By Seema Gupta, MD, MSPH

Clinical Assistant Professor, Department of Family and Community Health, Joan C. Edwards School of Medicine, Marshall University, Huntington, WV

Dr. Gupta reports no financial relationships relevant to this field of study.

SYNOPSIS: In a case-control study, among older adults, repeated vaccination for influenza was twice as effective in preventing severe influenza compared to non-severe influenza in patients who were admitted to the hospital.

SOURCE: Casado I, Dominguez A, Toledo D, et al. Repeated influenza vaccination for preventing severe and fatal influenza infection in older adults: A multicentre case-control study. CMAJ 2018;190:E3-E12.

Although seasonal influenza can cause mild to severe illness, serious illness, including hospitalization and death, occurs more frequently among older adults. In fact, it has been established that during most seasons, people ≥ 65 years of age bear the greatest burden of severe influenza. For instance, it has been estimated in recent years that 71-85% of seasonal influenza-related deaths have occurred in people ≥ 65 years of age and 54-70% of seasonal influenza-related hospitalizations have occurred among people in that age group.1

Based on data indicating that influenza vaccination programs produce a substantial health benefit in terms of averted cases, clinic visits, and hospitalizations, since 2010, the CDC and the CDC’s Advisory Committee on Immunization Practices (ACIP) have recommended routine annual influenza vaccinations for all persons aged ≥ 6 months who do not have contraindications.2 While vaccine effectiveness can vary each season, recent studies show that influenza vaccination reduces the risk of illness by 40-60% among the overall population during seasons when most circulating influenza viruses are well-matched to the vaccine. For persons ≥ 65 years of age, any age-appropriate inactivated influenza vaccine formulation (standard-dose or high-dose, trivalent or quadrivalent, unadjuvanted or adjuvanted) or recombinant influenza vaccines are acceptable options. ACIP makes no preferential recommendation for any specific vaccine product.3

With aging, several factors may interfere with a robust vaccine response in older adults, including the effects of immunosenescence and concomitant major chronic conditions. However, recent data demonstrate that vaccination might reduce the severity of illness among people who are vaccinated but still become ill, including reduced deaths, ICU admissions, ICU length of stay, and overall duration of hospitalization among hospitalized influenza patients, especially among patients ≥ 65 years of age.4 But while influenza vaccination in previous seasons may retain some preventive effectiveness, there is a lack of data on the effectiveness of repeated influenza vaccination in averting severe influenza in the elderly.

Casado et al conducted a case-control study during the 2013-14 and 2014-15 influenza seasons to assess the effectiveness of vaccination in preventing influenza among community-dwelling adults (age ≥ 65 years) who were admitted to one of 20 hospitals in Spain for laboratory-confirmed influenza (130 inpatients with severe influenza and 598 inpatients with non-severe influenza). Cases were matched with inpatient controls by sex, age, hospital, and admission date. Researchers compared vaccinated patients with patients who were unvaccinated in the current and previous three seasons.

The adjusted effectiveness of vaccination in the current and any previous season was 31% (95% confidence interval [CI], 13-46%) in preventing admission to the hospital for nonsevere influenza, 74% (95% CI, 42-88%) in preventing admissions to the ICU, and 70% (95% CI, 34-87%) in preventing death. There was no significant effect on cases of severe influenza for vaccination in the current season only. Among inpatients with influenza, vaccination in the current and any previous season reduced the risk of severe outcomes (adjusted odds ratio, 0.45; 95% CI, 0.26-0.76).


The overall effect of seasonal influenza can vary from year to year and is based on several factors, including match of the vaccine strains to the circulating viruses. However, it is clear that influenza places a substantial burden on people’s health and the U.S. economy each year. The CDC estimates that influenza has resulted in between 9.2 million and 35.6 million illnesses, between 140,000 and 710,000 hospitalizations, and between 12,000 and 56,000 deaths annually since 2010, with the elderly bearing the highest burden.5

Casado et al demonstrated that repeated vaccination for seasonal influenza in older adults may be highly effective in preventing severe and fatal infection caused by influenza. As this was observed mainly in patients who were vaccinated in both the current and previous seasons, the study clearly highlights another reason for annual influenza vaccination in older adults, thus reinforcing the existing ACIP recommendation.


  1. Kostova D, et al. Influenza illness and hospitalizations averted by influenza vaccination in the United States, 2005-2011. PLoS One 2013;8:e66312.
  2. Fiore AE, et al. Prevention and control of influenza with vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2010. MMWR Recomm Rep 2010;59:1-62.
  3. Grohskopf LA, et al. Prevention and control of seasonal influenza with vaccines: Recommendations of the advisory committee on immunization practices — United States, 2017-18 influenza season. MMWR Recomm Rep 2017;66:1-20.
  4. Arriola C, et al. Influenza vaccination modifies disease severity among community-dwelling adults hospitalized with influenza. Clin Infect Dis 2017;65:1289-1297.
  5. Rolfes MA, et al. Annual estimates of the burden of seasonal influenza in the United States: A tool for strengthening influenza surveillance and preparedness. Influenza Other Respir Viruses 2018;12:132-137.