showed that vaccination in the current and any three previous seasons reduced the odds of severe disease by 55% (adjusted OR = 0.45; 95% CI, 0.26-0.76), ICU admission by 65% (aOR = 0.35; 95% CI, 0.17-0.7) and morality by 56% (aOR = 0.44; 95% CI, 0.23-0.86). In contrast, vaccination in the current season only increased the risk for morality (aOR = 3.35; 95% CI, 1.06-10.58).
“Repeated vaccination for influenza was highly effective in preventing severe and fatal infection caused by influenza in older adults,” the researchers concluded. “This high effectiveness may be explained by the combination of prevented admissions to the hospital for influenza and reduced disease severity in patients admitted to the hospital for influenza. Because severe cases of influenza may be prevented by two mechanisms, the effectiveness of vaccination against severe influenza may be greater than that for mild cases, and the benefit of influenza vaccination may be greater than that estimated in previous studies.” – by Stephanie Viguers
Disclosures: Casado reports receiving research grants from the Horizon 2020 Program of the European Commission and the Carlos III Health Institute through the European Regional Development Fund. Please see the study for all other authors’ relevant financial disclosures.