Temporal dynamics of IgM and IgG after COVID-19 vaccination: a pilot cohort study on breakthrough infections

Abstract

Background

During the coronavirus disease 2019 (COVID-19) pandemic, we conducted a cohort study analyzing changes in immunoglobulin (Ig) M and IgG antibody levels before and after COVID-19 vaccination.

Methods

The study compared two groups: participants who were infected with severe acute respiratory syndrome coronavirus 2 within 14 days of vaccination and those who were not. We evaluated dynamic changes in IgM and IgG levels immediately following vaccination and assessed their potential correlation with infection risk.

Results

Participants who contracted COVID-19 within 14 days of vaccination exhibited significantly higher IgM antibody levels than uninfected individuals, likely due to combined stimulation by both the vaccine and the virus. In the infected group, IgG antibody levels were lower than those in the uninfected group during the first 1 to 5 days post-vaccination. Moreover, across all participants, IgG antibody levels were generally lower than baseline during the first 1 to 3 days post-vaccination.

Conclusions

These findings suggest that, in the early stages following vaccination, the immune system may experience a temporary “immune trough,” potentially even below pre-vaccination levels. As a result, individuals should exercise additional caution during this period to mitigate infection risk.

INTRODUCTION

The coronavirus disease 2019 (COVID-19) pandemic has posed an unprecedented global health challenge, leading to the rapid development and deployment of vaccines [1]. These vaccines have proven effective in reducing morbidity and mortality [2]; however, the nature and durability of the immune response remain a subject of ongoing investigation [3]. In particular, understanding the dynamics of immunoglobulin (Ig) M and IgG production after vaccination is crucial for optimizing vaccination strategies and interpreting serological results [4,5].

IgM antibodies typically emerge during the early stages of the immune response and are considered markers of recent antigen exposure [6], whereas IgG antibodies reflect long-term immunity and are strongly associated with clinical protection [5]. While numerous studies have described the induction and persistence of IgG after COVID-19 vaccination [3,5], much less is known about the kinetics of IgM [7], the interplay between IgM and IgG, and the influence of prior infection on these responses [8].

Moreover, factors such as vaccine platform (e.g., messenger ribonucleic acid [mRNA] vs. inactivated), demographic characteristics (age, sex), comorbidities, and booster doses may significantly modulate antibody dynamics [8,9]. Identifying these determinants is essential for tailoring vaccination schedules and ensuring adequate protection in vulnerable populations [10,11].

This study therefore aims to provide a comparative analysis of IgM and IgG responses after COVID-19 vaccination in a well-characterized cohort, with particular attention to the early post-vaccination period and differences between infected and uninfected individuals. By systematically examining temporal antibody patterns, vaccine-specific effects, and host factors, the study seeks to clarify the clinical relevance of IgM, highlight the durability of IgG, and generate evidence to guide vaccination policies, booster timing, and post-vaccination risk mitigation [12,13].

MATERIALS AND METHODS

Participants

A total of 17 participants were enrolled. The focus of the study was to track changes in severe acute respiratory syndrome coronavirus 2 spike antigen-specific IgM and IgG antibody levels over time following vaccination. Blood samples were collected from participants at multiple time points: before vaccination and on days 1, 2, 3, 5, 7, 10, and 14 post-vaccination. For each collection, 3 cc of blood was drawn via venipuncture using vacuum blood collection tubes. The blood samples were then processed to separate the serum, which was stored at −80°C until analysis. Antibody levels were measured using chemiluminescent immunoassays. Throughout the study, ethical considerations were rigorously adhered to, ensuring the safety and confidentiality of all participants.

Antibody measurements

Serum samples were collected from all participants at 7 specific time points: before vaccination (baseline) and on days 1, 2, 3, 5, 7, 10, and 14 post-vaccination. After collection, the tubes were gently inverted 5–6 times to ensure proper mixing and clot formation. The samples were allowed to clot at room temperature (15°C–25°C) for 30 minutes to 1 hour. Following clotting, the samples were centrifuged at 1,500–2,000 × g for 10–15 minutes to separate the serum from the clot. The serum was transferred into sterile tubes and stored at −20°C or lower for long-term storage. For antibody detection, a chemiluminescence immunoassay (CLIA) was used, employing commercial assay kits from MikeBio (MikeBio, Chengdu, China) and the i1000 CLIA analyzer (i1000; MikeBio). Serum samples were thawed to room temperature before testing. The i1000 analyzer was calibrated with control samples provided in the kits, and then the serum samples were analyzed according to the manufacturer’s protocol for IgG and IgM detection. The analyzer performed the chemiluminescent reaction, and antibody levels were quantified based on the emitted luminescence. The IgG and IgM levels at each time point were recorded and analyzed to assess the immune response trajectory following vaccination.

Throat swab sampling and quantitative polymerase chain reaction (qPCR)

At designated time points, throat swab samples were collected from participants. To ensure sample quality, participants were instructed to refrain from eating, drinking, or smoking for at least 30 minutes prior to sampling. Healthcare professionals, equipped with personal protective equipment, collected the samples by gently swabbing the pharyngeal area or tonsils. The collected throat swabs were immediately placed into viral preservation liquid (MGIEasy Sample Release Reagent, Catalog Number 1000027692; BGI Genomics, Shenzhen, China) and securely sealed. The samples were transported to the laboratory within 2 hours, maintaining a temperature range of 2°C–8°C to preserve sample integrity.

In the laboratory, nucleic acid extraction was performed using MGIEasy Nucleic Acid Extraction Reagents (Catalog Number 1000023938, BGI Genomics) and the MGISP-NE32 automated nucleic acid extraction and purification instrument (BGI Genomics). The extraction, enrichment, and purification procedures were conducted according to the manufacturer’s instructions provided for the reagents and equipment.

Following purification, qPCR was conducted using a QuantStudio 5 instrument (Catalog Number A28137; Thermo Fisher Scientific, Waltham, MA, USA). The qPCR reaction mixture was prepared, including specific primers, probes, qPCR Master Mix, and the extracted RNA samples. The reaction mixture was added to the wells of a qPCR plate, which was then placed in the qPCR instrument. The thermal cycling conditions included denaturation, annealing, and extension steps, and the real-time PCR instrument monitored fluorescence signals during each cycle. Cycle thresholds values were used to calculate the number of viral RNA copies in the samples.

Statistical analysis

Antibody level changes over time were analyzed using repeated measures analysis of variance (ANOVA) to assess the effects of vaccination across multiple time points. To compare differences between groups A and B, the Mann-Whitney U test was employed for non-parametric data due to the non-normal distribution of some variables. All statistical analyses were performed using the latest version of SPSS (Version 28.0; IBM Corporation, Armonk, NY, USA). A p-value of <0.05 was considered statistically significant for all tests.

Ethics approval statement

The study was conducted in accordance with ethical guidelines and received approval from Ethics Committee of the Berlin Medical Chamber (Reference number: Eth-24/17). All participants provided written informed consent before joining the study. All experiments were performed in compliance with relevant laws and institutional guidelines and in accordance with the ethical standards of the Declaration of Helsinki.

RESULTS

Baseline characteristics and vaccination status of study participants

The study included a total of 17 participants, comprising 9 males and 8 females. The average age of the cohort was 34.65 years, with a standard deviation of ±11.15, reflecting a generally young demographic with some variability in age. Concerning vaccination status, 2 participants were in the process of receiving their first dose, while 15 participants had completed their second dose. The vaccines administered varied in type: 11 participants received mRNA-based vaccines, 2 participants were given viral vector vaccines, and 2 participants received inactivated vaccines (Table 1).

Table 1. Study population (all).

Subject	Gender	Age (yr)	BMI	Smoke	Which dose	Type of this dose	Type of previous dose	History of infection	Post-vaccination infection
1	Male	21	22.73	None	First dose	mRNA	/	Yes	None
2	Male	22	26.20	Yes	First dose	mRNA	/	None	Yes
3	Male	61	17.82	Yes	Second dose	mRNA	mRNA	None	None
4	Female	30	19.54	None	Second dose	mRNA	mRNA	None	None
5	Male	53	21.80	Yes	Second dose	mRNA	mRNA	None	None
6	Male	37	18.25	None	Second dose	mRNA	mRNA	None	None
7	Male	28	21.02	None	Second dose	mRNA	mRNA	None	None
8	Male	35	18.69	None	Second dose	mRNA	mRNA	None	None
9	Female	29	18.23	Yes	Second dose	mRNA	mRNA	Yes	None
10	Female	47	21.51	None	Second dose	mRNA	mRNA	None	None
11	Female	35	24.26	None	Second dose	mRNA	mRNA	Yes	None
12	Female	21	25.47	Yes	Second dose	mRNA	mRNA	None	None
13	Female	27	19.57	None	Second dose	mRNA	mRNA	None	Yes
14	Male	31	19.47	None	Second dose	mRNA	Viral vector	None	None
15	Male	32	22.51	Yes	Second dose	mRNA	Viral vector	None	Yes
16	Male	36	23.03	None	Second dose	mRNA	Inactivated	None	Yes
17	Female	44	23.28	Yes	Second dose	mRNA	Inactivated	None	None

Post-vaccination infection: infection within 2 weeks of vaccination.

Data from 17 cases included 9 males with an overall mean age of 34.65±11.15 years and BMI of 21.38±2.60.

BMI, body mass index; mRNA, messenger ribonucleic acid.

Regarding medical history, 3 participants had a documented history of COVID-19 infection prior to the study, while 14 participants had not previously been infected. Post-vaccination, 4 participants experienced COVID-19 infection, whereas 13 participants did not contract the virus (Table 1). This detailed characterization of the study population is crucial for understanding and contextualizing the immune response data collected throughout the study.

Dynamic changes in IgM and IgG antibody levels before and after COVID-19 vaccination

We measured the serum antibody levels of 17 subjects before and after vaccination, specifically examining the signal-to-cutoff ratio (S/CO) for IgM and IgG at different time points (Supplementary Table 1). Statistical analysis was performed using 1-way ANOVA, and the results indicate distinct trends in the S/CO values for IgM and IgG over time (Fig. 1).

Fig. 1. Changes in antibody (IgG/IgM) levels before and after vaccination. The horizontal coordinate represents the number of days, with 0 representing the day before the injection, 1 representing the day of the injection, and so on. It can be seen that IgG decreased on the day of vaccination compared to the day before vaccination, and then increased, and was significantly higher after 7 days compared to the day before vaccination. IgM did not change significantly in the days after vaccination, but showed an overall increasing trend and was significantly different from the day before vaccination after the 10th day of vaccination.

Ig, immunoglobulin; S/CO, signal-to-cutoff ratio.

^*p<0.05 vs. day before vaccination (IgM); ^##p<0.01, ^####p<0.0001 vs. day before vaccination (IgG).

For IgM, the results show a gradual increase in S/CO values following vaccination. The baseline level (pre) was 0.4565±0.0516, and on Day 1, although there was a slight increase to 0.4712±0.0419, this change was not statistically significant (p>0.9999). The IgM S/CO values continued to rise at subsequent time points, reaching 1.2994±0.3507 on Day 7, although this increase was still not statistically significant (p=0.3211). However, by Day 10 (1.8571±0.5980, p=0.0205) and Day 14 (1.8965±0.6150, p=0.0160), IgM levels showed significant elevation compared to baseline, indicating a marked immune response at these time points.

In contrast, the dynamics of IgG S/CO values exhibited a different pattern. The baseline level (pre) was 4.2700±0.5529, which significantly decreased to 2.0706±0.2813 on Day 1 (p=0.0018), suggesting a transient drop in IgG levels immediately after vaccination. However, by Day 5 (5.5376±0.3521, p=0.1602), IgG levels surpassed the baseline and continued to rise in the following days. Notably, significant increases were observed on Day 7 (8.6894±0.3718, p<0.0001), Day 10 (10.2350±0.4600, p<0.0001), and Day 14 (11.1990±0.5653, p<0.0001), demonstrating a robust IgG immune response compared to baseline.

Fig. 1 illustrates these dynamics, where the S/CO curves for IgM and IgG display markedly different patterns. IgM S/CO values increased steadily over the first few days post-vaccination, peaking on Days 10 and 14, while IgG S/CO values initially decreased, followed by a sharp rise, exceeding baseline levels from Day 7 onward. This trend suggests that IgG and IgM exhibit distinct temporal dynamics in response to the vaccine. In summary, IgM responses reached statistical significance on Days 10 and 14, while IgG responses showed significant increases starting on Day 7 and continued to intensify thereafter. These data highlight the different temporal characteristics of the immune responses, with IgG likely playing a key role in the establishment of long-term immune memory following vaccination.

Analysis of differential antibody responses between post-vaccination healthy and infection groups

We further divided the 17 subjects into 2 groups based on whether they remained healthy or contracted COVID-19 after vaccination: the post-vaccination healthy group (n=13) (Supplementary Table 2) and the post-vaccination infection group (n=4) (Supplementary Table 3). This division allows us to explore the differential antibody responses between individuals who remained healthy and those who were infected after vaccination.

In the post-vaccination healthy group, the IgM levels showed minimal fluctuation over the 14 days following vaccination. As shown in Fig. 2A, the S/CO values of IgM remained relatively stable, with only slight increases observed. For instance, at Day 7, IgM levels rose modestly (0.6038±0.0347, p=0.1518) but did not reach statistical significance. By Day 14, the IgM S/CO values remained steady (0.6300±0.0421, p=0.6064), suggesting that the immune response, as measured by IgM, was not robust in these individuals. This pattern indicates that the production of IgM was relatively moderate, consistent with the early immune response typically expected following vaccination, where IgM is produced as the first line of defense.

Fig. 2. Changes in antibody (IgG/IgM) levels before and after vaccination (subgroup situation analysis). Cases were divided into 2 groups according to whether or not they were infected after vaccination. There was no significant change in IgM in the uninfected population after vaccination, but IgM was significantly higher in the infected population. The IgG all showed a decrease and then an increase after vaccination, regardless of whether or not they were infected with the virus after injection, but it can be seen that the momentum of the rise of IgG was significantly higher in the infected group after vaccination than in the healthy group after vaccination.

Ig, immunoglobulin; S/CO, signal-to-cutoff ratio.

^*p<0.05 and ^****p<0.0001 vs. pre-vaccination (healthy group); ^#p<0.05, ^##p<0.01, and ^####p<0.0001 vs. pre-vaccination (infection group).

In contrast, the post-vaccination infection group exhibited a more pronounced increase in IgM levels. As shown in Fig. 2B, IgM levels began to rise significantly by Day 7 (3.5600±0.7375, p=0.0069), and this increase continued through Day 14 (6.0125±1.0796, p<0.0001). This marked elevation in IgM levels suggests a heightened immune response in individuals who contracted COVID-19 post-vaccination. Given that IgM is an early antibody produced in response to viral infection, its rapid and significant increase in the infection group highlights the body’s urgent response to viral exposure, as IgM plays a key role in the initial immune defense. The faster and sharper increase in IgM levels in the infection group may also indicate the activation of a more robust immune response upon infection, reflecting the body's attempt to combat the virus after exposure.

For the IgG response, a different dynamic was observed. In the post-vaccination healthy group, IgG levels initially decreased during the first 3 days following vaccination, falling below the pre-vaccination baseline (Fig. 2B). This drop was particularly significant on Day 1 (2.3838±0.3186, p=0.0300), suggesting a transient suppression or delay in IgG production immediately after vaccination. However, by Day 5, IgG levels began to rise above the baseline (5.7762±0.4107, p=0.1063), and by Day 14, the levels showed a marked increase (10.4633±0.5287, p<0.0001), indicating the establishment of a strong adaptive immune response. This trend reflects the gradual maturation of the immune system’s memory response, as IgG is typically associated with long-term immunity and memory B-cell activation.

In the post-vaccination infection group, IgG levels exhibited a similar overall pattern, with an initial drop followed by a subsequent rise. However, it is noteworthy that from Day 1 to Day 5, IgG levels in the infection group were lower than those in the healthy group. On Day 1, the IgG S/CO value was 1.0525±0.1156 (p=0.0476), which was significantly lower compared to the healthy group, indicating a delayed or suppressed early IgG response in those who became infected. This could suggest that individuals who eventually contracted COVID-19 may have had a delayed humoral response to the vaccine.

However, by Day 7, IgG levels in the infection group began to exceed those of the healthy group, with a significant rise (8.7750±0.6878, p=0.0051) that continued through Day 14 (13.5930±1.0557, p<0.0001). This surge in IgG levels post-infection likely reflects the combined effect of the vaccine-induced immunity and the body’s response to active infection, as IgG is crucial in neutralizing the virus and forming long-term immune memory. The more pronounced IgG response in the infection group after Day 7 suggests that infection triggers a more robust secondary immune response, with higher levels of IgG being produced in response to the virus.

Correlation between prior infection history and breakthrough infection

Among the 17 participants, 3 had a documented history of prior COVID-19 infection, whereas the remaining 14 did not. All 4 breakthrough infections that occurred within 2 weeks after vaccination were observed exclusively in the group without prior infection; none of the previously infected participants experienced reinfection following vaccination. When considered alongside the antibody dynamics, these findings suggest that individuals with prior infection were able to mount and sustain higher IgG levels more rapidly after vaccination, thereby avoiding the transient susceptibility window associated with the early post-vaccination decline in IgG. In contrast, participants without prior infection appeared more vulnerable to breakthrough infections during this period.

COVID-19 infection risk analysis: vaccination history and previous infection history

In this study, we further analyzed the vaccination history of 17 participants and their post-vaccination COVID-19 infection status (specifics of each case). Among the 17 participants, 4 individuals were infected with COVID-19 shortly after vaccination (defined as infection within 2 weeks of vaccination, labeled as post-vaccination infection). Notably, one of these individuals had received the first dose, while the remaining 3 had received the second dose. Furthermore, all of the infected individuals had no prior history of COVID-19 infection before vaccination.

Further analysis indicated a higher rate of infection after vaccination among those who had never been infected with COVID-19. This suggests that individuals who have not previously contracted COVID-19 may lack the immune memory generated by natural infection, making them more susceptible to viral infection even after vaccination. This finding aligns with previous research, which suggests that dual immunity, derived from both previous infection and vaccination, may provide stronger protection. For individuals who have never been infected with the virus, relying solely on vaccination may not be sufficient to fully prevent infection, particularly in the early stages after vaccination when immune protection is not yet fully established.

Moreover, although 3 out of the 4 individuals who became infected post-vaccination had received the second dose, infection still occurred, indicating that in certain cases, even after completing 2 doses, individuals may remain at risk of infection in the short-term following vaccination. This could be due to individual variations in immune response or the vaccine's efficacy against specific viral variants. These results underscore the importance of maintaining protective measures after vaccination, especially during the initial period when the immune response may not have reached its optimal level.

These findings suggest that vaccination strategies should pay particular attention to individuals with no prior history of infection, especially during the early stages post-vaccination, to enhance their protection against infection.

DISCUSSION

Our study, though preliminary, suggests the existence of a transient ‘immune trough’ in IgG levels immediately following COVID-19 vaccination, which may be associated with an increased short-term risk of breakthrough infection.

Our findings are broadly consistent with prior reports showing robust IgG but only weak or transient IgM responses following COVID-19 vaccination [14,15]. Similar to other studies, previously infected individuals mounted faster and stronger IgG responses, while infection-naïve participants required completion of the full vaccination series to achieve comparable titers. Reports on antibody durability remain variable: some observed rapid waning within 3 to 4 months [14], whereas others noted persistence of elevated levels for longer durations [16,17]. These discrepancies may reflect differences in demographics, prior infection prevalence, vaccine platforms, assay methodologies, and follow-up time frames [18,19]. The extended follow-up in our study (6 months) provides additional clarity regarding the decline of IgG compared with studies with shorter observation periods.

The antibody kinetics observed in our study align with fundamental immunology. The initial, transient decline in IgG levels immediately post-vaccination can be mechanistically explained by the principles of antibody-antigen equivalence. Vaccination, particularly with mRNA platforms, induces a swift and substantial production of spike antigen. This sudden state of antigen excess leads to the rapid formation of small, soluble immune complexes as the newly synthesized antigen binds to pre-existing, circulating IgG antibodies from previous vaccination or infection. These complexes are efficiently cleared by phagocytic cells, resulting in a temporary net decrease in measurable circulating IgG—an immune trough—until a de novo humoral response can be mounted. Subsequently, IgM, secreted by short-lived plasmablasts, wanes quickly once class switching to IgG occurs [15]. By contrast, the durability of the new IgG response is supported by long-lived plasma cells and memory B cells, explaining its persistence beyond the early phase. Booster vaccinations predominantly recall these memory responses, accounting for the minimal IgM but strong IgG induction observed thereafter. The stronger performance of mRNA vaccines likely reflects more efficient germinal center reactions, which underpin this sustained response, while age and comorbidities impair B- and T-cell function, reducing antibody magnitude and durability in vulnerable groups [18,20,21].

The transient decline in IgG immediately after vaccination—followed by robust elevation—suggests the existence of a short-term vulnerability window, during which infection risk, including breakthrough infections, may increase. While cellular immunity and memory B cells continue to protect against severe disease, waning circulating antibodies may allow viral replication in the upper respiratory tract, driving mild illness and transmission [20].

These insights support 3 actions: 1) prioritizing IgG measurement in serological monitoring, as IgM adds little diagnostic value in vaccinated populations; 2) timely booster administration to minimize temporary immunity gaps and sustain protection, especially among older adults and immunocompromised individuals; 3) preferential use of mRNA vaccines where possible, given their superior induction of IgG.

Our study is limited by its cohort size, which reduced statistical power for some subgroup analyses. The very small sample size (n=17), particularly the low number of breakthrough infections (n=4), represents the primary limitation. As a result, the findings should be regarded as preliminary and primarily hypothesis-generating. They need to be validated in much larger, prospective cohorts to confirm the observed effects and ensure their generalizability to broader populations.

In addition, the heterogeneous vaccination status of participants, including differences in vaccine platforms (mRNA, viral vector, inactivated) and dose numbers (first vs. second), may significantly influence the strength and kinetics of the immune response. This variability limits our ability to draw unified conclusions about antibody dynamics. Future studies should aim to investigate more homogeneous vaccine groups and dosing schedules, which will allow for more consistent comparisons and clearer interpretations of immune response patterns.

Furthermore, asymptomatic prior infections may have gone undetected despite screening, and follow-up was limited to 6 months, leaving the longer-term durability of IgG unresolved. Neutralization assays and cellular immune measurements were also not included, though these represent critical correlates of protection.

COVID-19 vaccination induces a predictable pattern of transient IgM and durable IgG responses. Importantly, our findings highlight that waning IgG levels create a transient window of increased susceptibility to infection, although protection against severe outcomes is largely maintained through cellular and memory immune mechanisms. This underscores the critical role of booster vaccinations in sustaining high levels of circulating antibodies and minimizing breakthrough infections, especially among vulnerable populations.

In a broader context, our data reinforce the importance of prioritizing IgG as the primary serological marker post-vaccination, optimizing booster timing, and tailoring strategies for high-risk groups. Future work should define antibody thresholds for infection versus disease protection, assess immunity beyond 1 year, and integrate neutralizing antibody and T-cell analyses. Ultimately, such knowledge will inform next-generation vaccines aimed at inducing more durable humoral immunity and reducing the intervals of transient vulnerability. We expect that our findng of a transient decrease in free vaccine IgGs directly after 2nd vaccination and the associated transient increased breakthrog infection risk might be a general phenomenone also in other vaccinations. However, this needs to be proven in further studes.

SUPPLEMENTARY MATERIALS

Supplementary Table 1

Antibody concentration (all)

Supplementary Table 2

Antibody concentration (post-vaccination healthy)

Supplementary Table 3

Antibody concentration (healthy group)