The rapid development of mRNA vaccines, such as the Pfizer-BioNTech BNT162b2 vaccine, against SARS-CoV-2, the virus responsible for the COVID-19 pandemic, has been instrumental in controlling disease transmission, morbidity, and mortality (Polack et al., 2020). These vaccines have been shown to induce innate and adaptive immune responses rapidly. These responses include the production of neutralizing antibodies and activating antigen-specific CD4 and CD8 T cells (Gebre et al., 2021; Cagigi and Loré, 2021). The innate immune response is enhanced following booster immunization, leading to a more potent immune response (Arunachalam et al., 2021). The Pfizer-BioNTech vaccine, in particular, has demonstrated high efficacy in preventing symptomatic COVID-19 and reducing the risk of severe disease, hospitalization, and death (Polack et al., 2020).

The immune response comes at the cost of changes in inflammatory and coagulation profiles. These changes have been implied to affect the reported adverse events following immunization. Commonly reported adverse events include pain, redness, swelling at the injection site, fatigue, headache, muscle pain, chills, fever, and nausea. These are generally short-lived, not serious, and indicate the body’s immune response to the vaccine. However, serious adverse events, though rare, have been observed, including allergic reactions, myocarditis, and pericarditis, particularly in young men after the second dose (Yasmin et al., 2023; Seyedalinaghi et al., 2022; Klein et al., 2021). Thromboembolic events and Guillain-Barré syndrome have also been reported, but their association with the vaccine is well elucidated. Although the biological mechanisms underlying the adverse events have not been fully understood, they are postulated to be closely related to inflammatory profile and coagulation system changes that go beyond their normal, necessary threshold.

This study aims to investigate the nuanced biochemical and hematological responses following the third dose of a COVID-19 mRNA vaccine, with a particular focus on the longitudinal immune response. By investigating the dynamics of immunoglobulins (IgG and IgA), inflammatory biomarkers (IL-6, IFN-γ, CRP, hs-CRP), coagulation factors (PT, APTT, D-dimers), and hematological parameters (neutrophils, leukocytes, platelets), we aim to uncover novel insights into the complex mechanisms underlying vaccine-induced immunity and make a way for further research to demonstrate the interplay between these closely linked systems. This research seeks to bridge critical knowledge gaps regarding the long-term effects of booster vaccination, ultimately informing evidence-based strategies for optimizing vaccine efficacy and safety, particularly in high-risk populations.

In this study, 68 individuals were enrolled. Their ages ranged between 20 and 30. Among the 68 participants, 41% (n=28) were male, while 59% (n=40) were female. No differences were made based on age or gender. Moreover, 28% (n=19) reported that they had not contracted symptomatic infection, while 72% (n=49) reported that they had suffered from symptomatic infection before the third dose of vaccine, which PCR confirmed.

This study aimed to assess the effects of the 3rd dose of an mRNA COVID-19 vaccine on antibody response, inflammatory biomarkers, coagulation profile, and blood cell counts. These were the immunoglobulins, i.e., Anti-SARS-CoV-2 IgG and IgA; the inflammation biomarkers, i.e., IL-6, IFN-γ, CRP & hsCRP; the coagulation profile, i.e., D-dimers, PT & APTT and cell counts, i.e., lymphocytes, neutrophils & platelets counts. A total of 68 healthy participants were enrolled who had received two doses of the COVID-19 vaccine earlier, with a second dose six months but no more than eight months than this study.

As the results demonstrate, the anti-SARS-IgG and IgA levels increased after the dose of the mRNA vaccine. Although the increase in IgG levels was rapid, a significant increase was noted even 48 hours after vaccination. Levels of both IgG and IgA didn’t reach a plateau till day 180, i.e., six months after the vaccine dose. This confirms the prolonged protection conferred by the COVID-19 mRNA vaccines. Similar findings are reported by Sabina Zurac et al., who determined the IgG and IgA at day 21 and day 45 and showed that the increase in mean IgA was lower than mean IgG after 2nd dose of the mRNA vaccine at day 21. Another study by Herzberg et al. that investigated the immune response to third dose of an mRNA vaccine also indicated a significant increase in the immune response after 4 weeks of the 3^rd dose (Herzberg et al., 2022).

However, in contrast to our study, they showed that the mean IgA levels at day 45 were higher than IgG levels, while in our study, no such increase in levels of IgA compared to IgG was observed throughout the study period (Zurac et al., 2021). As we have no record of the IgG and IgA levels of the participants after the second dose, it can’t be concluded whether the immunoglobulin levels reported before the third dose decreased or not compared to those after the second dose. Some studies in Europe and Japan have shown that the IgG levels had dropped to 10% six months after the second dose (Naaber et al., 2021; Kato et al., 2022). However, a much larger study of more than 18 thousand Polish healthcare workers indicated that mRNA vaccines provided protection till 8–9 months after the second dose (Skorupa et al., 2022). The same study showed that with the 3^rd dose, the IgG Levels increased significantly at around the fifth day and maximized on the fourteenth day (Skorupa et al., 2022). In our research, like the study by Monika Skorupa et al (Skorupa et al., 2022), there was an abrupt increase in IgG levels, indicating that the immunity granted by the earlier two doses has significant memory that rapidly boosts upon any future encounter with the virus or any of its components. As shown by the results in Figure 1a , a constant rise was observed for six months, and no plateau was reached till that time.

Immune response is always preceded and/or accompanied by inflammation, whether the inflammation is initiated by infection or transfection. Inflammatory markers assessed in our study were IL-6, IFN-γ, CRP & hs-CRP. All these are mutually inclusive, as CRP is an acute inflammatory marker produced by the liver when signaled by IL-6. They both assess low-grade inflammation (Del Giudice and Gangestad, 2018). Also, IFN-γ and IL-6 have been hypothesized to have functionally opposing roles in inflammation, immune response, and cell proliferation (Qi et al., 2013). IFN-γ has pro-inflammatory effects, while those of IL-6 are anti-inflammatory (Qi et al., 2013). The study cited above by Herzberg et al. and another study by Thomas et al. reported that IFN-γ increases significantly after the booster dose suggesting the T-cell immune response. In study by Herzberg et al., they reported that 96.3% of the participants showed detectable T-cell response evident by the rise of IFN-γ. Thomas et al. delved deeper into the matter and reported that significant increase was shown in the IFN-γ along with granzyme B production, neutralizing antibodies and class-switched B cells (Thomas et al., 2025). The stratification protocols and sophisticated techniques of both of these studies gives them increased value and our findings are in accordance with the relative aspects of these studies. In our study, a rise in CRP was noted 48 hours after the third dose of the vaccine, Table 1 . A survey by Ugur Sahin also reported the same results and found that the CRP levels dropped to normal on day eight after the vaccination (Sahin et al., 2020). hs-CRP also increased with statistical significance (p = <0.0001) in our study 48 hours after the vaccine dose. For quite some time, hs-CRP has been considered an independent marker of cardiovascular risks (Moutachakkir et al., 2017). In line with this, several studies have concluded from the published data that there can be a possible association between COVID-19 vaccination and cardiac events, markedly visible in otherwise healthy, younger male populations and with repeated doses of mRNA vaccines (Fatima et al., 2023). Though a temporal association can be established between the vaccine injection and the development of myocarditis, the milder nature of adverse events, low incidence rate, and lack of experimental studies make it difficult to pose a cause-effect association.

COVID-19 infection has been associated with disturbances in coagulation pathways. The disturbed coagulation profile expressed by the PT, APTT, and D-dimers has been a significant indicator of disease severity and is used to guide treatment plans (Mucha et al., 2020). S-protein has been pointed out as the virus’s main antigenic and inflammatory antigen. As the mRNA vaccines contain S-protein or subunits of S-protein, the prolonged antigen presence and excessive immune response can trigger sustained inflammation that can harm the endothelium, thus disturbing anti-thrombogenic properties in multiple vascular beds (Trougakos et al., 2022). Studies have shown remarkable disturbances in coagulation profiles after the vaccine. A narrative review by Emmanuel J. Favaloro demonstrated that in most studies included in the review, D-dimers in patients diagnosed with suspected COVID-19 vaccine-induced thrombotic thrombocytopenia were almost always abnormal (Favaloro, 2021). In our research, like the observations of Favaloro, a statistically significant increase was reported in D-dimer levels ( Table 2 ). Our study noted a rise of lesser statistical significance in PT and APTT. The study already mentioned by Favaloro also pointed out that in most cases with COVID-19-induced thrombotic thrombocytopenia, PT and APTT were also deranged (Favaloro, 2021). Another study in Nature by Jiping Liu et al. reported that the mRNA vaccine significantly alters the coagulation profile (Liu et al., 2021). They found that PT was shortened on day 7 after the vaccine, but on days 28 and 42, both PT and APTT were prolonged. They followed the patients and reported that the coagulation profile returned to normal at day 90. These changes in coagulation profile for a significantly longer time suggest that however clinically less significant these changes may look, they should be looked at with great care, especially with repetitive doses and patients at risk for bleeding diathesis.

Another important aspect related to inflammatory, immunologic, and coagulation profiles is the blood cells, as they play an essential role in inflammation, immune response, and the cross-talk between various process stakeholders in the language of cytokines and interleukins and coagulation profile. Our study focused on neutrophils, lymphocytes, and platelet count changes. A decrease of statistical significance in lymphocyte count was noted 48 hours after the vaccine dose ( Table 3 ). A study by Ugur Sahin et al. also reported the decrease of a similar pattern in lymphocytes on day two after the vaccine dose that returned to normal on day 8 (Sahin et al., 2020). The same study, on the other hand, had reported an increase in neutrophil count on day 2, returning to normal on day 8 after the vaccine dose (Sahin et al., 2020).

In contrast to the findings of Ugur Sahin et al., a decline in neutrophil count was noted in this study on day 2 after the vaccine dose, but that was statistically not significant ( Table 3 ). Our study reported a statistically insignificant increase in platelet count on day 2 after the vaccine dose. Although not statistically significant, the increase in platelet count contrasted with the general observation of a decrease after COVID-19 vaccination. However, rare cases have been reported where patients had presented with thrombocytosis after COVID-19 vaccination, but those vaccines were adenoviral based (Graça et al., 2021).

It is essential to acknowledge the limitations of this study. Firstly, the sample size was small. The small sample size affects the generalizability of these results to wider segments of population with different ethnic, geographic and genetic backgrounds. Secondly, children and old aged patients were not included. In addition, patients with immunodeficiencies, comorbidities, coagulation abnormalities, and inflammatory diseases were not enrolled, and the results cannot be applied to them. Thirdly, the study could not assess the IgG and IgA levels after six months of the vaccine dose. Similarly, changes in inflammatory cytokines and coagulation profile were not followed within 48 hours of the booster dose, and the study cannot shed light on what could have been observed after this specified time.

Further research should be carried out to address these limitations and better understand the kinetics of antibodies, changes in inflammatory markers, coagulation profiles, and cellular components of blood, as well as the interaction of these related mechanisms.

A booster dose with an mRNA vaccine at the recommended time causes the anti-SARS-CoV-2 IgG and IgA levels to increase rapidly and for at least six months. The increase in inflammatory cytokines and changes in the coagulation profile are consistent with the vaccine’s normal immune induction mechanism. Mild AEFIs were reported in a minority of vaccine recipients. The consistent subclinical changes in inflammatory and coagulation markers suggest that these pathways may be involved in the pathogenesis of severe ARFIs reported with mRNA vaccines.