Figure 1.

Scheme of enrollment and collected samples from healthy and type 2 diabetes mellitus (T2D) vaccine recipients. A schematic representation of groups, number of study participants and timeline of sample collection are depicted. “CoV_only” group (in grey) comprised of healthy subjects (HS) (n = 24) to whom only the third booster dose of anti-COVID-19 BNT162b2 mRNA vaccine was administered. “Flu_beforeCoV” group comprised of HS (n = 18) (in light blue) to whom the 2021–2022 seasonal cell-based quadrivalent influenza (Flu) vaccine (Flucelvax) was administered 1 month before immunization with the third booster dose of anti-COVID-19 BNT162b2 mRNA vaccine. “Flu_plusCoV” groups comprised of HS (n = 19) (in blue) and of T2D (n = 6) (in petrol green), to whom 2021–2022 seasonal cell-based quadrivalent flu vaccine (Flucelvax) was simultaneously inoculated in different limbs with the third booster dose of anti-COVID-19 BNT162b2 mRNA vaccine. Demographic and clinical information as well as serum samples from longitudinal peripheral blood withdrawals were collected at the following time points: immediately before (day 0, d0), as well as 1 day and 30 days (d1 and d30, respectively) after the third booster dose of COVID-19 mRNA vaccine in the presence or absence of the flu vaccine. Only for “Flu_beforeCoV” group sera were stored immediately before (day 0, d0) quadrivalent flu vaccine, as well as before (day 30, d30) and 1 and 30 days after (d30 + 1 and d30 + 30, respectively) the third booster dose of the COVID-19 mRNA vaccine.