Serum anti-Spike antibody titers before and after heterologous booster with mRNA-1273 SARS-CoV-2 vaccine following two doses of inactivated whole-virus CoronaVac vaccine
Authors: Sinto Robert , Utomo Dwi , Suwarti  , Nelwan Erni J. , Surendra Henry , Natasha Cindy , Fransiska  , Theresia Deborah , Ranitria Adella Faiqa , Subekti Decy , Nuraeni Nunung , Handayani Winahyu , Rahardjani Mutia , Baird J. Kevin , Dunachie Susanna , Shankar Anuraj H. , Hamers Raph L. 
Published: December 2021

SummaryBackgroundThe inactivated whole-virus vaccine CoronaVac (SinoVac) is the COVID-19 vaccine most administered worldwide. However, data on its immunogenicity and reactogenicity to heterologous boosting with mRNA vaccines are lacking.MethodsIn a cohort of hospital staff in Jakarta, Indonesia, who received two-dose CoronaVac six months prior (median 190 days, IQR165-232), we measured anti-Spike IgG titers on paired serum samples taken before and 28 days after a 100μg mRNA-1273 (Moderna) booster. We performed correlations and multivariable ordinal regressions.FindingsAmong 304 participants, the median age was 31 years (range 21-59), 235 (77.3%) were women, 197 (64.8%) had one or more previous SARS-CoV-2 infections (including 155 [51.0%] who had a post-CoronaVac breakthrough infection. Pre-boost IgG titers correlated negatively with the time since the latest documented “virus exposure” (either by the second CoronaVac or SARS-CoV-2-infection whichever most recent). Previous SARS-CoV-2 infection and a longer time interval between second vaccine and mRNA-1273 boost were associated with a higher pre-boost IgG titer. Post-booster, the median IgG titer increased 9.3-fold, from 250 (IQR32-1389) to 2313 (IQR1226-4324) binding antibody units (BAU/mL) (p<0.001). All participants, including seven whose pre-boost IgG was below assay detection limits, became seropositive and all reached a substantial post-boost titer (≥364 BAU/mL). Post-boost IgG was not associated with pre-boost titer or previous SARS-CoV-2 infection. Booster reactogenicity was acceptable, with 7.9% of participants experiencing short-lived impairment of activities of daily living (ADL).InterpretationA heterologous, high-dose mRNA-1273 booster after two-dose CoronaVac was highly immunogenic and safe, including in those most in need of improved immunity.FundingWellcome Trust, UKResearch in contextEvidence before this studyThe inactivated whole-virus vaccine CoronaVac (SinoVac) is the COVID-19 vaccine most administered worldwide, at around 2 billion doses in 54 countries. Concerns that CoronaVac has lower immunogenicity than virus vector or mRNA vaccines, with pronounced decreases of neutralising antibody titres within a few months, and reduced effectiveness in the older population, highlight the urgent need for immunogenic, safe and well-tolerated booster schedules, especially with Omicron rapidly emerging.We used the terms “SARS-CoV-2”, “COVID-19”, “vaccine”, “booster” to search PubMed and medRxiv up to Dec 22th, 2021, with no language or date restrictions, to identify clinical trials and real-world studies reporting on the immune responses and reactogenicity to a “third booster” of currently approved COVID-19 vaccines. Previous research reported that neutralising antibody responses elicited by all currently approved vaccines (mRNA, adenovirus-vectored, inactivated, and protein subunit) declined to varying degrees after 6-8 months after full-schedule vaccination. Several clinical trials have evaluated heterologous (“mix and match”) vaccination schedules, demonstrating robust immune responses in adults. After two-dose CoronaVac, BNT162b2 (Pfizer-BioNTech) boost was significantly more immunogenic than a homologous booster against wild-type and Variants of Concern (VOCs) Beta, Gamma and Delta, and AZD1222 boost increased spike RBD-specific IgG 9-10-fold, with high neutralizing activity against the wild type and VOCs. Compared to previous SARS-CoV-2 variants, current vaccine boosters appeared to neutralise Delta to a slightly lesser degree, and Omicron to a substantially lesser degree, although preliminary data from Moderna found that the authorised dose (50μg) of the mRNA-1273 boost increased antibodies 37-fold and the high-dose (100μg) boost 83-fold.Added value of this studyTo our knowledge, this study is the first to provide critical real-world evidence that heterologous boosting with high-dose mRNA-1273 vaccine after CoronaVac is highly immunogenic, safe and well-tolerated in adults. After a primary course of two-dose CoronaVac, we found that a high-dose (100μg) mRNA-1273 booster was immunogenic for all participants in a highly exposed cohort of hospital staff in Jakarta, Indonesia, in the context of Delta predominance, particularly for those with the lowest pre-boost antibody levels. All participants became seropositive and all reached a substantial post-boost titer (≥364 BAU/mL), up to a median 9.3-fold increase. Booster reactogenicity was acceptable, with 7.9% of participants experiencing short-lived impairment of activities of daily livingImplications of all the available evidenceThe study findings contribute to informing policy makers on flexible options in deploying COVID-19 vaccines in mix-and-match schedules, with particular relevance for countries that are largely dependent on inactivated vaccines. Further trials are warranted that assess clinical endpoints of optimized doses of mRNA-1273 booster, and variant-specific or multivalent vaccines in response to decreased protection against emerging SARS-CoV-2 VOCs.