Memory B cell and humoral responses elicited by Sputnik V in naïve and COVID- 1 19-recovered vaccine recipients 2

The development of effective vaccines against SARS-CoV-2 remains a global health priority. Despite 14 extensive use, the effects of Sputnik V on B cell immunity need to be explored in detail. We show that B 15 memory cell (MBC) and antibody responses to Sputnik V were heavily dependent on whether the 16 vaccinee had a history of SARS-CoV-2 infection or not. In vitro stimulated MBCs from previously infected 17 recipients of Sputnik V secreted a significant amount of anti-RBD IgG both on days 28 and 85 from the 18 beginning of vaccination. These antibodies demonstrated robust neutralization of the Wuhan Spike- 19 pseudotyped lentivirus. In the naïve group of vaccinees, the level of anti-RBD IgG secretion was five- to 20 six-fold reduced compared to that of the recovered group, and maximum virus neutralization (Wuhan 21 spike) was achieved only on day 85. Sera from all the recovered and most naïve Sputnik V recipients 22 were neutralizing against the ancestral Wuhan and mutant B.1.351 viruses. Thus, our in-depth analysis 23 of MBC responses in Sputnik V vaccinees complements traditional serological approaches and may 24 provide important outlook into future B cell responses upon re-encounter with the emerging variants of 25 as ± IQR. significant difference between groups determined using 523 the Kruskal–Wallis test, *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001, ns = not significant.


Introduction 27
Presently, the therapeutic options for COVID-19 patients remain limited, emphasizing the necessity of 28 concerted mass vaccination campaigns to counteract the pandemic. An ideal vaccine must induce long-29 lasting protective cellular and humoral immunity, which should translate into reduced rates of infection 30 and mortality. Importantly, an ideal vaccine should, in addition, retain activity against emerging viral 31 lineages. 32 Three anti-SARS-CoV-2 vaccines, Moderna mRNA-1273, BioNTech BNT162b2, and Janssen 33 Ad26.COV2.S, are now being extensively used around the world and have received the most public 34 attention and validation (Goel et al., 2021), while less is known about immunity after Sputnik V 35 vaccination (Logunov et al., 2021). Although humoral responses to Sputnik V have recently been 36 reported for a limited number of study participants (Ikegame et al., 2021;Rossi et al., 2021), data 37 regarding the B cell response in Sputnik V-vaccinated subjects are presently lacking. Clearly, these data 38 are central to the comprehensive assessment of current vaccines (Goel et al., 2021), provide important 39 clues to the development of new vaccines, and impact the epidemiological models of immunity. 40 In late 2020, multiple SARS-CoV-2 lineages were reported across the globe, of which В. 1.1.7, 41 В.1.351, В. 1.1.248, and В.1.617 are now referred to as variants of concern (VOCs) (Davies et al., 2021;42 Tegally et al., 2021). В. 1.351 and В.1.1.248 display profound resistance to most of the approved highly 43 potent neutralizing monoclonal antibodies, as well as to the polyclonal antisera induced by infection 44 with the ancestral SARS-CoV-2 and by all the vaccines developed to date (Dejnirattisai, Zhou, Supasa, et 45 al., 2021;Z. Wang, Muecksch, et al., 2021;Wibmer et al., 2021;Zhou et al., 2021). It is generally believed 46 that it was the emergence and rapid spread of VOCs that are largely responsible for the documented 47 cases of SARS-CoV-2 re-infection (Kustin et al., 2021;Naveca et al., 2021;Ortloff, Harsch 2021;Tegally et 48 al., 2021). Specifically, post-vaccination antisera from Moderna and BioNTech vaccinees were 6.5-40-49 fold less potent against the В.1.351 VOC, but most typically neutralization was reduced 3-8-fold 50 compared to that of the Wuhan-1 SARS-CoV-2 strain (Abdool Karim, de Oliveira 2021; Garcia-Beltran et 51 al., 2021). So far, qualitative and quantitative data on the VOC neutralization by Sputnik V-induced 52 antisera have been very limited (Gushchin et al., 2021;Ikegame et al., 2021). 53 Notably, in contrast to Moderna mRNA-1273, BioNTech BNT162b2, and Janssen Ad26.COV2.S 54 vaccines that were designed to present the SARS-CoV-2 Spike protein in its pre-fusion conformation, 55 Sputnik V is based on a native Spike protein lacking such modifications. This, in turn, may underlie 56 distinct immune responses, upon cross-platform comparisons, and warrants in-depth analysis. 57 In this study, we aimed to determine i) whether Sputnik V vaccination is efficient in inducing 58 durable memory B cell ( Five individuals recruited in October-November 2020 had experienced mild COVID-19 symptoms prior to 70 vaccination (53-120 days). Although no virus-containing samples were available for these patients, it 71 must be noted that the SARS-CoV-2 viral lineage B.1.1 was predominant in Moscow at that time and it 72 was only in March 2021 that the first B.1.351-associated infections were reported in Moscow (Gushchin 73 et al., 2021). Before vaccination, no nucleocapsid (N)-or receptor-binding domain (RBD)-specific IgGs 74 were detected in the sera of naïve individuals without prior COVID-19 symptoms. In contrast, all 75 recovered recipients with self-reported COVID-19 symptoms had both N-and RBD-specific IgG prior to 76 vaccination (Supplementary figure 2). 77 RBD-specific IgM levels was most pronounced in naïve individuals at T2 (T0 vs. T2, P<0.0001), consistent 88 with the primary nature of their immune response. Taken together, these results are consistent with 89 those of previous reports exploring antibody responses to mRNA vaccines (Bos et al., 2020;Goel et al., 90 2021). 91 Next, we investigated if plasma from Sputnik V-vaccinated subjects, with or without prior 92 COVID-19 history, was active in terms of virus neutralization against the wild-type Wuhan strain WA1 93 and B.1.351 VOC. To address this question, a SARS-CoV-2 Spike-pseudotyped virus-neutralization test 94 (pVNT) was used to analyze the sera collected at T3 (Figure 1c) Figure  104 3). In contrast, in the naïve group, only one out of the total 17 samples (subject 20) displayed similar 105 neutralization potency. Nonetheless, the undiluted sera could achieve 90% neutralization in all but one 106 of the naïve samples. The neutralization activity in that exceptional sample (subject 22) could not be 107 reliably measured. However, the neutralization titers we measured in the cohort of naïve Sputnik V 108 vaccinees were significantly higher than those reported by Ikegame et al. (Ikegame et al., 2021). 109 Specifically, they were three-fold higher for the ancestral WA1 Spike (ID50 GMT 150; 95% CI -90.  248.5) and seven-fold higher for В. is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint  is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint The copyright holder for this this version posted October 17, 2021. ;https://doi.org/10.1101https://doi.org/10. /2021 Total and RBD-specific plasmablast response 134 One of the earliest manifestations of the B cell response is the emergence of circulating total and 135 antigen-specific plasmablasts, which peak around the seventh day post-immunization (Wrammert et al., 136 2008). Plasmablasts were defined here as CD3 − CD16 − CD19 + IgD − CD27 hi CD38 hi cells (Wrammert et al.,137 2008) (Figure 2a, left panel). 138 Before vaccination, the plasmablast frequencies were the same as those in normal donors but 139 increased markedly after the primary immunization (T0 vs. T1 P=0.0076 and P=0.0289 for naïve and 140 recovered individuals, respectively) ( Figure 2b). Booster immunization resulted in an insignificant 141 increase in the percentage of total plasmablasts compared to the baseline level. Positive plasmablast 142 response was detected in 81% (17/21) of cases at T1 and only in 40% (8/20) of cases at T2. 143 Subsequently, as can be seen from the measurements at T3, plasmablasts completely disappeared from 144 the circulation. 145 The dynamics of SARS-CoV-2-specific plasmablasts during vaccination were of special interest to 146 us. Since virus-neutralizing antibodies are known to predominantly target the RBD (Dejnirattisai,Zhou,147 Ginn of Sputnik V and only in eight participants (40%) after the booster immunization. In only one subject 156 from the recovered subgroup, it exceeded 1%, which is still significantly lower than what was observed 157 in patients with moderate COVID-19 (Byazrova et al., 2021). 158 Since plasmablasts are antibody-secreting cells (ASCs), it is logical to detect and enumerate 159 them using an enzyme-linked immunosorbent spot (ELISpot) assay. Representative ELISpot images of 160 circulating Spike-and RBD-specific IgG ASCs are presented in Figure 2d. Historic control wells showed 161 only rare spontaneous total ASCs and no SARS-CoV-2-specific ASCs. The magnitude of the IgG ASC 162 response was the largest in the recovered group after the first vaccine dose, when RBD-and S-specific 163 IgG ASCs were detected in all vaccine recipients (median 69, IQR 27.33-772.8 and median 459, IQR 318-164 1439 for RBD-and S-specific ASCs, respectively) ( Figure 2e). In contrast, no RBD-or S-specific IgG ASCs 165 were found in all but one naïve subject after the first dose. After the second vaccination, 7 and 11 naive 166 participants (n=15) had RBD-and S-specific IgG ASCs above the baseline, respectively. The fact that after 167 the first vaccine dose, the frequencies of anti-RBD IgG ASCs were higher in SARS-CoV-2-recovered 168 individuals than in individuals in the naïve group is consistent with the idea that, in the former cohort, 169 this increase is due to the re-activation of MBCs. The SARS-CoV-2-specific IgM responses mediated by 170 the circulating ASCs were generally lower in their magnitude than the IgG responses and we did not 171 observe significant differences between the naive and recovered samples, nor between the T1 and T2 172 time points. 173 Plasmablasts are a heterogeneous population of cells that are usually subdivided into early and 174 later plasmablasts based on their ability to express the B cell receptor (BCR) on their surface and secrete 175 antibodies (Sanz et al., 2019). For the most part, plasmablasts express membrane bound BCR and 176 simultaneously secrete antibodies. However, surface BCR expression is more a characteristic of early 177 plasmablasts, while antibody-secreting capacity is more associated with later plasmablasts. Accordingly, 178 we found a modest correlation between the frequencies of RBD + plasmablasts and RBD-specific Ig (IgG + 179 IgA + IgM) circulating ASCs (Spearman's r=0.6505, P=0.0468 at T1; r=0.5574, P=0.0162 at T2) ( Figure 2f). 180 . CC-BY-NC-ND 4.0 International license It is made available under a perpetuity.
is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint The copyright holder for this this version posted October 17, 2021. ; https://doi.org/10.1101/2021.10.13.21264894 doi: medRxiv preprint Results are shown for individual samples (symbols) from naïve (n = 17) and recovered (n = 5) recipients. 195 Data are presented as median IQR. Asterisks indicate significant difference between groups 196 determined using the Kruskal-Wallis test, *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001, ns = not 197 significant. ACS, antibody-secreting cell; IQR, interquartile range; RBD, receptor-binding domain. is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint The copyright holder for this this version posted October 17, 2021. shown in Figure 3b. Even before vaccination, the recovered individuals had a noticeable number of RBD + 206 MBCs, which was higher than that of naive subjects (P=0.0362), and above the level of the negative 207 control defined by staining with an irrelevant Bet v 1 protein (0.01%). Until day 85, the level of RBD + 208 MBCs in recovered individuals remained on average stable. In naïve individuals, baseline frequencies of 209 RBD+ MBCs were observed at T0 and T1. At T2, RBD + MBCs were detected above the threshold in 37.5% 210 of naïve individuals, displayed a further increase at T3 (T1 vs. T3, P=0.0023), and approached the level 211 observed in recovered individuals. 212 Measurements of the SARS-CoV-2-specific circulating MBC numbers were supplemented with a 213 more functional ELISpot assay. In contrast to plasmablasts, MBCs are resting cells and do not secrete 214 antibodies without stimulation. For MBC activation and induction of antibody secretion, 215 immunomagnetically purified B cells were stimulated in vitro with IL-21/CD40L. These conditions mimic 216 the germinal center environment in which B cells differentiate into plasma cells (Ding et al., 2013). After 217 stimulation for 7 days, the frequencies of S-and RBD-specific ASCs were evaluated using an ELISpot 218 assay. As shown in the representative ELISpot images (Figure 3c), our protocol for polyclonal B cell 219 activation was highly efficient and resulted in the secretion of both total and SARS-CoV-2-specific 220 antibodies. In pre-pandemic control samples, the frequencies of SARS-CoV-2-specific IgG ASCs were 221 below 200 and 215 spots per million B cells were seeded in Spike-and RBD-coated wells. These values 222 served as a cut-off for positivity. The MBC-derived ASC numbers were measured at two time points, 28 223 (T2) and 85 (T3) days after the first dose of the vaccine, when MBCs become detectable (T2) and 224 undergo maturation (T3). 225 MBC-derived IgG ASCs displayed the strongest response. At T2, in all recovered (5/5) and in 226 some naive subjects (7/17) the SARS-CoV-2-specific MBC-derived ASC numbers were above the baseline; 227 however, recovered subjects had a higher level of ASCs than naïve subjects (P=0.034 and P=0.0373 for 228 RBD-and S-specific ASCs, respectively) ( Figure 3d). At T3, the numbers of ASCs in recovered vaccinees 229 remained stable, while in naïve subjects they increased 14-fold and approached the recovered group 230 level (in the naïve group T2 vs. T3, P<0.0001 and P=0.0002 for RBD-and S-specific ASCs, respectively). 231 On average, at the peak of the response, approximately 3,000 RBD-specific IgG ASCs per million B cells 232 (0.3% of total B cells) were detected, which was similar to the number of RBD + MBCs detected using 233 flow cytometry in the recovered group (median 0.23%) (Figure 3b). The dynamics of the RBD-specific IgG 234 ASC response was also consistent with the flow cytometry data. RBD-specific ASCs constituted 8-100% 235 of all S-specific ASCs (Figure 3d), suggesting that most of the MBC-derived antibodies were directed 236 against the RBD. 237 The frequencies of IgA MBC-derived ASCs targeting the Spike and RBD followed a similar 238 pattern, namely, recovered subjects responded more quickly than naïve subjects, but the responses of 239 both groups were roughly comparable at T3. Although the anti-SARS-CoV-2 IgM ASC responses were 240 above the threshold, no significant differences were found between the naive and recovered groups, or 241 between T2 and T3. Thus, our results indicate that in naive recipients, the maximum number of vaccine-242 induced MBCs is reached only 85 days after the first vaccination, and in terms of kinetics, this process 243 significantly lags behind the formation of serum antigen-specific antibodies. Thus, on day 85 after the 244 first dose of Sputnik V, almost all the vaccinees developed RBD-and S-specific MBCs. 245 . CC-BY-NC-ND 4.0 International license It is made available under a perpetuity.
is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint  is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint individuals secreted a significant amount of anti-RBD IgG both at T2 and T3 (Figure 4a). In the naïve 268 group at T3, the level of anti-RBD IgG secretion was five-to six-fold reduced compared to that of the 269 recovered group. Stimulated MBCs produced anti-RBD IgA and IgM less efficiently than IgG, and the 270 difference between samples from naïve and recovered subjects was less pronounced. 271 Next, we tested the virus-neutralizing activity of MBC-derived antibodies using a pVNT assay. 272 Since the concentration of antibodies in the supernatants was approximately two orders of magnitude 273 lower than that in the plasma, undiluted supernatants were used. MBC-derived antibodies from 274 recovered individuals inhibited pseudovirus entry in the range of 37-99% at both time points T2 and T3 275 (Figure 4b). At T2, 12/17 individuals from the naïve group were also responders in the pVNT assay 276 ( 4d). Based on these findings, it may be possible to assess the quality of antibodies that MBCs will 282 produce upon re-exposure to an antigen. 283 We observed that naïve and recovered vaccine recipients differed in several key parameters of 284 humoral and B cell immunity. To investigate if study participants could be divided into subgroups, we 285 carried out hierarchical cluster and principal component analyses. Based on the 12 humoral and B cell 286 response measurements, these analyses, which included 15 naïve and five recovered vaccine recipients 287 indicated the existence of two well-separated clusters (Figure 4e, f). The first compact cluster 288 encompassed exclusively naïve vaccine recipients; the second, somewhat loose cluster included all the 289 recovered and three naïve vaccine recipients. Thus, when several humoral and B cell parameters were 290 taken into consideration, the distinct immune profiles of the naïve and recovered vaccine recipients 291 became clearly apparent. Most interestingly, our comprehensive B cell profiling analysis has uncovered 292 the existence of two categories of vaccine recipients, namely, the high-and low-responders. This 293 highlights the underappreciated heterogeneity of the human immune response to Sputnik V, thereby 294 warranting a systematic identification of predictors and modifiers of this response. 295 . CC-BY-NC-ND 4.0 International license It is made available under a perpetuity.
is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint  shown. Two distinct clusters are indicated by the ovals. 310 Results are shown for individual samples (symbols) from naïve (n = 17) and recovered (n = 5) recipients. 311 Data are presented as median IQR. Asterisks indicate significant difference between groups 312 determined using the Kruskal-Wallis test, *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001, ns = not 313 significant. ASC, antibody-secreting cell; IQR, interquartile range; RBD, receptor-binding domain. is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint  Rossi et al., 2021). In the present study, a comprehensive analysis of B cell immunity 319 was performed in a cohort of 22 vaccinated subjects, among whom 5 had previously recovered from 320 mild COVID-19. Several parameters were analyzed, namely, (i) the serum antibody titers to RBD, (ii) 321 activity of serum antibodies in the pVNT assay with wild-type SARS-CoV-2 and its mutant variant В1.351, 322 (iii) detection of RBD-specific plasmablasts and MBCs, (iv) enumeration of circulating and MBC-derived 323 ASCs, and the (v)  This is in line with the results obtained in mRNA vaccine studies (Ebinger et al., 2021;Goel et al., 2021;332 Krammer et al., 2021;Stamatatos et al., 2021;Z. Wang, Muecksch, et al., 2021). Importantly, we, for the 333 first time, demonstrate this effect at the level of both plasmablasts and MBCs rather than just via 334 quantification of antibody responses and assessment of their neutralization activity. Combined with 335 previously reported data (Rossi et al., 2021;Sasikala et al., 2021), our results indicate that a single dose 336 of the adenovirus-based anti-SARS-CoV-2 vaccine, like mRNA-based vaccines, may be sufficient for 337 protective immunity, when used in subjects with pre-existing immunity to SARS-CoV-2. 338 Among naive vaccine recipients, the response to Sputnik V vaccination was overall slower than 339 that of recovered vaccine recipients, and a fraction of vaccinees who received both doses of Sputnik V 340 never displayed the concentration and neutralization potency of antibodies that would match the levels 341 observed in recovered individuals. Importantly, after the second dose of Sputnik V, the antisera of naïve 342 vaccine recipients showed virus-neutralizing activity against the wild-type virus in all but one individual 343 (recipient 22). On average, the antisera from naïve Sputnik V recipients were less potent in terms of 344 their neutralizing ability as compared to the antisera from the mRNA vaccines (Anderson et al., 2020;Liu 345 et al., 2021;Wu et al., 2021), yet they were on par with the numbers reported for other adenovirus-346 based vaccines (Folegatti et al., 2020;Zhu et al., 2020 The neutralizing potency of the antisera from recovered subjects against the wild-type variant 359 was more than four-fold higher than that of the naive group. Predictably, these samples were also much 360 more active against the В.1.351 variant. This is in excellent agreement with the data reported for mRNA 361 . CC-BY-NC-ND 4.0 International license It is made available under a perpetuity. is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint The copyright holder for this this version posted October 17, 2021. ;https://doi.org/10.1101https://doi.org/10. /2021 vaccines (Goel et al., 2021) and is indicative of the higher level of protection against emerging viral 362 variants in the SARS-CoV-2 pre-exposed vaccinees. 363 It is interesting to compare the B cell immunity elicited by Sputnik V vaccination with that 364 induced by natural SARS-CoV-2 infection. The dynamics of plasmablast numbers is of special interest 365 because it can be used as a predictor of successful humoral immunity (Appanna et al., 2016). We found 366 low levels of RBD + plasmablasts and circulating RBD-specific ASCs in vaccinated naive individuals 367 compared to those observed in the acute phase in COVID-19 patients (Byazrova et al., 2021;Kuri-368 Cervantes et al., 2020). Perhaps these differences are associated with the extrafollicular pathway of B 369 cell activation in the acute phase of COVID-19, which is characterized by massive plasmablast expansion 370 (Woodruff et al., 2020). It has been suggested that this pathway may contribute to the pathogenesis of 371 acute COVID-19. From this standpoint, the modest plasmablast response observed during vaccination 372 can be viewed as beneficial. 373 Unlike the plasmablast response, the MBC response was well-pronounced, and the MBC 374 numbers observed are comparable to those found in acute COVID-19 patients. This is important because 375 vaccine-induced MBCs are known to be central to the longevity of immune memory and are among the 376 first cells to produce massive amounts of antibodies upon antigen re-encounter. Although the MBC 377 numbers are informative descriptors of B cell immunity, the functional activity of the antibodies that will 378 be produced during secondary immune responses is key to our understanding of vaccine-induced 379 protection. To address this question, antigen-specific MBCs were isolated followed by single-cell 380 sequencing of Ig genes and expression of recombinant MBC-derived antibodies (Abayasingam et al., 381 2021;Dejnirattisai, Zhou, Supasa, et al., 2021;Scheid et al., 2021;Z. Wang, Schmidt, et al., 2021). 382 Alternatively, antibody secretion can be induced in cultures of polyclonally stimulated B cells. Seven-day 383 cultures of IL-21/CD40L-stimulated MBCs isolated from the blood of Sputnik V vaccinees secreted anti-384 RBD IgG at approximately the same level as MBCs from acute COVID-19 patients. 385 One of the most interesting observations we made is that the ancestral variant of SARS-CoV-2 386 could be neutralized by MBC-derived antibodies from the vast majority of Sputnik V vaccinees. Two 387 months following vaccination, the differences between the naïve and recovered vaccine recipient 388 cohorts in terms of MBC numbers and their ability to differentiate into ASCs that secrete RBD-specific 389 and virus-neutralizing antibodies, became less pronounced, unlike the differences in the neutralizing 390 titers of serum antibodies. This indicated the gradual maturation and continued evolution of the MBC 391 population. Thus, we show that in vitro stimulation of virus-specific MBCs can considerably extend the 392 traditional serological analysis of vaccinated donors. This will allow the study of the dynamics and 393 longevity of antigen-specific MBCs in the course of infection and vaccination. Using this approach, we 394 provide experimental evidence indicating that both recovered and naïve vaccinees accumulate similar 395 numbers of virus-specific MBCs at 2 months after the second dose of Sputnik V. Upon antigen 396 stimulation, these cells differentiate into ASCs that secrete virus-specific antibodies, of which a 397 significant proportion is virus-neutralizing. Based on these data, we conclude that the MBCs elicited by 398 Sputnik vaccination, both in their number and productivity, are comparable to those generated during 399 natural infection. 400 As in the scenario of natural infection, some individuals, classified as low responders, fail to 401 mount robust immune response upon Sputnik V vaccination. The reasons underlying the poor vaccine 402 immunogenicity in these subjects are presently unknown but are likely related to individual features of 403 the immune system. However, our study group was rather limited; 2 months after vaccination, one 404 recipient with the lowest levels of virus-neutralizing antibodies and other indicators of poor B cell 405 immunity developed PCR-confirmed COVID-19 (to be published). Currently, the minimum levels of 406 serum virus-binding and virus-neutralizing activity that are protective against vaccine breakthrough 407 . CC-BY-NC-ND 4.0 International license It is made available under a perpetuity.
is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint The copyright holder for this this version posted October 17, 2021. ; https://doi.org/10.1101/2021.10.13.21264894 doi: medRxiv preprint infections remain to be determined. Hence, additional studies are required to establish this minimum 408 level of immunity. 409

Materials and Methods 410
Volunteers 411 A cohort of 22 Sputnik V recipients was enrolled in December 2020 at the National Research Center 412 Institute of Immunology of The Federal Medical Biological Agency of Russia. None of the participants 413 were pregnant, immunodeficient, or receiving immunosuppressive treatment. Subjects were immunized 414 by intramuscular injection into the deltoid muscle with a 21-day interval between the doses. All subjects 415 received two doses of Gam-COVID-Vac (Sputnik V) vaccine. None of the volunteers had experienced 416 serious adverse events after vaccination. Written informed consent was obtained from each of the study 417 participants before performing any study procedures. The study protocol was reviewed and approved by 418 the Medical Ethical Committee of Institute of Immunology (#12-1, December 29, 2020). 419

Blood sample collection and processing 420
Whole-blood samples were collected into heparinized vacutainer tubes (Sarstedt, Cat. No. 04.1927) four 421 times: one day before vaccination, on day 7 after the first and the second doses of vaccine, and on day 422 85 from the start of vaccination (T0, T1, T2, and T3 time points, respectively) ( Supplementary Fig. 1). is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint The copyright holder for this this version posted October 17, 2021. ;https://doi.org/10.1101https://doi.org/10. /2021  is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint The copyright holder for this this version posted October 17, 2021. ; https://doi.org/10. 1101 were immediately used in neutralization tests or stored at -70°C for no more than a month. Viral yield 498 was quantified using titration on HEK293T-hACE2 cells. 499 Prior to analysis, plasma samples were heated for 30 min at 56°C to inactivate complement. After that, 500 serial two-fold plasma dilutions ranging 1:10 -1:1280 were prepared in a 96-well plate and 20,000 501 lentiviral particles were added in an equal volume of Opti-Mem supplemented with 2.5% of heat-502 inactivated FBS. Plasma and viral particles were co-incubated for 30 min at 37°C, and added to HEK293T-503 hACE2 cells. 72 hours following transduction, the percentage of transduced cells was measured in the 504 cultures using flow cytometry. Half-maximal inhibitory dilution (ID50) was determined by non-linear 505 regression as the serum dilution that neutralized 50% of the pseudotyped lentivirus. 506 Cloning of the B.1.351 Spike variant 507 The construct pCAGGS-SΔ19 carrying a codon-optimized cassette encoding a SARS-CoV-2 Spike protein 508 (reference Wuhan-Hu-1 isolate) lacking 19 C-terminal residues, which has been shown to boost the viral 509 titers (Johnson et al., 2020), has been described (Gorchakov et al., 2021 in press presented as median ± IQR. Asterisks indicate significant difference between groups determined using 523 the Kruskal-Wallis test, *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001, ns = not significant. 524 525 Acknowledgments 526 The authors thank all the individuals in the study for the kind donation of both their time and biological 527 material. We thank Gaukhar Yusubalieva, Vladimir Baklaushev, Yuri Lebedin and Rudolf Valenta for their 528 kind help with experiments and for providing reagents. Fig. 1a  is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint