Immune Response to Vaccination against COVID-19 at Different Second-Dose Intervals and Their Associations with Metabolic Parameters

COVID-19, caused by SARS-CoV-2, has led to substantial global mortality. Rapid development of vaccines, particularly mRNA-based platforms, has reduced disease severity and deaths. mRNA vaccines encode the spike protein and induce anti-spike antibodies, while infection induces anti-nucleocapsid antibodies, enabling differentiation of infection versus vaccination. Standard mRNA vaccine protocols require two doses spaced by about 3 weeks, but the effect of extending the interval on long-term immunity is unclear. Older age, obesity, and metabolic dysregulation (diabetes, dyslipidemia, hypertension) are major determinants of COVID-19 severity, yet their roles in vaccine-induced immune responses remain uncertain. This study evaluated whether the interval between the first two BNT162b2 doses and metabolic parameters affect the humoral (anti-spike) response to vaccination.

The paper notes prior evidence that extended dosing intervals for mRNA vaccines can enhance peak antibody responses, though the long-term impact is uncertain. It highlights established associations of age, obesity, and metabolic dysregulation with severe COVID-19 and mixed findings regarding the influence of BMI and other metabolic factors on vaccine-induced humoral responses. The discussion contextualizes results with studies showing higher peak titers after extended intervals and reports linking adiposity and age to reduced antibody responses, while acknowledging studies reporting no BMI association.

Design and setting: Prospective observational study at the Clinical Research Center, Medical University of Bialystok (Poland). Participants: 67 adults (18–65 years; 21 male, 46 female) without contraindications to vaccination and without prior or incident COVID-19 (defined by positive PCR/antigen test or anti-nucleocapsid antibodies). Vaccination: BNT162b2 mRNA COVID-19 vaccine; second dose administered after 3 weeks (n=54) or 5 weeks (n=13), per national program availability. Follow-up: Antibody and clinical assessments at baseline (pre-dose 1), weekly for 7 weeks (visits 2–8), then every 4 weeks (visits 9–11), total follow-up 19 weeks after the first dose. Laboratory measures: Anti-spike antibodies (Cobas e411, Roche; double-antigen sandwich ECLIA; measuring range 0.40–250 U/mL; <80 U/mL negative, ≥80 U/mL positive). Also measured: vitamin D, aTPO, IL-6, ferritin, TSH, fT3, fT4 (Cobas e411). Biochemistry: TG, total cholesterol, HDL, LDL, creatinine, AST, ALT, homocysteine, CRP (Cobas c111 colorimetric methods). HbA1c by HPLC (Bio-Rad D-10). Anthropometry and body composition: Height (SECA 264), weight (SECA 769); BMI (kg/m2); bioimpedance (InBody 720) for skeletal muscle mass, fat mass, percent body fat, visceral fat area; waist and hip circumferences; blood pressure. Ethics: Informed consent obtained; approved by Bioethics Committee (APK.002.79.2021); conducted per Declaration of Helsinki and GCP. Statistics: Normality by Shapiro–Wilk. Group comparisons by unpaired t-test or Mann–Whitney U as appropriate; sex distribution by chi-squared. Correlations by Spearman’s rank. Significance threshold p<0.05. Analyses performed in R.

• Group characteristics: No sex distribution differences. The 5-week interval group was older and had a more unfavorable metabolic profile (higher BMI, adverse body composition, higher systolic blood pressure, and triglycerides) but similar HbA1c, total/HDL/LDL cholesterol, thyroid hormones (TSH, fT3, fT4), liver enzymes (AST, ALT), and inflammation markers (CRP, IL-6). - Antibody kinetics: Peak anti-spike titers occurred ~2 weeks after dose 2 in both groups. Maximal geometric mean titers were significantly higher with a 5-week interval vs 3-week interval (6762.20 vs 1916.53 U/mL; p<0.001; ~3.5-fold higher). At 19 weeks after dose 1, no significant difference in titers (geometric means: 1203.70 vs 934.07 U/mL; p=0.202). - Age correlations: End-of-observation titers negatively correlated with age in both groups and overall (5-week: r=−0.57, p<0.05; 3-week: r=−0.29, p<0.05; all: r=−0.26, p<0.05). In the 5-week group, age also negatively correlated with maximal titers (r=−0.74, p<0.01). - Anthropometry: In the 3-week group, end-of-observation titers negatively correlated with hip circumference (r=−0.28, p<0.05), waist circumference (r=−0.35, p<0.05), and BMI (r=−0.31, p<0.05). No significant correlations in the 5-week group or when pooling all participants. No significant associations with weight, fat content, visceral fat, or skeletal muscle mass. - Cardiometabolic markers: fT4 positively correlated with end-of-observation titers in the 5-week group (r=0.60, p<0.05) and in all participants (r=0.37, p<0.01), and with maximal titers in all participants (r=0.35, p<0.01). ALT negatively correlated with maximal titers in the 5-week group (r=−0.57, p<0.05). No significant correlations for blood pressure, HbA1c, lipids, AST, creatinine, vitamin D3, IL-6, homocysteine, or ferritin. - Overall: Extending the second-dose interval to 5 weeks increased peak antibody response but did not affect titers 19 weeks after the first dose.

Extending the interval between BNT162b2 doses to 5 weeks substantially enhanced peak anti-spike antibody titers, aligning with prior reports, but this advantage did not persist at 19 weeks post–first dose. The clinical relevance of higher peak titers remains uncertain given limited evidence linking peak humoral responses to protection or cellular immunity, which was not measured here. Age consistently showed an adverse association with humoral responses, corroborating known immune senescence effects. Indicators of adiposity (BMI, waist, hip) were negatively associated with end-of-observation titers in the 3-week group, consistent with literature suggesting obesity may impair vaccine responses; this pattern was not observed in the 5-week group or overall, potentially reflecting group differences and the small size of the 5-week cohort. Notably, fT4 levels positively associated with both end-point and peak titers, suggesting thyroid hormone status may modulate vaccine-induced humoral responses; this requires validation in larger cohorts and across broader thyroid function spectra. Traditional cardiometabolic risk markers (HbA1c, lipids, blood pressure), inflammation markers (CRP, IL-6), homocysteine, and ferritin did not show meaningful associations with antibody responses in this cohort. Policy-wise, dose-interval extension may transiently boost humoral immunity and could be considered in contexts of vaccine scarcity, though implications for variant evolution and protection durability warrant careful evaluation.

Administering the second BNT162b2 dose after 5 weeks produced higher peak anti-spike antibody titers than the standard 3-week interval but did not alter titers at 19 weeks after the first dose. Age, BMI, and waist/hip circumferences were associated with lower antibody responses, while fT4 positively associated with both peak and total responses; homocysteine and ferritin were not associated with humoral responses. These findings inform optimization of dosing intervals and highlight potential influences of anthropometry and thyroid hormone status on vaccine-induced immunity. Further studies with larger cohorts, balanced group characteristics, and assessments of cellular immunity are needed.

• Small sample size, particularly in the 5-week interval group (n=13), limits statistical power and generalizability. - Baseline differences: the 5-week group was older and had a less favorable metabolic profile than the 3-week group, potentially confounding comparisons. - Only humoral responses were measured; cellular immune responses were not assessed. - Follow-up limited to 19 weeks after the first dose; longer-term durability not evaluated.