Assessment of SARS-CoV-2 Infection, Vaccination, and Immunity Status Among a Population of Dentists/Academic Professors in a Clinical Setting: One-Year Findings

Abstract

Background: This study aimed to assess the prevalence of SARS-CoV-2 infection, vaccination, and immune status among a population, both Dentists and University Professors, within a clinical setting at one and at 12 months after COVID-19 vaccination. Methods: A cross-sectional study involving 47 professionals (aged 27–52) was conducted in the University Fernando Pessoa. Participants completed an online survey on SARS-CoV-2 infection status and vaccination, received and provided plasma samples for serological analysis. The protocol was approved by the UFP-Ethics Committee. Anti-S1-RBD SARS-CoV-2 IgM and IgG antibody titration values (AU/mL) were measured, by enzyme-linked-immunosorbent assay (ELISA), with reactive immunoglobulins (Ig) seropositivity for values ≥1 AU/mL. Results: SARS-CoV-2 infection rate increased from 8.5% in July 2021 to 48.9% in June 2022, with 8.5% experiencing reinfection. Vaccination rate was 91.5% by July 2021 and increased slightly to 93.6% by June 2022; 72.3% of the sample received a third dose. IgG seropositivity increased from 91.5% to 95.7% in June 2022. After one-year, significant associations were found between IgG seropositivity and both participant’s age (p = 0.009; <50 years) and vaccine doses (p = 0.003; 1–3 doses) received. Conclusions: SARS-CoV-2 infection rate, vaccination, and IgG seropositivity rates were high and increased over one year. The age and vaccination status were associated with the immunity status at 12th month follow-up. Findings highlight variability in IgG seroprevalence due to multiple influencing factors, which justifies future studies.

1. Introduction

The global pandemic of coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), led to severe health crises, including pneumonia, respiratory failure, and widespread mortality [1]. Since its initial outbreak in Wuhan, China in December 2019, COVID-19 has resulted in 7,087,731 deaths and 777,368,929 reported cases worldwide as of 2 February 2025 [2]. In response to this unprecedented crisis, the medical and scientific community has made significant efforts to understand the transmission dynamics, clinicopathological characteristics, diagnostic tools, prevention strategies, and educational reforms related to COVID-19 [3].

Healthcare professionals face elevated risks of SARS-CoV-2 infection due to direct patient contact and potential exposure to asymptomatic carriers [4,5,6]. Dental settings pose a high transmission risk owing to close interaction with oral and respiratory secretions and frequent aerosol-generating procedures (AGPs) such as drilling and scaling. These procedures increase the potential for airborne viral spread. To mitigate these risks, stringent infection control protocols have been adopted, including enhanced ventilation, high-volume aerosol evacuation, and consistent use of personal protective equipment (PPE) [7,8]. Additionally, dental academics have experienced increased pressure during the pandemic, balancing teaching and research responsibilities while managing concerns about virus transmission [9].

Since 2020, COVID-19 vaccines authorized by the FDA (Food and Drug Administration) and EMA (European Medicines Agency) have elicited neutralizing immune responses to prevent SARS-CoV-2 infection. Various vaccine platforms—including live attenuated, inactivated, protein subunit, viral vector, DNA, and mRNA vaccines—have been critical in curbing the pandemic. However, the emergence of SARS-CoV-2 variants of concern has challenged vaccine efficacy by partially evading neutralizing antibodies and cellular immunity, raising concerns about long-term protection [10].

Real-time reverse transcriptase polymerase chain reaction (RT-PCR) has been the gold standard for SARS-CoV-2 diagnosis, though it faces limitations such as false results due to contamination, sample quality, and cross-reactivity [11,12]. Enzyme-linked immunosorbent assay (ELISA), commonly used to quantify antibodies rather than detect the virus, plays a key role in assessing immune responses [13,14]. SARS-CoV-2 infection triggers IgM and IgG production, and quantitative assays targeting antibodies against the receptor-binding domain (RBD) of the spike protein have been essential for monitoring vaccine-induced immunity and individual antibody titers over time [15,16,17]. These assays also help differentiate between vaccine-induced and infection-induced immunity [18]. Assessing immune responses in specific occupational groups, such as dental professionals and academics, is critical for understanding vaccination impact and infection risk.

Therefore, the aims of this research were as follows: (1) to determine the prevalence of SARS-CoV-2 past or current infection and COVID-19 vaccination status (number of vaccine doses received) among Portuguese dental school faculty members, who also practice clinically. Self-reported data were collected in July 2021 and June 2022—corresponding to one month and 12 months after the initial phase of vaccine distribution in Portugal, respectively; (2) to analyze associations between SARS-CoV-2 infection and vaccination status with participants’ demographic characteristics, general health, and professional activities, under the null hypothesis that no such associations exist; (3) to assess humoral immune status at both time points by measuring anti-S1-RBD IgM and IgG antibody levels (AU/mL) using ELISA; and (4) to evaluate whether IgG titers are associated with age, gender, SARS-CoV-2 infection history, or number of vaccine doses received.

2. Materials and Methods

This research protocol was approved (FCS/PI-501/23-3) by the Ethics Committee of the University Fernando Pessoa (UFP). Information on the study protocol, the rights to withdraw at any point, and the informed consent was provided by institutional email to all (N = 62), both Dentists and Dentistry University Fernando Pessoa professionals, that at the time were working at the Dentistry Master-UFP. After signing the consent forms, the subjects who freely and voluntarily accepted to participate completed a brief online self-report survey, in two-time frames, July 2021 and June 2022.

2.1. Research Polulation, Sample, Inclusion and Exclusion Criteria

The population of this research involved both Dentists and University Fernando Pessoa Professors, professionals (N = 62), that were at the time (July of 2021 year) the workforce in the dental pedagogical clinical setting of the Dentistry Master program at UFP. Only professionals who had either received COVID-19 vaccines or remained unvaccinated until July 2021 and voluntarily agreed to participate in the study by completing the entire online self-report survey and by providing plasma samples for IgG and IgM antibodies titration (within the same week of survey completion), were included. Nine professionals did not respond to the self-report survey and were therefore excluded. Among the remaining 53 participants, only 47 met all inclusion criteria and were enrolled based on the population proportion (75.8%). The cross-sectional study was carried out in July 2021 (baseline data) and in June 2022 (final data) to the same sample of professional sample (N = 47) in the clinical setting of Faculty of Health Sciences of UFP.

2.2. Data Collection

The participants completed a brief online self-reported form that gathered demographic, general health, and professional clinical setting activity information, and assessed the SARS-CoV-2 past or present infection history and SARS-CoV-2 vaccination status (number of doses received). The self-reported infections were confirmed with testing by means of rapid tests (immunochromatography) or laboratory testing (RT-PCR) and were never based on personal assumptions. The asymptomatic patients were only included in the infected group if there was serological evidence of SARS-CoV-2.

After this and within the same weeks, plasma sample collection was carried out, by duly qualified and trained professionals, the clinical laboratory technicians (M.D.) and nurses from the UFP-Hospital (UFP-HE) at the UFP-Dentistry Pedagogical Clinic across two periods (July 2021 and June 2022), over one year. The collected samples were analyzed by the Unilabs laboratory at HE-UFP. Participants were informed in advance that the primary potential complications were those inherent to routine venous blood sampling. No complications occurred during the biological sample collection procedure. Sample collection, monitoring, and ensuring appropriate storage and preservation were in accordance with the National Health regulations. Samples were stored by HE-UFP and managed in terms of prescription, analytical results, and disposal following the final collection in July 2022. Sample management was all overseen by the physician (D.A.) of the UFP and HE-UFP Occupational Health Service.

For in vitro diagnostics, the Atellica^® IM SARS-CoV-2 IgG (sCOVG) assay (Siemens Healthcare Diagnostics) was used for the qualitative and quantitative detection of IgG antibodies, including neutralizing antibodies against SARS-CoV-2, in serum and lithium-heparin plasma obtained via venous or capillary puncture using the Atellica^® IM Analyzer. For this study, the Atellica^® IM sCOVG assay categorized IgG and IgM quantification results as follows: (1) Serum levels < 1.00 AU/mL were considered non-reactive, indicating negativity for SARS-CoV-2 IgG/IgM antibodies; (2) Serum levels ≥ 1.00 AU/mL were considered reactive, indicating positivity for SARS-CoV-2 IgG/IgM antibodies [19]. The ≥1 AU/mL threshold was established by the manufacturer following ROC curve analysis to optimize clinical sensitivity and specificity. It is part of the FDA/CE-marked assay specifications. Manufacturer data reports sensitivity: 96.4% at ≥21 days post-PCR, specificity 99.9%. The Public Health England (PHE) evaluation reports sensitivity 78.3% (≥21 days) and specificity 100% at the ≥1 threshold. The cut-off correlates well with functional neutralization titers, as confirmed by correlation studies involving surrogate virus neutralization tests [19,20,21].

Participation in the study was entirely voluntary and without financial compensation. All analytical results remained strictly confidential, and the identities of the participants were neither disclosed nor revealed at any stage. Personal data were handled in full compliance with current data protection regulations. To ensure confidentiality, collected personal data were coded and pseudonymized. Details regarding data confidentiality and research ethics were explicitly outlined in the informed consent form.

2.3. Data Statistical Analysis

Data analysis was performed using IBM^® SPSS^® Statistics for Windows, Version 29.0 (IBM Corporation, Armonk, NY, USA) [22] considering a significance level set at p < 0.05 for all inferences. Categorical variables were expressed as counts and percentages, and quantitative variables as mean and standard deviation (SD) and/or median and interquartile range (IQR) as well as minimum and maximum values, unless otherwise specified. Descriptive and inferential analyses were conducted to evaluate the effect of the variables collected from the self-reported surveys and the two-plasma sample performed in July 2021 and June 2022, for each participant. Inferential analysis was performed using appropriate statistical techniques, selected after verifying the assumptions of parametric methods, using the chi-square or Fisher tests. If not applicable, non-parametric tests were conducted to assess differences between study groups based on population medians. Prevalence measures were expressed as percentages with corresponding 95% confidence intervals, calculated using either the Wald method or the exact method, depending on the applicability of each case. In most cases, the normality assumption for quantitative variables was not met. Therefore, comparisons of central tendency measures were performed using non-parametric tests, the Kruskal–Wallis test for comparisons involving more than two variable groups and the Mann–Whitney U test for comparisons between two independent groups.

3. Results

3.1. Prevalence of SARS-CoV-2 Infection

By July 2021, 8.5% (n = 4) of individuals reported a prior SARS-CoV-2 infection. By June 2022, another 20 individuals reported having had infection, totaling 24 by the end of the study (51.1%). The incidence of reported SARS-CoV-2 infection across age groups throughout the study period was as follows: 57.7% among individuals aged 40–49 years, 45.5% in the 30–39 age group, 42.9% in those aged ≥50 years, and 33.3% in individuals under 30 years of age. (Table S1—Supplementary file)

3.2. SARS-CoV-2 Vaccination Status

The vaccination rate was 91.5% (n = 43) at baseline (July of 2021) and 93.6% (n = 44) until June of 2022 (Table S2—Supplementary file). No statistically significant relationships were found (p > 0.05) for the total number of vaccine doses administered and the participants’ demographic and general health characteristics.

3.3. Analysis of SARS-CoV-2 Infection and Participants Vaccination Status

No significant associations were found between age, gender, presence of chronic conditions or comorbidities, clinical activity and SARS-CoV-2infection history, or number of vaccine doses administered (Table S3—Supplementary file).

3.4. Analysis of Immunity Status, IgM and IgG Antibody Titration Values and Participants Age, Gender, SARS-CoV-2 Infection and Vaccination Status

In the first sample collection, conducted in July of 2021 (

Table 1. Descriptive analysis by age, gender, SARS-CoV-2 infection history, vaccination status of participants, for the reactive IgG antibody categorization (positive/negative) and for the IgG and IgM titration values—1st plasma sample, in July 21.

Reactive IgG Antibody Categorization and IgG and IgM Median and Level Titration Values (AU/mL)—July 2021
Participants (Subjects)		SARS-CoV-2 Infection July 2021	Vaccine Status July 2021	N	IgG			IgM
Age (Years)	Gender				Pos/Neg *	Median Value (AU/mL)	Ig G Level or Min–Max (AU/mL)	Median Value (AU/mL)	IgM Level or Min–Max (AU/mL)
<30 years	F	No	Yes	1	Pos	-	29.900	-	0.300
	M	Yes	Yes	1	Pos	-	105.500	-	0.400
		No	Yes	1	Pos	-	84.100	-	0.100
30–39 years	F	No	Yes	5	Pos	8.100	1.4–73.0	0.200	0.100–0.400
			No	2	Neg	0.500	0.500	0.300	0.300
	M	No	Yes	4	Pos	29.500	2.2–142.4	0.250	0.200–0.400
40–49 years	F	Yes	Yes	1	Pos	-	84.100	-	0.100
		No	Yes	18	Pos	18.100	1.1–308.9	0.400 a	<0.10–1.20 a
	M	Yes	Yes	1	Pos	-	10.300	-	0.900
		No	Yes	6	Pos	32.050	6.2–65.6	0.200 b	<0.10–1.30 b
≥50 years	F	Yes	Yes	1	Pos	-	288.100	-	0.400
		No	Yes	2	Pos	5.950	4.5–7.4	0.850 c	0.30–1.40 c
			No	2	Neg	0.500	0.500	0.250	0.100–0.400
	M	No	Yes	2	Pos	8.350	6.8–9.9	0.149	<0.10–0.200

* IgG Pos/neg—Categorization of Positive/Reactive IgG Antibody for values ≥1 AU/mL; Categorization of Negative/Non-reactive IgG Antibody for values <1 AU/mL; ^a—Positive IgM value in 3 individuals (IgM = 1.1; 2; 1.2 AU/mL); N = Number of subjects; ^b—Positive IgM value in one subject (IgM = 1.3 AU/mL); ^c—Positive IgM value in 1 individual (IgM = 1.4 AU/mL). F—Female; M—Male.

), IgG positivity was detected in 91.5% (n = 43) of the participants, with only four subjects presenting IgG anti-S1-RBD SARS-CoV-2 antibody titration values below the assay’s minimum reference threshold (serum values < 1.00 AU/mL). With respect to IgM positivity, 10.6% (n = 5) presented values above the minimum reference threshold (Table 1). Four unvaccinated individuals—all female (two aged 30–39 and two over 50)—were identified. None of the participants had prior SARS-CoV-2 infection, and all exhibited IgG negativity (≤0.5 AU/mL) for SARS-CoV-2 S1 subunit spike protein antibodies. Among vaccinated individuals (n = 43), all demonstrated positive IgG reactivity. Only four individuals reported prior SARS-CoV-2 infection: two males (one <30 years, one 40–49 years) and two females (one 40–49 years, one >50 years). Compared with those without infection history, this subgroup generally presented the highest median IgG titration values. Median IgG levels were predominantly higher in vaccinated individuals and those with prior SARS-CoV-2 infection.

In the final plasma collection (June of 2022), the IgG reactivity rate increased to 95.7% (n = 45), while IgM reactivity was detected in 4.3% (n = 2) of the participants (

Table 2. Descriptive analysis by age, gender, SARS-CoV-2 infection history, and number of vaccine doses of participants for the reactive IgG antibody categorization (positive/negative) and for the IgG and IgM titration values—final plasma sample, in June 2022.

Reactive IgG Antibody Categorization and IgG and IgM Median and Titration Values (AU/mL)—June 2022
Participant’s (Subjects)		SARS-CoV-2 Infection June 2022	No. of Vaccines Doses June 2022	N	IgG Value (AU/mL)			IgM Value (AU/mL)
Age (Years)	Gender				Pos/Neg *	Me	Level Min–Max	Me	Level Min–Max
<30 year	F	No	3	1	Pos	-	244.00	-	0.200
	M	Yes	2	1	Pos	-	118.10	-	0.400
		No	3	1	Pos	-	66.40	-	0.100
30–39 years	F	Yes	2	1	Pos	-	75.40	-	0.400
			3	2	Pos	152.60	39–266.2	0.3	0.200–0.400
		No	0	1	Pos	-	29.00	-	0.300
			3	3	Pos	157.10	77.5–173.6	0.3	0.100–0.300
	M	Yes	3	2	Pos	191.70	151.4–232	0.2	0.200
		No	2	1	Pos	-	163.20	-	0.600
			3	1	Pos	-	13.00	-	0.300
40–49 years	F	Yes	2	2	Pos	84.30	73.4–95.2	0.2	0.200
			3	7	Pos	109.70	66.7–560.1	0.30 b	0.200–1.100 b
		No	3	10	Pos	101.05	18.6–480.2	0.50 a	0.100–2.200 a
	M	Yes	2	4	Pos	475.65	67–730.7	0.2	<0.100–0.900
			3	2	Pos	162.45	114.4–210.5	0.15	0.100–0.200
		No	3	1	Pos	-	4.50	-	0.20
≥50 years	F	Yes	0	1	Neg	-	0.50	-	0.30
			3	1	Pos	-	110.20	-	0.50
		No	0	1	Neg	-	<0.50	-	0.10
			3	2	Pos	86.60	21.5–151.7	0.5	0.400–0.600
	M	Yes	3	1	Pos	-	302.80	-	0.30
		No	3	1	Pos	-	332.5	-	0.10

N = Number of subjects; * IgG Pos/neg—Categorization of Positive/Reactive IgG Antibody for values ≥1 AU/mL; Categorization of Negative/Non-reactive IgG Antibody for values <1 AU/mL; ^a—Positive IgM value in one subject (IgM = 2.2 AU/mL); ^b—Positive IgM value in one subject (IgM = 1.1 AU/mL): F—Female; M—Male.

).

A total of 45 individuals (95.7%), with a mean age of 41.71 ± 8.3 years, tested positive for IgG. Only two individuals, averaging 57.5 ± 0.7 years, had IgG levels below 1 AU/mL. Among IgG-positive individuals (n = 45), 44 received an average of 2.7 ± 0.6 doses of the SARS-CoV-2 vaccine (Table 2). One unvaccinated woman (aged 30–39) had no history of SARS-CoV-2 infection, suggesting a possible asymptomatic case. The unvaccinated group (n = 3, all female) included two individuals aged ≥50 and one aged 30–39. The latter, despite reporting no SARS-CoV-2 infection, tested IgG-positive, indicating prior virus exposure. This individual’s non-reactive IgM suggests past asymptomatic infection. The other two unvaccinated individuals had non-reactive median IgG levels, though one had a confirmed history of SARS-CoV-2 infection.

By June 2022 (Table 2), 51.0% of the sample (n = 24) had been diagnosed with SARS-CoV-2 infection, including four reinfections. Moreover, 23 individuals remained infection-free at the conclusion of the study in June 2022.

A statistically significant relationship was observed between participant age (p = 0.019), median age (42 years for IgG ≥ 1 AU/mL; 57.5 years for IgG < 1 AU/mL; p = 0.017), the number of vaccine doses (p = 0.003), and the median number of vaccine doses (Me = 3 doses; Q1/3 = 3–3; p = 0.004) with recorded IgG positivity/reactivity (

Table 3. Inferential analysis of participants’ age, gender, SARS-CoV-2 infection history, number of vaccines doses, and the prevalence of IgG (AU/mL) positivity/reactivity or negativity/non-reactivity, final plasma sample in June 2022.

Participant’s Gender, Age, SARS-CoV-2 Infection and Number of Vaccine Doses (June 2022 yr.)		IgG (AU/mL) Reactive *	IgG (AU/mL)	p Value
			No Reactive *
		n (%)	n (%)
Age (years)	<50 years	40 (88.9%)	0 (0%)	0.019 1
	≥50 years	5 (11.1%)	2 (100%)
	Median (Q1–Q3)	42 (37–46)	57.5	0.017 2
	Min-Max	26–65	57–58
Gender	Female	30 (66.7%)	2 (100%)	1.000 1
	Male	15 (33.3%)	0 (0%)
SARS-CoV-2 Infection history	No	22 (48.9%)	1 (50%)	1.000 1
	Yes	23 (51.1%)	1 (50%)
No. of vaccine doses	0	1 (2.2%)	2 (100%)	0.003 1
	1 to 3	44 (97.8%)	0 (0%)
	Median (Q1–Q3)	3 (3–3)	0	0.004 2
	Min-Max	0–3	0

* IgG Pos/neg—Categorization for Positive/Reactive antibody IgG for values ≥1 AU/mL; Categorization for negative/non-reactive antibody IgG for values <1 AU/mL.; ¹—Fisher test; ²—Mann-Whitney test.

). Individuals over 50 years of age presented significantly lower reactivity/positivity (AU/mL) than unvaccinated individuals did (zero vaccine doses or a median of zero doses).

4. Discussion

This study aimed to evaluate SARS-CoV-2 infection, vaccination status, and humoral immunity among dentists and university faculty at the UFP clinical setting, one month (July 2021) and 12 months (June 2022) after the initial COVID-19 vaccine rollout in Portugal. Anti-S1-RBD IgG and IgM titers (AU/mL) were measured by ELISA, with seropositivity defined as antibody levels ≥ 1 AU/mL. Associations between prior infection, number of vaccine doses received, and IgG/IgM responses were analyzed in relation to seroreactivity over time.

At the time of the first sample collection, no international standard for antibody testing was available [16]. Diagnostic tools included RT-PCR, semi-quantitative ELISA assays, and rapid tests. While rapid immunochromatographic tests offered quick results, their lower sensitivity compared to automated serological assays limited their reliability. The absence of standardized protocols and proper monitoring may have led to overestimated assumptions regarding immunity, highlighting the need for careful interpretation of early serological data.

Healthcare professionals are considered a risk group for various infectious diseases, including SARS-CoV-2. Consequently, several studies have focused on this population [23,24,25,26]. However, a lack of research on the prevalence of SARS-CoV-2 infection and vaccination status could be identified among oral health professionals, including professionals, dentists, and university professors. This gap motivated the present academic research. When this research was conducted, no references were identified in the literature regarding validated surveys for SARS-CoV-2 that specifically targeted the academic community, including professors and/or dentists. However, other studies have utilized similar surveys, validating their applicability and scope for use in relevant populations [25,27,28]. Given their clinical training and familiarity with medical concepts, dentists are capable of providing informed and accurate self-assessments of their health conditions. While independent verification adds rigor, self-reporting remains a practical and ethical method for collecting health data in studies involving healthcare professionals.

In the present study, participants were predominantly female, with an average age of 42.38 years and a low comorbidity prevalence; however, owing to the small sample size, no significant associations were found between comorbidities reported and SARS-CoV-2 infection or vaccination status. Cohort studies with multivariate analyses have consistently linked greater disease severity to demographic factors such as older age and male gender. Strong associations have also been observed with obesity, diabetes [29,30], and chronic obstructive pulmonary disease [31,32,33,34]. Conversely, evidence linking hypertension, cancer, smoking, and kidney disease to COVID-19 severity is limited [35].

Our study found that 40.4% of participants reported SARS-CoV-2 infection between December 2021 and June 2022, coinciding with national infection peaks in January 2021 and January 2022 reported by Portugal’s National Health Service [36]. Infection rates among dentists have varied from 1% to 15% in prior studies [37,38,39]. During the early pandemic (March–May 2020), 63.8% of 47 participants maintained continuous clinical activity, increasing to 93.6% engaging in both clinical practice and academic roles by June 2021. No significant association was observed between professional activity and SARS-CoV-2 infection or vaccination status, suggesting effective infection control measures in clinical and academic settings.

Vaccination rates were high, with 91.5% vaccinated by July 2021 and 93.6% at 12 months, surpassing the 68% rate in the general population aged 25–49 years [36]. Comparable vaccination rates have been reported among dentists globally: 86% in Lebanon (February 2021), 85.8% in the Czech Republic (May 2022), 85% in Israel (April 2020), 78.5% in Greece (December 2020), and 71.6% across French-speaking countries (December 2020) [40,41,42,43,44]. These findings align with the 81.1% vaccine acceptance rate among dentists reported by Lin et al. [45]. Our findings showed no significant association between SARS-CoV-2 infection, vaccination status, and participants’ demographic, health, or professional characteristics, supporting acceptance of the null hypothesis.

Measuring IgG antibody levels is essential for evaluating vaccine-induced humoral responses [46]. Correlations between antibody titers and functional immunity against SARS-CoV-2 have been documented, though these levels fluctuate over time [47]. Immunity evolution is influenced by viral infection, viral dynamics, and host factors. Sustained protection may persist despite declining antibody levels due to memory B cells that rapidly respond upon reinfection, and T cells that support humoral immunity. However, viral mutations can alter antigenic sites, potentially evading existing antibody recognition [48]. IgM values above the reference threshold were considered indicators of recent SARS-CoV-2 infection, as IgM levels typically peak around 14 days after symptom onset and decline to undetectable levels approximately 35 days later [49]. The low IgM levels found may be explained by three possible reasons: a rapid class-switching to IgG (especially in secondary or reinfections) [50], an overall weak or absent IgM response (particularly in mild or asymptomatic infections) [51], or individual immune variability. Additionally, the timing of sample collection relative to symptom onset or assay sensitivity may influence the measured IgM level. Elevated serum IgG levels may result from prior SARS-CoV-2 infection or vaccination. Therefore, it was crucial to analyze data on individuals’ infection history and vaccination status, including the number of vaccine doses received, and correlate these factors with IgG antibody levels measured across different collection periods.

A major challenge in COVID-19 vaccination is determining the duration and strength of antibody-mediated immunity following infection or vaccination. IgG antibody levels typically peak one month after the second mRNA vaccine dose but can decline rapidly to 6% of peak levels within four months [52]. After natural infection, IgG antibodies provide protection against reinfection for up to six months, though seroreversion may occur 3–4 months post-infection [24,53]. Vaccinated individuals with prior SARS-CoV-2 infection exhibit significantly higher antibody titers than those without previous infection [54]. Hansen et al. reported a faster and stronger immune response post-vaccination in previously infected individuals, with weaker responses linked to older age and male gender [55], partially aligning with our findings.

Some studies explored SARS-CoV-2 vaccines and IgG antibody serological outcomes in healthcare groups and specified the serologic outcome units, such as AU/mL, IU/mL, and BAU/mL. Different IgG positivity reference outputs, such as mean titers, serological positivity, or negative reactivity, have been reported [56,57,58,59,60]. The ≥1.0 index threshold for positivity on the Atellica IM SARS-CoV-2 IgG assay is grounded in Siemens’s CE-IVD (in vitro and FDA-EUA approval, based on rigorous ROC and specificity analyses). Independent validation by Public Health England showed that the cut-off aligns with mean + 3 SD of negatives (~0.94 index), yielding ~99.9% specificity and strong sensitivity at ≥21 days post-infection. This threshold is analytically sound (exceeding LOD and within linear range), reproducible (CVs < 5%), and correlates well with neutralizing titers when standardized to WHO BAU/mL [61,62]. Using WHO standard calibration, the index corresponds to ~21.8 BAU/mL [19].

The prevalence values of IgG seropositivity varied across studies, from higher rates of 68.42% to 100% [56,58,63]. Low seropositivity rates of 20% (39 participants) [64], 19.1% (62 participants) [28], 10.8% (54 participants) [65], 6.2% (7 participants) [59], 5.2% (146 participants) [66], and 0.43% (4 participants) [67] have also been reported.

Potential correlation of IgG seropositivity prevalence rates have also been explored in some studies, with variables such as COVID-19 history [56,59]; hematological parameters [56]; ethnicity [57,64]; diabetes [57]; smoking habits [57,65]; blood type [58]; comorbidities [24,63]; working conditions; virus infection outside work [63]; use of protective equipment [63,66]; infection-related symptoms [64,67]; household cohabitants [28,59,67]; body mass index [65]; geographic location [65,66]; and educational level [28].

This study found that IgG seropositivity increased from 91.5% in July 2021 to 95.7% in June 2022. A significant association was observed between IgG seropositivity and both participant age (<50 years) and the number of vaccine doses received (1–3 doses), leading to a partial rejection of the null hypothesis.

Despite the relatively small sample size, the cohort is representative of dental professionals and academic staff in clinical settings. Findings should be interpreted cautiously and not broadly generalized. The sample size may have limited the ability to detect causal links between occupational risk factors and immune responses. The complex interplay between vaccination, prior infection, and individual immune variability complicates the assessment of long-term immunity. Current literature also reflects ongoing uncertainties regarding vaccine durability and efficacy, reinforcing the need for further research on humoral and cellular immune responses. Notably, participants showed effective protection through adherence to preventive measures, and those with both vaccination and prior infection exhibited higher IgG titers. These results support the value of booster vaccination strategies to sustain protective immunity in clinical academic settings.

Future research should focus on longitudinal studies to assess the long-term effectiveness of infection control measures, vaccination programs, and antibody dynamics. A deeper understanding of both humoral and cellular immunity in dental professionals, staff, and students is crucial. Enhancing diagnostic accuracy in seroprevalence studies will further support targeted prevention strategies and strengthen occupational health in clinical and academic settings.

5. Conclusions

This study found that SARS-CoV-2 infection prevalence increased from 8.5% to 51.1% over one year among dentists and university professors in clinical settings. Vaccination rates were high, rising slightly from 91.5% to 93.6% at 12 months. No significant associations were observed between infection prevalence or vaccine doses and participants’ demographics, health, or clinical activity. IgG seropositivity also increased from 91.5% to 95.7%, with a significant correlation between seropositivity, age (<50 years), and vaccine doses (1–3). These findings underscore the variability in humoral immunity and highlight the need for ongoing surveillance and targeted risk assessment, especially in clinical academic workforces, to guide occupational health strategies amid evolving SARS-CoV-2 variants.