Flu shot efficacy in older adults depends on vaccine-circulating strain matching

For older adults, the efficacy of influenza vaccination is associated with vaccine-circulating strain matching, as reported in a study. If the strains are all matched, the vaccine can reduce the risk of hospitalization for community-acquired pneumonia (CAP) by up to 28 percent.

Using Taiwan’s National Health Insurance Research Dataset, researchers conducted a case-crossover study design where each person served as their own control (ie, individuals were a ‘case’ in vaccinated years and a ‘control’ in unvaccinated years). The study periods were the 2006/2007 season, when virus-vaccine strains were matching, and the 2007/2008 season, when virus-vaccine strains did not match.

The participants were aged >65 years and grouped into two: those admitted for CAP during the prevaccination period (admitted, n=311) and those who were not (nonadmitted, n=572,432). The outcome was hospitalization for CAP during the influenza period.

Influenza vaccination showed no protective association with the outcome in the admitted group. Nevertheless, the sample size was small, thus limiting the drawing of conclusions.

In the nonadmitted group, on the other hand, influenza vaccination was associated with a lower likelihood of CAP hospitalization only during the vaccine-circulating strain-matched year (odds ratio, 0.72, 95 percent confidence interval, 0.64–0.83). However, vaccination was not protective in the subgroup of individuals with a Charlson Comorbidity Index score of >2, suggesting that comorbidity reduced vaccine efficacy.

In light of the findings, older adults should be urged to receive annual influenza vaccination and in combination with nonpharmaceutical interventions to reduce the risk of CAP.