Do Past Flu Vaccines Provide Protective Benefits in Current Seasons?

the MPR take:

Few studies on influenza vaccine effectiveness have evaluated the effect of vaccinations received in prior seasons, but a new study suggests that current and previous-season vaccination may generate similar levels of protection against influenza. From the 2004–2005 influenza season through 2012–2013, observational studies of community-dwelling residents were conducted and participants were recruited during a clinical encounter for acute respiratory illness (ARI). A total of 7,315 individuals ages ≥9 with medically attended ARI were included in the analysis over the eight influenza seasons (1056 with H3N2; 650 with influenza B, 3 co-infected with both), with an average age of 27 and 40 for influenza B and H3N2, respectively. Current- and previous-season vaccinations were shown to be equally effective against medically attended H3N2 and B virus-associated illnesses, regardless of prior vaccination history; the vaccine-induced protection was greatest among those not vaccinated during the prior five years. An observed protection was seen among individuals vaccinated during the previous (but not current season), which could indicate some residual protection. Regardless of whether the influenza vaccine offers long-term protective effects, it is still recommended for all persons ages ≥6 months each and every year.

Recent studies suggest that influenza vaccination in the previous season may influence the effectiveness of current-season vaccination, but this has not been assessed in a single population over multiple years.