Humoral and cellular responses to vaccination with homologous CoronaVac or ChAdOx1 and heterologous third dose with BNT162b2.

Dear Editor,

We read with interest the recent manuscript of Mak et al. reporting SARS-CoV-2 antibody and T cell responses one year after COVID-19 convalescence and vaccination. The authors showed that T cell and humoral responses persisted for up to one year after mild to moderate COVID-19, and that a single dose of COVID-19 vaccine induced robust responses, irrespective of the vaccine type (Ad26.CoV2, ChAdOx1, mRNA-1273 or BNT162b2). A second dose did not further increase cellular and humoral responses.

Despite the benefits of vaccination and immune protection due to convalescence, the spread of new variants such as Omicron raise new questions about vaccine efficacy, immunity waning and the need of booster doses. Studies on heterologous third dose with BNT162b2 after two doses of CoronaVac (CV/CV/BNT) or ChAdOx1 (Ch/Ch/BNT) showed increased immunogenicity in both cases. Here, we corroborate these data and show that both heterologous vaccination schemes, largely adopted in Brazil, produced consistent increases in humoral and cellular responses after the third booster dose in our cohorts.

We conducted an observational study with a non-randomized cohort of 48 healthcare workers of regional hospitals and institutions, vaccinated with two homologous doses of CoronaVac (CV, n = 25, 72% females) or ChAdOx1 (Ch, n = 23, 87% females), and with a third heterologous dose of BNT162b2. For CV/CV/BNT and Ch/Ch/BNT, blood samples were collected according to the scheme shown on Fig. 1 A. The dates of sample collection could not be completely synchronized between the CoronaVac and ChAdOx1 groups due to delays in vaccine distribution and due to differences in the vaccination schedules. Participants were tested for anti-S1 IgG at all time points. Neutralizing antibodies and cellular responses were evaluated at t3, t3’, t4, and t4’. Assays were performed with Anti-S1 QuantiVac IgG, NeutraLISA and SARS-CoV-2 IGRA kits (EUROIMMUN). Statistical significance tests were performed using the non-parametric Wilcoxon–Mann–Whitney test (details on Supplementary Methods). All individuals enrolled in this study provided written informed consent as part of the protocols approved by the Ethics Committee of the Federal University of São Paulo and by the National Ethics Committee (CONEP, study number CAAE: 32,571,720.0.0000.5505).

Fig. 1

(A) Experimental design of this study. Healthcare professional volunteers vaccinated with two homologous doses of CoronaVac and with a heterologous third dose of BNT162b2 (CV/CV/BNT) or with two homologous doses of ChAdOx1 and with a heterologous third dose of BNT162b2 (Ch/Ch/BNT). Blood samples were collected at t0–t4 for CV/CV/BNT and at t0’–t4’ for Ch/Ch/BNT. Humoral and cellular responses were evaluated by the levels of anti-S1 IgG, neutralizing antibodies, and IFN-γ. (B) Dynamics of humoral responses in vaccinated volunteers after two homologous doses of CoronaVac followed by a third dose with BNT162b2 (CV/CV/BNT) or after two homologous doses of ChAdOx1 followed by a third dose with BNT162b2 (Ch/Ch/BNT).

The median age was 30 (25th–75th percentile: 24–41) years and 40 (35–53) years for the CV/CV/BNT and Ch/Ch/BNT groups, respectively. In the CV/CV/BNT group, the median anti-S1 IgG values increased from 19.8 BAU/ml (6.0–38.7, 7/24 positives) at t1 after the first dose to 429.0 BAU/ml (227.3–578.5, 25/25 positives) at t2 after the second dose (p < 0.0001) (Fig. 1B). From t2 to t3, the concentrations significantly decreased (p < 0.01) to 115.7 BAU/ml (62.3–184.5, 22/25 positives) (Fig. 1B). However, after the third booster dose at t4, the anti-S1 IgG concentration increased 25-fold (p < 0.0001) to 2843.0 BAU/ml (2738.2–2956.0, 19/19 positives) (Fig. 1B). The levels of neutralizing antibodies significantly increased (p < 0.0001) from 23.5% (13.4%–38.3%, 8/25 positives) at t3 to 99.3% (99.2%–99.3%, 19/19 positives) at t4 (Fig. 2 A).

Fig. 2

(A) Levels of neutralizing antibodies and (B) IFN-γ after homologous vaccination with CoronaVac followed by a third dose with BNT162b2 (CV/CV/BNT) or after homologous vaccination with ChAdOx1 followed by a third dose with BNT162b2 (Ch/Ch/BNT).

In the Ch/Ch/BNT group, the median anti-S1 IgG responses increased from 86.8 BAU/ml (53.0–280.1, 16/19 positives) at t1’ to 648.9 BAU/ml (588.3–721.4, 21/21 positives) at t2’ (p < 0.0001) (Fig. 1B). The anti-S1 IgG levels also decreased significantly (p < 0.01) to 390.9 BAU/ml (231.6–484.9, 19/20 positives) from t2’ to t3’ (Fig. 1B). After the third booster dose at t4’, anti-S1 IgG levels increased 7-fold (p < 0.0001) to 2799.2 BAU/ml (2182.8–2832.3, 11/11 positives) (Fig. 1B). The levels of neutralizing antibodies increased (p < 0.0001) from 63.2% (46.8%–87.6%, 16/20 positives) at t3’ to 98.9% (range 98.6%–99.1%, 11/11 positives) at t4’ (Fig. 2B). Additional anti-S1 IgA and anti-NCP IgG assays were also performed (Supplementary Figure).

Both CoronaVac and ChAdOx1 vaccines induced high cellular responses at t3 and t3’, presenting median IFN-γ values of 778.9 mIU/ml (340.0–1092.2 mIU/ml, 21/25 positives) and 1232.7 mIU/ml (579.3–2663.2 mIU/ml, 19/20 positives) (Fig. 2B), respectively. The third booster dose with BNT162b2 significantly increased the IFN-γ levels to 4906.4 mIU/ml (4423.1–4928.6 mIU/ml, 19/19 positives) and 12,197.2 mIU/ml (3041.7–12,277.3 mIU/ml, 9/11 positives) in the CoronaVac (p < 0.0001 from t3 to t4) and ChAdOx1 (p < 0.01 from t3’ to t4’) groups, respectively (Fig. 2B).

A large population study in Brazil showed the importance of the massive vaccination campaign and of all vaccines for the prevention of severe COVID-19 and deaths. However, it also indicated immune senescence for three of the vaccines currently in use in the country: CoronaVac, ChAdOx1, and BNT162b2, especially for CoronaVac. We observed waning immunity >75 days after the second dose in CoronaVac or ChAdOx1 groups (Fig. 1B). However, the heterologous vaccination schemes CV/CV/BNT or Ch/Ch/BNT resulted in consistent increases in humoral (Figs. 1B and 2A) and cellular responses (Fig. 2B) after the third booster dose in both groups.

This study has several limitations. The small sample size of our cohorts did not allow the analysis of possible differences between sexes and stratification of age groups. Information on previous medical conditions was not systematically collected. The dates of sample collection could not be completely synchronized between the CoronaVac and ChAdOx1 groups. The trial is non-randomised and unblinded, which inhibits direct comparisons between the two vaccine groups. The age differences between the two groups also limits comparisons.

In conclusion, our study provides evidence that waning immunity after >75 days of the second doses of CoronaVac or ChAdOx1 vaccines can be strongly recovered by the administration of a heterologous booster dose of BNT162b2.

Declaration of Competing Interest

NBSB, RH, LDG and MMS are employees of EUROIMMUN Brasil. All other authors have nothing to declare.