Table 4.

Outcomes of the included studies based on diabetic characteristics.

Study (Author, year)	Assessed variables	Association between the variables and immunogenicity
Ali,H. et al, 2021 (43)	Age	Age (above or below 60) had no significant interaction with the effect of DM on antibody response (P=0.103).
	Gender	Gender did not show any significant interaction with the effect of DM on antibody response (P=0.634).
	BMI	BMI (above or below 30) had no significant effect on the association between DM and antibody response (P=0.563)
	Hypertension	Hypertension had no significant interaction with the effect of DM status on antibody response (P=0.393).
	Previous COVID infection	Previous COVID infection, did not significantly affect the antibody response among patients with DM compared to patient without DM.
Singh A. K. et al, 2021 (34)	Duration of diabetes	Percentage of positive seroconversion was 81.8% and 100% among patients with DM duration < 5 years, 67.4% and 76.7% among those with 5–10 years’ duration and 73.7% and 92.9% among patients with over 10 years of DM, after first and second dose respectively. There was no statistically significant difference between the subgroups.
	Glycemic control	Percentage of seroconversion was 72.7% and 84.3% in DM patients with optimum glycemic control and 0% and 100% in those with poor control, after first and second dose respectively. There was no statistically significant difference between the subgroups.
	Diabetes management	Percentage of seroconversion was 87.5% in DM patients with monotherapy, 68.6% and 93.3% and 81.2% in those with combination therapy, 100% among patients on insulin and 60% and 80% among DM patients with no medication, after first and second dose respectively. There was no statistically significant difference between the subgroups.
Marfella, R. et al, 2022 (32)	Glycemic control and HbA1c	Patients with poor glycemic control (HbA1c >7%) has significantly lower levels of neutralizing antibody levels compared to patients with good glycemic control (HbA1c <7%).
Papadokostaki, E. et al, 2022 (38)	Age	Twenty-one days after the first dose, age was significantly correlated with RBD-IgG levels (r = -0.327, p = 0.020), however this association was no longer significant 7-15 days and 70-75 days after the second dose of the vaccine.
	Duration of diabetes	There were no significant correlation between duration of diabetes antibody response in patients with DM after the first or the second dose of the vaccine.
	HbA1c	HbA1c had no significant correlation with antibody response in patients with DM status after the first or the second dose of the vaccine.
Sourij C. et al, 2022 (35)	Age	Age had a moderate to strong negative correlation with antibody response (r= -0.45, P < 0.001) in patients with DM. This association was significant and stronger among patients with T1DM compared to patients with T2DM (r= -0.53, P < 0.001 vs. r= -0.20, P=0.087).
	Gender	Gender was not significantly associated with antibody response in patients with DM.
	BMI	BMI of patients with DM had a weak negative correlation with antibody response (r = -0.18, P = 0.027).
	Type of DM	T1DM patients had higher seroconversion compared T2DM patients (P=0.013), however this association did not remain significant after adjusting for age and sex.
	Glycemic control and HbA1c	Seroconversion was not significantly associated with glycemic control (HbA1c cut off point=58mmol/l) among either T1DM or T2DM patients. Antibody response in patients with DM was not significantly correlated with either baseline HbA1c levels (r = 0.07, P = 0.398) or changes of HbA1c levels between baseline and the follow-up visit after the second dose of the vaccine (r = 0.06, P = 0.509).
	Insulin therapy	Insulin therapy was not significantly associated with seroconversion in patients with T2DM.
	Duration of diabetes	Duration of diabetes was not significantly associated with antibody response in patients with DM.
	GFR	There was a significant positive association between eGFR and antibody response among patients with DM (r=0.28, P=0.001).