Immunogenicity of third dose of anti-SARS-CoV-2 vaccine co-administered with influenza vaccine: An open question.

In October 2021, the Italian Ministry of Health has planned the offer of a booster dose of anti-SARS-CoV-2 vaccine for healthcare workers (HCWs), recommending the simultaneous administration of the third anti-SARS-CoV-2 dose and the influenza vaccine. The immunogenicity and serological response of co-administration are questioned. This is a retrospective cohort pilot study. We evaluated in a sample of HCWs the serological response 1 month after the administration of the third dose, comparing it between subjects who chose for co-administration (Cominarty+Flucelvax) and subjects who preferred the administration of the anti-SARS-CoV-2 vaccine. The study population comprised 20 HCWs, 9 (45.0%) chose co-administration (Group 1), and 11 (55.0%) preferred the administration of the COVID-19 vaccine alone (Group 2). A statistical significant difference of the variation of IgG anti-spike-protein antibodies between the serological evaluation at 1 month after the third dose and the serological evaluation 1 month after the basal routine with Comirnaty between Group 1 (-4,842.9; 95%CI = -15,799.2-6,113.2) and Group 2 (9,258.9; 95%CI = 1,081.0-17,435.9; -value = 0.029) was reported. New scientific evidences are necessary to clarify this critical issue to guarantee both the best immunogenicity of COVID-19 vaccination and an high vaccine coverage for influenza vaccination.