Towards robust immune responses after heterologous COVID-19 vaccination and its application perspectives

The coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) [1], has seriously impacted the global health and economy. Effective vaccination, with homologous or heterologous prime-boost strategies, is the key to controlling the ongoing COVID-19 pandemic [2]. Homologous vaccinations, which are the administration of the same type of COVID-19 vaccine, have been shown to be highly efficient in inducing robust immune responses [3–7]. Due to safety concerns regarding the rare cases of vaccine-induced immune thrombotic thrombocytopenia after the first dose of the ChAdOx1 nCov-19 (AZD1222) vaccine [8], heterologous booster vaccination was assessed and approved in some European countries. There are limited data on the immune responses induced by heterologous vaccination regimens. We summarize the latest data on the immunogenicity, reactogenicity, and neutralizing activity of heterologous COVID-19 vaccinations against variants of concern (VoCs); we further discuss the distinct advantages of their application and critical factors that may influence their efficiency. Currently, different countries have approved multiple types of COVID-19 vaccines, including mRNA-based vaccines (BNT162b2/BNT and mRNA-1273), adenovirus-vectored vaccines (ChAdOx1

actogenicity remain largely unknown.To date, there are limited studies comparing immune responses to homologous and heterologous vaccination schedules in humans (Table 1).In February 2021, Logunov et al. [3] reported the interim clinical efficacy results of a heterologous prime-boost vaccination regimen using the rAd26-and rAd5-vectored COVID-19 vaccine Gam-COVID-Vac separately in a randomized, double-blind, placebo-controlled multicentre study.The study found that this rAd26/rAd5 vector-based heterologous vaccination induced robust humoral and cellular immune responses with good safety profiles and had a 91.6% protective efficacy against COVID-19.Homologous immunizations with the ChAd or BNT vaccines have been evaluated in American and European countries.Two observational studies of heterologous primeboost vaccination regimens, incorporating ChAd and BNT vaccines, have been conducted in healthy adult individuals in Germany [4,5].In late July 2021, Tenbusch et al. [5] reported the vaccine-induced antibody responses in healthcare workers or volunteers who received homologous or heterologous vaccinations using ChAd and BNT as a prime and/or boost vaccine in Germany.Interestingly, the heterologous ChAd/BNT vaccination at a 10-week interval induced a significantly higher surrogate neutralization activity than homologous ChAd/ChAd (at a 10-week interval) or homologous BNT/BNT (at a 3-week interval) vaccination.In September 2021, Schmidt et al. [4] reported that a heterologous ChAd/BNT vaccination (10-week interval) induced higher T-cell responses than a homologous BNT/BNT vaccination (3-week interval); nevertheless, the two vaccination strategies yielded similar levels of anti-spike (S) IgG and neutralizing antibodies, consistent with the findings of Tenbusch et al. [5].Moreover, two critical clinical trials, with 1,506 participants from Spain and the UK, were conducted to study the immunogenicity and reactogenicity of heterologous COVID-19 vaccinations [6,7].Borobia et al. [6] conducted the CombiVacS study, a multicentre, open-label, randomized, controlled phase 2 trial in Spain.The study demonstrated significant increases in SARS-CoV-2 anti-S IgG and neutralizing antibody levels and T-cell responses in the ChAdprimed, BNT-boosted participants compared with those in participants not administered a BNT booster, which indicated that humoral immune responses increased following a BNT booster vaccination.However, this study lacked the data regarding the homologous ChAd/ChAd vaccination comparator.Liu et al. [7] also observed in their single-blind, randomized, noninferiority trial that heterologous ChAd/BNT and homologous BNT/BNT vaccinations induced higher immunogenicity than a homologous ChAd/ChAd vaccination did.The participants who were vaccinated with ChAd/BNT or BNT/BNT had similar levels of anti-S IgG and neutralizing antibodies, while T-cell responses were higher in the ChAd-primed, BNT-boosted participants.Although both heterologous vaccinations (ChAd/BNT and BNT/ChAd) led to higher levels of anti-S IgG than that induced by the homologous ChAd/ChAd vaccination comparator, the BNT/ChAd regimen did not meet the noninferiority criteria [8].Collectively, accumulating evidence suggests that heterologous COVID-19 vaccinations with ChAd and BNT boost the vaccine's effectiveness by triggering stronger or broader immune responses.
Recently, the rapid worldwide spread of the B.1.617.2 (Delta) variant has partially nullified the protection effectiveness afforded by COVID-19 vaccines, suggesting that booster vaccination would be beneficial for combating emerging VoCs.Keskin et al. designed a small clinical trial to investigate the differences of in antibody titres to S and nucleocapsid (N) proteins in 45 healthcare workers who already received two doses of an inactivated virus vaccine (2IVV) and were administered a third dose of the CoronaVac (3IVV) or BNT (2IVV+BNT) vaccines in Turkey [11].They found that the levels of anti-S and anti-N IgG antibodies in the 2IVV+BNT group were significantly higher than those in the 3IVV group.Notably, two recent clinical studies separately, conducted by Cao et al. [9] and Ai et al. [10], evaluated the safety and immunogenicity of a heterologous third booster dose with the recombinant protein subunit vaccine ZF2001 primed with two doses of IVVs (CoronaVac or BBIBP-CorV) in China.Both studies showed that the third booster dose of ZF2001 was well tolerated and could induce a significantly high humoral immunogenicity in adult recipients.Therefore, these studies indicate that a heterologous third booster dose induces a much stronger B-cell recall response than its homologous counterpart.
To date, there are limited data regarding the neutralization capacities of heterologous vaccination regimens against different emerging VoCs [12], including B.1.1.7 (Alpha), B.1.351(Beta), P.1 (Gamma, formerly B.1.1.28.1), and B.1.617.2 (Delta) variants.Barros-Martins et al. [13] reported that the heterologous ChAd/ BNT vaccination elicited significantly higher neutralizing antibody titres against the B.1.1.7,B.1.351,and P.1 variants than homologous ChAd/ChAd vaccination.Interestingly, their subsequent study further found that the homologous BNT/BNT vaccination more efficiently induced neutralizing antibodies against the B.1.617.2 variant than the heterologous ChAd/BNT vaccination.However, the heterologous ChAd/BNT vaccination also supported strong inhibition of the B.1.617.2 variant by inducing robust neutralizing antibodies.Recently, two independent clinical studies reported that a heterologous third booster dose with ZF2001 was more efficient than a homologous third booster dose with IVVs in terms of inducing humoral immunity against different SARS-CoV-2 variants, including the Delta strain [9,10].Heterologous primeboost COVID-19 vaccination as well as additional heterologous vaccine doses might be a suitable strategy to combat emerging VoCs.Therefore, there is an urgent need for more detailed studies to evaluate and compare the immunogenicity of heterologous and homologous COVID-19 vaccinations against emerging VoCs.

Distinct advantages of heterologous vaccinations
Given the ongoing COVID-19 pandemic, heterologous COVID-19 vaccination may have distinct advantages.First, mRNA vaccines may serve as an alternative heterologous booster for the ChAd vaccine to overcome their safety concerns.Second, heterologous vaccinations may boost more durable and robust humoral and cellular immune responses than those induced by already authorized homologous vaccinations and greatly contribute to effective control of emerging or future VoCs [4,6,10].Finally, as a feasible and reasonable vaccination strategy, heterologous vaccinations also have the potential to compensate for vaccine shortfalls and accelerate the worldwide vaccine rollout, especially in low-and middle-income countries.

Factors influencing the efficiency of heterologous vaccinations
Some of the critical factors that may influence the high efficiency of heterologous COVID-19 vaccinations are as follows: 1) the vaccine types used in the heterologous prime-boost schedules [9][10][11]; 2) the order of candidate vaccines in the prime-boost process of a heterologous vaccination regimen [7]; and 3) the differences in time interval between doses of the vaccines in heterologous vaccinations [14].
Overall, accumulating evidence supports the fact that certain heterologous COVID-19 vaccination regimens have good safety profiles and represent a feasible and promising vaccination strategy to boost durable Natl.Sci.Open, 2022, Vol.1,20220008 and robust humoral and cellular immune responses and effectively control the emerging VoCs [4,6,10].Notably, a feasible heterologous vaccination is a powerful weapon against the ongoing COVID-19 pandemic.Further studies are needed to confirm the clinical effectiveness of heterologous vaccinations against emerging VoCs and optimize the combinations, orders, doses, and intervals of heterologous prime-boost vaccination schedules.

Table 1
Comparison of humoral immune responses after heterologous COVID-19 vaccinations ChAd/BNT: prime with ChAd followed by a BNT booster; ChAd/ChAd: homologous vaccination with ChAd as both prime and booster; IVV, inactivated whole virus vaccine; PBMCs, peripheral blood mononuclear cells; S, spike protein; 2IVV, two-dose inactivated whole virus vaccine; 2IVV+BNT, two-dose inactivated whole virus vaccine followed by a third BNT booster; 3IVV, three times inactivated whole virus vaccine.