key: cord-0997251-7rr65ik6
authors: Asamoah-Boaheng, Michael; Goldfarb, David M.; Prusinkiewicz, Martin A.; Golding, Liam; Karim, Mohammad Ehsanul; Barakauskas, Vilte; Wall, Nechelle; Jassem, Agatha N.; Marquez, Ana Citlali; MacDonald, Chris; O’Brien, Sheila F.; Lavoie, Pascal; Grunau, Brian
title: Determining the optimal SARS-CoV-2 mRNA vaccine dosing interval for maximum immunogenicity
date: 2022-03-03
journal: bioRxiv
DOI: 10.1101/2022.03.01.482592
sha: 3779ab299517aed3867358eff810d180b13184b8
doc_id: 997251
cord_uid: 7rr65ik6

Objective Emerging evidence indicates that longer SARS-CoV-2 vaccine dosing intervals results in an enhanced immune response. However, the optimal vaccine dosing interval for achieving maximum immunogenicity is unclear. Methods This study included samples from adult paramedics in Canada who received two doses of either BNT162b2 or mRNA-1273 vaccines and provided blood samples 6 months (170 to 190 days) after the first vaccine dose. The main exposure variable was vaccine dosing interval (days), categorized as “short” (first quartile), “moderate” (second quartile), “long” (third quartile), and “longest” interval (fourth quartile). The primary outcome was total spike antibody concentrations, measured using the Elecsys SARS-CoV-2 total antibody assay. Secondary outcomes included: spike and RBD IgG antibody concentrations, and inhibition of angiotensin-converting enzyme 2 (ACE-2) binding to wild-type spike protein and several different Delta variant spike proteins. We fit a multiple log-linear regression model to investigate the association between vaccine dosing intervals and the antibody concentrations. Results A total of 564 adult paramedics (mean age 40 years, SD=10) were included. Compared to “short interval” (≤30 days), higher dosing interval quartiles (moderate: 31-38 days; long: 39-73 days and longest: ≥74 days) were all associated with increased Elescys spike total antibody concentration. Compared to the short interval, “long” and “longest” interval quartiles were associated with higher spike and RBD IgG antibody concentrations. Similarly, increasing dosing intervals increased inhibition of ACE-2 binding to viral spike protein, regardless of the vaccine type. Conclusion Increased mRNA vaccine dosing intervals longer than 30 days result in higher levels of circulating antibodies and viral neutralization when assessed at 6 months.

The coronavirus disease 2019 caused by the SARS-CoV-2 virus was declared as a 61 global pandemic on March 11, 2021 by the World Health Organization. SARS-CoV-2 vaccines 62 were subsequently developed, with high efficacy for preventing short-term disease. 1,2 The two 63 mRNA vaccines, BNT162b2 (Pfizer) and mRNA-1273 (Moderna), were tested and have been 64 approved for use as a two-dose schedule, administered 21 and 28 days apart, respectively. 65

Existing evidence indicates that extended vaccine dosing intervals for mRNA 3, 4,5 and Oxford-66

AstraZeneca 6 vaccines enhance vaccine immunogenicity. However, while previous studies have 67 investigated specific intervals, the exact optimal dosing interval for achieving maximum 68 immunogenicity is unclear. Further, while extended dosing intervals may lead to an enhanced 69 post-vaccine immune response, the trade-off is sub-total immunity in the between-dose period. 70

Thus, the shortest interval to achieve the highest long-term immune response would be the 71 optimal scenario. 72

For these reasons, we sought to investigate a range of dosing intervals and resultant 73 immunogenicity (measured at 6-months post first vaccine), focusing on the wild-type and delta 74 variant strains, as delta variants were the predominant strains at the time of blood sampling. We 75 hypothesized that the benefits in increasing immunogenicity with increasing dosing intervals 76 would plateau at a certain dosing interval, and thus sought to identify the shortest interval to 77 achieve the maximum post-vaccine immune response. 

We used samples from the COVID-19 Occupational Risks, Seroprevalence, and Immunity 83 among Paramedics in Canada (CORSIP) observational cohort study. CORSIP is a longitudinal 84 prospective study examining the workplace risks and seroprevalence of SARS-CoV-2 exposure 85 among paramedics (aged ≥ 19 years and older) working in the Canadian provinces of British 86 Columbia, Ontario, Saskatchewan and Manitoba. Study participants provided responses to an 87 administered questionnaire including past medical history (i.e., hypertension, diabetes, asthma, 88 chronic lung disease, chronic heart disease, liver disease, malignancy, and immunosuppression), 89 data pertaining to results and dates of vaccination and SARS-CoV-2 nucleic acid amplification 90 tests (NAAT), and blood samples for serology testing. All samples used in this study were 91 collected 170-190 days after the participant's first vaccine dose. The CORSIP study was 92 approved by the University of British Columbia and University of Toronto research ethics 93

For this investigation we included samples from CORSIP participants who: (1) had two doses of 96 either BNT162b2 or mRNA-1273 vaccines; and, (2) provided a blood sample 6-months ± 10 97 days (170-190 days) after the first vaccine. We excluded those with evidence of previous 98 COVID-19 (defined as a positive NAAT COVID-19 test or presence of anti-nucleocapsid 99 antibodies in the blood [Roche, IND, USA] 7 ) given the known differential impact on antibody 100 responses post-vaccination. 8 We also excluded cases for whom the second vaccine was >130 101 days after the first, in order to separate the vaccine and the blood collection date by >40 days, 102

given the expected post-vaccine antibody surge. 6, 9 103 Serological Testing 104

All samples were tested with: (1) the Roche Nucleocapsid Elecsys Anti-SARS-Cov-2 (Roche, 105 IND, USA) assay (to confirm eligibility); (2) the quantitative Roche Spike Elecsys Anti-SARS-106

Cov-2 S assay (Roche, IND, USA), which measures spike total antibody concentrations; (3) 

We performed statistical analyses with SPSS (IBM, USA) and R (V. 3.6.1) software. For 120 participant characteristics we described categorical variables as counts (with percentages) and 121 continuous variables as mean (with standard deviation [SD]) or median (with interquartile range 122

[IQR]) values. To visualize the trend of the antibody response over the range of dosing intervals, 123

we created: (1) error plots of the means (with 95% confidence intervals) of outcomes values; 124 and, (2) created scatter-plots with cubic spline curves (with 95% confidence intervals). 10 125

To investigate the relationship between vaccine dosing intervals and antibody outcomes, we 126 divided participants into vaccine dosing interval quartiles: "short interval" (first quartile), 127 "moderate interval" (second quartile), "long interval" (third quartile), and "longest interval" 128 (fourth quartile). We compared characteristics and outcomes between quartiles using ANOVA to 129 test for differences in the mean ages of participants, Chi-square test to test for differences 130 between categorical variables, and the Kruskal Wallis H test for the difference between outcome 131

We fit a multiple log-linear regression model to estimate the association between the dosing 133 interval quartiles (with the first quartile as the reference) and both the primary and secondary 134 outcome variables, adjusted for type of vaccine, sex, age, and medical history (each past medical 135 history category was modeled as a binary variable). As sensitivity analyses, we repeated the 136 primary analyses incorporating an interaction term with dosing interval category and vaccine 137 type. We also repeated the primary analysis within subgroups based on vaccine type. 138 139

The study included a total of 564 adult paramedics, with a mean age of 40 years; 253 (45%) were 141 female sex at birth and 79 (14%) were from Asia and other ethnic groups (Table 1) . Overall, 469 142 (83%) received two doses of the BNT162b2 vaccine and 95 (17%) received two doses of the 143 mRNA-1273 SARS-CoV-2 vaccine. 144

The median vaccine dosing interval was 38 days (IQR 31-73), and thus cases were classified 145 "short interval" (≤30 days), "moderate interval" (31-38 days), "long interval" (39-73 days), and 146 "longest interval" (≥74 days). Table 1 shows participant characteristics, which were similar 147 across dosing interval quartiles. All outcome measures increased with each vaccine dosing 148 interval quartile. Figures 1, 2 , 3 and Supplementary figures S1, S3, S5, S7, S9, S10, S12-S13 The primary regression analyses demonstrated that, with reference to the first quartile, higher 154 quartiles were all associated with increased log Elecsys total spike antibody concentrations 155 (Table 2) . Models examining secondary outcomes of V-PLEX spike and RBD IgG showed that 156 "long" and "longest" quartiles, with reference to the first quartile, were associated with 157 significantly increased log antibody concentrations. Models examining secondary outcomes of 158 ACE-2 inhibition to the wild type spike protein and Delta strain spike variant proteins showed 159 similar results (see Table 3 ). 160

In the sensitivity analysis, we introduced an interaction term of vaccine type and the vaccine 161 dosing interval into all regression models; however, none were significant (Tables S1 and S2). 162

We repeated the models in subgroups defined by vaccine type; although limited by smaller 163 sample sizes, results were similar. 164

We investigated 6-month immunogenicity among 564 adults who received SARS-COV-2 166 mRNA vaccines, to identify the vaccine dosing interval for achieving maximum 167 immunogenicity. We found that longer vaccine dosing intervals resulted in higher measures of 168 immunogenicity 6-months post-vaccination, including antibodies against spike protein and its 169 RBD domain, as well as surrogate measures of viral neutralization. Whereas we hypothesized 170 that the rise in immune measures would plateau with a certain vaccine dosing interval, antibody 171 levels continued to rise with extended vaccine intervals up to 130 days. This data will assist 172 clinicians and policy-makers by demonstrating that increasing vaccine dosing intervals (up to 173 130 days) results in corresponding increased 6-month immunogenicity. 174

Our data demonstrated a continuous increasing trend of immunogenicity as the vaccine dosing 175 interval increased, which was consistent with all immune measures. Our regression models were 176 consistent. In deciding the optimal vaccine dosing interval, there is a trade-off between decreased 177 immunity between the first and second dose with longer dosing intervals, and enhanced 178 immunity in the post-second dose period. We had hoped to identify a plateau in the 179 immunogenicity measures with increasing dosing intervals-however this was not apparent. 180

Instead, public health providers need to balance these two competing priorities, the balance of 181 which may change based on the incidence of COVID-19 in the community, and the risk of 182 severe disease in individual patients. Of additional consideration is the availability of booster 183 (third) vaccine dosing-i.e. if readily available then decreased immunity in the post-second dose 184 period among those with short vaccine dosing intervals can be addressed by a booster dose. 11, 12 185

Community level-immunity may also play a role in vaccine dosing interval decisions. Previous 186 modeling studies have demonstrated that extending SARS-CoV-2 vaccine dosing intervals 187 results in reduced cumulative mortality. This is due to earlier access to the first vaccine dose, 188 even without considering the long-term benefit of post-second dose enhanced 189 immunogenicity. 13,14 Global-level immunity and vaccine distribution also deserve consideration. 190

While booster doses may serve as a method to augment waning immunity in the months after the BNT162b2 vaccine up to 12 weeks generated higher antibody levels with 3.5-fold higher peak 201 antibody values than the standard 3-week interval. Additionally, higher two dose vaccine 202 effectiveness was observed when more than 6 weeks dosing interval was adopted between 203

BNT162b2 doses compared to the standard schedule. 19 respectively. Few countries, including the United Kingdom (UK) and parts of Canada have 208 approved guidelines for delaying the second dose schedules of the vaccines to up to 12 and 16 209 weeks respectively 21, 22 . In contrast, the world health organization (WHO) has recently 210 recommended an interval of at least 6 weeks between same mRNA vaccines. 23 The existence of 211 these varying dosing interval strategies raises some concerns as to which dosing interval is 212 optimal in achieving maximal immunogenicity. Our study sought to investigate this phenomenon 213 to identify the minimum optimal threshold to improve immunity against COVID-19; we found 214 that increasing dosing intervals resulted in a continual increase in immunogenicity measures at 6 215 months. 216

This was an observational study, and it is possible that unaccounted for confounders affected our 218 results. We measured antibody concentrations at 6-months, as we believed that immunogenicity 219 at a standard time interval after the beginning of the vaccination series is the most clinically 220 relevant end-point. Although we excluded samples for which the second vaccine-to-blood 221 collection interval was <40 days, due to the expected post-vaccination antibody surge, it is 222 possible that proximity of the second dose to our blood collection time affected the results. We 223 excluded cases with previous COVID-19 using nucleocapsid serological testing-which, 224 although a high-performance test 7 , may have mis-classified samples. Our study participants 225 consist of paramedics that were primarily mid-aged and white, and thus these data may not be 226 generalizable to other groups. Although antibody levels have been correlated with vaccine 227 effectiveness 24 , we did not examine clinical endpoints such as break-through infections or severe 228 disease. 229

Extending the interval between SARS-CoV-2 mRNA vaccine doses from 31 to 123 days is 231 associated with a continuous increase in immunogenicity when measured at 6-months. 232

The authors declare no conflict of interest. Determining the optimal SARS-COV-2 mRNA vaccine dosing interval for maximum immunogenicity All models were adjusted for type of vaccine administered, age, "sex at birth", and comorbidities (hypertension, diabetes, asthma, chronic lung diseases, heart diseases, kidney diseases, liver diseases, cancer, and immune suppressed). Low dosing interval (<31 days) was set as a reference category Model 1: Outcome variable in model 1 is "Wild-type"; Model 2: Outcome variable in model 2 is "Spike (AY.1)"; Model 3: Outcome variable in model 3 is "Spike (AY.12)"; Model 3: Outcome variable for model 4 is "Spike (AY.12)"; *Indicates model is significant at p<0.05 All models were adjusted for type of vaccine administered, age, "sex at birth", and comorbidities (hypertension, diabetes, asthma, chronic lung diseases, heart diseases, kidney diseases, liver diseases, cancer, and immune suppressed). Low dosing interval (<31 days) was set as a reference category

Geometric mean; gSD: Geometric standard deviation; SD: Standard deviation

Chi-square test was used to test for the significant difference between the categorical variables and the dosing interval, ANOVA for the differences in the mean ages of participants

All models were adjusted for type of vaccine administered, age, "sex at birth", and comorbidities (hypertension, diabetes, asthma, chronic lung diseases, heart diseases, kidney diseases, liver diseases, cancer, and immune suppressed). Low dosing interval (<31 days) was set as a reference category