Clinical relevance of increased antibody titres in older adults upon vaccination with squalene-adjuvanted versus non-adjuvanted influenza vaccines

In older adults, the serum antibody response to inactivated influenza vaccine (IIV) is often lower than in adolescents and non-elderly adults which may translate into suboptimal protection against influenza. To counteract this expression of immunosenescence, the use of adjuvanted IIV formulations has been explored. Four recent studies (three meta-analyses and one clinical trial) found an antibody increase of up to 1.5-fold in older adults, when a squalene-adjuvanted (MF59™) IIV was used. The clinical relevance of this increase may well continue to be a matter of debate. We would favour a threshold of 1.5 to consider an adjuvanted vaccine formulation superior to standard aqueous IIV because it exceeds the inevitable variation of antibody responses to non-adjuvanted IIV. It is also the same as the upper FDA equivalence limit for IIV lot-to-lot consistency. A corresponding threshold for the seroresponse rate difference could then be +5%.