Covaxin/BBV152 is a whole virion inactivated vaccine formulated with a toll-like receptor ligand adsorbed to alum (1–4). A phase I trial of Covaxin reported an acceptable safety profile and enhanced immune responses. Neutralizing antibody responses remained elevated in all the participants at 3 months after the second vaccination (5). Phase II trial of Covaxin reported enhanced Th1 responses and antibody responses (6). Moreover, animal studies with this vaccine demonstrated significantly high neutralizing antibody and high-antigen binding titers as well as increased severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) specific IFNγ producing CD4+ T cells. Based on interim efficacy data from the Phase III trial, Covaxin was approved for use in India. Thus far over 340 million doses of the vaccine have been administered.

Multiple studies have shown that long-term innate immune responses can be either increased (trained immunity) or down-regulated (innate immune tolerance) after certain vaccines or infections (7–9). Since Covaxin is based on whole virus inactivation combined with a toll-like receptor (TLR) agonist and alum as adjuvants, it is to be expected that Covaxin would induce systemic immune responses in vaccine recipients. However, the nature of these systemic immune responses has not been explored. We, therefore, examined the plasma levels of a panel of cytokines and chemokines in vaccine recipients at baseline and at monthly intervals following administration of the prime-boost vaccine regimen.

Live attenuated vaccines, including BCG (Bacillus Calmette Guerin) and MMR (measles, mumps and rubella) are known to activate innate immune responses in addition to vaccine-specific immune responses, leading to a bystander or off-target effects on the immune system (10–14). The role of COVID vaccines in inducing such an effect remains unexplored. Covaxin is one of the first whole virion inactivated vaccine approved for use against SARS-CoV-2. As such, the immune responses engendered by this vaccine is not completely described, unlike the wealth of data available for the mRNA vaccines against COVID-19 (15–17). Hence, we examined the influence of Covaxin on systemic cytokine and chemokine response, which would reflect the underlying activation of the innate and adaptive immune system by the vaccine. Our data clearly delineate the kinetics of induction of cytokine and chemokine responses in vaccinated individuals. For the most part, the first dose of the vaccine does not induce any significant changes in plasma cytokine and chemokine levels. It is following the second (or booster) dose, that profound changes in the plasma cytokine and chemokine levels are observed. In addition, these alterations persist till the third month following the prime-boost vaccination.

Type 1, Type 17 and pro-inflammatory cytokines (including IL-6, IL-1 and IL-12) are known to play an important role in host immunity to viral infections (18–23). Published studies have reported that, Type 1, Type 17 and pro-inflammatory cytokines may confer long-lasting immune memory against novel coronaviruses (24, 25). In addition, recent studies have also reported that in mRNA-based vaccine (BNT162b2) also elicit better cytokine and chemokine response after 1^st and 2^nd dose of vaccination in SARS-CoV-2 naive individuals (Bergamaschi C et al. Cell Rep Aug 2021). Therefore, our finding that all of the cytokines examined in the above groups are elevated at a month following booster vaccination suggests the efficient induction of protective cytokine responses following Covaxin vaccination. In addition, while not protective in nature, we also observed induction of Type 2 and regulatory cytokines following prime-boost vaccination in vaccinated individuals. We postulate this to reflect the use of Alum as an adjuvant in the inactivated vaccine since Alum is a known inducer of Type 2 cytokines (26, 27). The effect of Type 2/regulatory cytokine induction in vaccinated individuals needs to be explored further. We also observed the induction of two cytokines with growth-promoting effects – IL-3 and IL-7. While IL-3 is a growth factor for hematopoietic progenitor cells (28), IL-7 is a growth factor for T cells (29), indicating that Covaxin might have off-target effects on progenitor cells as well. Similar to BBV152, other mRNA COVID-19 vaccines BNT162b1 and mRNA-1273 also elicited a profound cytokine response after two doses on vaccination indicating that these vaccines have the potential to protect against COVID-19 infection (30, 31). Interestingly, the only cytokines that were observed to be diminished in our study were IFNβ, GM-CSF and IL-1α. Again, the effect of diminished induction of these particular cytokines needs to examined further. While there are a number of studies in children showing that BCG scar can modulate pro-inflammatory responses, studies have not shown an effect of the BCG scar on cytokine responses in the adult population (32, 33).

CC and the CXC family of chemokines are known to play a vital role in host immunity to viral infections (34–37). These chemokines act by inducing the activation and migration of innate and adaptive immune effectors to the site of infection (38). Our data clearly reflect the elevated induction of CCL4, CXCL1, CXCL2 and CX3CL1, indicating the activation of innate immune cells by Covaxin. Interestingly, we also observed diminished plasma levels of CCL2 and CXCL10 following vaccination, the implications of which are not known currently. Nevertheless, the increased production of certain chemokines following prime-boost vaccination with Covaxin reflects the robust induction of host immunity that is of protective nature by this inactivated vaccine.

Our study has several limitations. We only examined systemic levels of cytokines and chemokines and did not examine vaccine-specific or ligand-specific immune responses. Our findings also suggested that there is no association between age and gender and cytokine responses at M0 and M3, indicating that the induction or reduction of different cytokines or chemokines are not correlated with the either age or gender. Our study is purely descriptive without any mechanistic underpinnings. However, despite these limitations, our study is one of the first, to our knowledge, to describe the induction of systemic cytokine and chemokine responses following COVID-19 vaccination. This data has important implications in terms of understanding the nature of the systemic immune response engendered by COVID-19 vaccines and its effect on bystander immune responses. Biomarkers we have reported in this study can be used as the surrogates of vaccine-induced innate and adaptive protective responses and these biomarkers could also help in the improvement of vaccine efficacy and pertinence

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Ethics Committee of ICMR-NIRT. The patients/participants provided their written informed consent to participate in this study.

SB, CP, and NK designed the study. NK and AN conducted the experiments. NK, KD, GC, and AN acquired data. NK and AN analyzed data. SB, CP, and KD contributed reagents and also revised subsequent drafts of the manuscript. CP, MM, VB, AM, and SM were responsible for the enrolment of the participants and also contributed to acquisition and interpretation of clinical data. SB and NK wrote the manuscript. All authors read and approved the final manuscript.

This work was supported by the Indian Council of Medical Research (ICMR).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.