Serological Mumps Evaluation of Medical Students in Catalonia (Spain): Is a Third Dose of Vaccine Needed?

ABSTRACT

While measles‐mumps and rubella vaccines (MMR) have reduced mumps incidence, outbreaks continue to occur in developed countries. The aim of the study was to evaluate the serological status against mumps and its association with vaccination history among medical students in Catalonia (Spain). A cross‐sectional study was conducted during 2023–2025 among medical students attending four university teaching‐units. Participants completed a questionnaire and provided blood samples for mumps IgG testing. Associations between positive serology and vaccination status and the independent variables were assessed calculating adjusted OR (aOR) and 95% confidence interval (CI) after adjustment for age, sex, and degree year. Among 235 participants, 94.5% had received 2‐MMR doses, and 94.9% had been vaccinated more than 15 years earlier. Complete vaccination was associated with being a 5th‐year student (aOR 5.45; 95%CI: 1.38–21.50); 80.4% showed positive IgG, being higher in women (aOR 2.52; 95%CI: 1.20–5.30). Seropositivity declined with increasing time since last vaccination. Although the proportion of students serologically protected is acceptable, more than 15 years have passed since the last vaccine dose in most of the students and waning immunity is expected. The recommendation of a third MMR dose for medical students and healthcare workers in mumps outbreak settings should be considered.

1. Introduction

Mumps is an acute viral infection characterized by non‐suppurative swelling and tenderness of parotid glands. The disease is self‐limiting, with one third of infected persons remaining asymptomatic [1]. Severity increases after puberty, when extra‐salivary gland involvement becomes more frequent. In the pre‐vaccine era, central nervous system involvement represented the most common extra‐salivary manifestation [2].

Prior to widespread vaccine use, annual incidence ranged from 100 to 1000/100,000 population, and most adolescents, had serological markers of prior infection. In controlled clinical trials it was observed that primary vaccine failure occurs in 5% of vaccinated persons, but most non‐responders seroconvert following a second dose [3, 4]. The estimated mumps vaccine effectiveness in the field use is 76%–78% for one dose and 86%–88% for two doses [5]. Gender does not appear to influence response to vaccination [4].

Despite the success of the 2‐dose measles, mumps, and rubella (MMR) vaccination schedule, long‐term immunity conferred by the mumps component is lower than that conferred by measles and rubella [6]. Although two doses of vaccine are more effective than one, their effectiveness remains insufficient to prevent infection in high‐exposure situations, and outbreaks can occur [4].

The mumps virus is considered serologically monotypic, but there is some evidence that antigenic differences between strains may allow for breakthrough infections by heterologous strains in persons with low neutralizing antibody levels [4].

In Catalonia, MMR vaccination was included into the immunization schedule in 1980 at age 12 months. The 2‐dose schedule (at 15 months and 11 years) was introduced in 1988. In 1999 the second dose was moved to 4 years, and since 2008 doses have been administered at 12 months and 3–4 years. The MMR vaccine containing the Rubini strain, associated with low seroconversion [5], was administered as the first dose for the 1993–1995 birth cohorts and as the second dose for the 1983–1985 birth cohorts [7]. Consequently, all persons born from 1985 (aged 38 years in 2023) onward should have received two doses of MMR vaccine.

In 2024, MMR vaccination coverage in Catalonia reached 95.7% for one dose and 93.2% for two doses [8].

While effective vaccines have reduced mumps incidence, mumps outbreaks continue to occur in developed countries. Before the current live‐attenuated vaccine, epidemics occurred every 2–5 years [1]. Current patterns suggest that secondary vaccine failure is the major contributor to outbreaks appearance, mainly involving persons aged 18–25 years [4].

Based on the effectiveness of a third MMR vaccine dose (MMR3) in controlling a mumps epidemic at the University of Iowa in the United States (US), where 98.1% of students had previously received two doses of MMR vaccine, in 2018 the Advisory Committee on Immunization Practices (ACIP) recommended MMR3 for exposed persons in outbreak settings [9].

Persons with prior natural infection are considered immune for life, whereas adults without evidence of immunity should receive, according to the US Centers for Disease Control and Prevention (CDC), at least one MMR vaccine dose. Serological screening is not routinely recommended because vaccinating immune individuals poses no risk [10].

This study of medical students attending four university teaching units in Catalonia (Spain) aimed to assess serological status against mumps and its association with vaccination history.

2. Materials and Methods

2.1. Study Design and Participants

A cross‐sectional study was conducted. Invited to participate during the academic years 2023–2024 and 2024–2025 were medical students attending four university teaching units in Catalonia (Hospital Clínic of Universitat de Barcelona, Hospital Universitari Vall d'Hebron and Hospital Universitari Germans Trias i Pujol of the Universitat Autònoma de Barcelona, and Hospital Universitari Arnau de Vilanova of Lleida).

2.2. Serology

Blood samples were drawn from the participants. Serological immunoglobulin G (IgG) determination performed with Lisaion XL (Diasorin) was considered positive for > 11 UA/mL, negative for < 9 UA/mL, and indeterminate for 9–11 UA/mL. For determinations using VIRCLIA (Vircell), the antibody index was considered reactive for > 1.10 units, non‐reactive for < 0.90 units, and indeterminate for 0.90‐ < 1.10 units. For analysis purposes, indeterminate values were considered negative.

2.3. Questionnaire

Students who agreed to participate were administered a questionnaire regarding compliance with both the Catalan recommended vaccination schedule and the official calendar of the Interterritorial Council of the Spanish Health System. The questionnaire, based on the European Joint Action on Vaccination (EU‐JAV) student survey [11], was adapted to ensure conceptual and semantic equivalence in the specific context of medical students in Catalonia. To check for clarity and readability, the questionnaire was pilot‐tested on 10 medical students and healthcare professionals.

2.4. Sample Zize

The minimum sample size calculated considering that the number of medical students from 3rd to 5th academic years rotating through each participating teaching unit was between 180 and 250 students, a prevalence of IgG against mumps of 76% [12] (worst‐case scenario), a precision of 0.5, and a design effect of 1.5 was 329 students.

2.5. Data Collected

Information was collected on age, sex, country of birth and future medical specialty (medical, surgical, other). Data on MMR vaccination (number of doses received and the dates of each dose) were obtained. In addition, attitudes to MMR vaccination were evaluated on a 5‐point Likert scale (strongly agree to strongly disagree), with response categories ultimately dichotomized into ‘agree’ (strongly agree, somewhat agree,) and ‘disagree’ (neither agree or disagree, somewhat disagree, and strongly disagree).

2.6. Statistical Analysis

Associations between the dependent variables (positive serology and vaccination status) and the independent variables were assessed using the odds ratio (OR) and 95% confidence interval (CI). A multivariate analysis to calculate adjusted OR (aOR) values was performed using logistic regression adjusted for age, sex, and degree year.

2.7. Ethical Considerations

This study was conducted in accordance with the Declaration of Helsinki and was approved by the ethics committees of the University of Barcelona (CBUB‐IRB00003099) and of each participating hospital (CEIm Hospital Universitari Vall d'Hebron: EOM(AG)034/2023; CEIm Hospital Universitari Arnau de Vilanova: CEIC‐2884; CEIm Hospital Clínic de Barcelona: HCB/2023/1154; and CEIm Hospital Universitari Germans Trias i Pujol: PI‐23‐237). All subjects included in the study received detailed information on the study and its aims, were guaranteed anonymity and confidentiality, and granted their consent to participate.

3. Results

A total of 255 students (77.51% of the estimated sample size) agreed to participate in the study. Finally, 235 participants were included in the analysis after 20 students were excluded, 19 because of missing vaccination data and 1 because of missing serological data.

Most participants were female (189, 80.4%), aged ≤ 24 years (216, 91.9%), born in Spain (224, 95.3%), and over half were 5th‐year medical students (143, 60.9%). The majority had received the complete schedule of two doses of mumps‐containing vaccine (222, 94.5%), and for most students ≥ 15 years had passed since the last dose received (223, 94.9%). The most frequent interval between doses was 1–4 years (156, 66.4%). The only variable associated with a completed vaccination schedule was being a 5th‐year student (aOR 5.45; 95%CI: 1.38–21.50). Positive attitudes to MMR vaccination were higher than 98% and no differences were observed in relation to a completed schedule (Table 1).

Table 1.

Factors associated with a completed MMR vaccination schedule.

	All N = 235 N (%)	Completed vaccination N = 222 N (%)	Incomplete vaccination N = 13 N (%)	OR (95% CI)	p value	aOR (95% CI)	p value
Sex
Male	46 (19.6)	43 (19.4)	3 (23.1)	Ref.		Ref.
Female	189 (80.4)	179 (80.6)	10 (76.9)	1.25 (0.33–4.73)	0.74	0.98 (0.24–3.95)	0.98
Age group
19–24	216 (91.9)	204 (91.9)	12 (92.3)	Ref.		Ref.
25–31	19 (8.1)	18 (8.1)	1 (7.7)	1.06 (0.13–8.61)	0.96	1.78 (0.21–15.42)	0.60
Country of birth
Spain	224 (95.3)	212 (95.5)	12 (92.3)	Ref.		Ref.
Other	11 (4.7)	10 (4.5)	1 (8.3)	0.57 (0.07–4.80)	0.60	0.41 (0.04–3.98)	0.44
Medical degree year
3rd	33 (14.0)	33 (14.9)	0 (0)	—	—	—	—
4th	37 (15.7)	32 (14.4)	5 (38.5)	Ref.		Ref.
5th	143 (60.9)	139 (62.6)	4 (30.8)	5.43 (1.38–21.36)	0.02	5.45 (1.38–21.50)	0.02
6th	22 (9.4)	18 (8.1)	4 (30.8)	0.70 (0.17–2.96)	0.63	0.76 (0.18–3.34)	0.72
Medical degree years combined
3rd–4th	70 (29.8)	65 (29.3)	5 (38.5)	Ref.		Ref.
5th–6th	165 (70.2)	157 (70.7)	8 (61.5)	1.51 (0.48–4.79)	0.48	1.83 (0.56–5.98)	0.32
Serology
Negative	46 (19.6)	43 (19.4)	3 (23.1)	Ref.		Ref.
Positive	189 (80.4)	179 (80.6)	10 (76.9)	1.25 (0.33–4.73)	0.74	1.17 (0.28–4.92)	0.83
Future specialty a
Medical	95 (41.3)	92 (42.4)	3 (23.1)	Ref.		Ref.
Surgical/medical‐surgical	123 (53.5)	113 (52.1)	10 (76.9)	0.37 (0.10–1.38)	0.14	0.36 (0.09–1.39)	0.14
Family/community	8 (3.5)	8 (3.7%)	0 (0)	—	—	—	—
Preventive	2 (0.9)	2 (0.9%)	0 (0)	—	—	—	—
Other	2 (0.9)	2 (0.9%)	0 (0)	—	—	—	—
MMR vaccine is safe a
Disagree	4 (1.8)	4 (1.8)	0 (0)	Ref.		Ref.
Agree	227 (98.3)	214 (98.2)	13 (100.0)	—	—	—	—
MMR vaccine is important a
Disagree	3 (1.3)	3 (1.4)	0 (0)	Ref.		Ref.
Agree	228 (98.7)	215 (98.6)	13 (100.0)	—	—	—	—
Would recommend the MMR vaccine to patients a
Disagree	1 (0.4)	1 (0.5)	0 (0)	Ref.		Ref.
Agree	230 (99.6)	217 (99.5)	13 (100.0)	—	—	—	—

Abbreviations: aOR, adjusted OR; CI, confidence interval; MMR, measles‐mumps‐rubella (vaccine); OR, odds ratio.

^aFour students excluded due to missing responses.

A positive serological result was observed in 80.4% of the students, and it was more frequent among females (aOR 2.52; 95%CI: 1.20–5.30). Students in the 5th academic year showed higher odds of having a positive serology compared with third‐year students, although this association did not remain statistically significant in the multivariate analysis (aOR 2.43; 95% CI: 0.92–6.44). None of the other independent variables, including years since last dose, were associated with positive serological results (Table 2).

Table 2.

Factors associated with positive serological results for mumps.

	Positive serology N = 189 N (%)	Negative serology N = 46 N (%)	OR (95% CI)	p value	aOR (95% CI)	p value
Sex
Male	31 (16.4)	15 (32.6)	Ref.		Ref.
Female	158 (83.6)	31 (67.4)	2.47 (1.19–5.10)	0.02	2.52 (1.20–5.30)	0.02
Age group
19–24	173 (91.5)	43 (93.5)	Ref.		Ref.
25–31	16 (8.5)	3 (6.5)	1.33 (0.37–4.76)	0.66	1.27 (0.34–4.75)	0.73
Country of birth
Spain	180 (95.2%)	44 (95.7%)	Ref.		Ref.
Other	9 (4.8%)	2 (4.3%)	1.10 (0.23–5.27)	0.90	1.12 (0.22–5.64)	0.89
Medical degree year
3rd	23 (12.2)	10 (21.7)	Ref.		Ref.
4th	28 (14.8)	9 (19.6)	1.35 (0.47–3.89)	0.58	1.41 (0.45–4.40)	0.56
5th	121 (64.0)	22 (47.8)	2.39 (1.00–5.71)	0.05	2.43 (0.92–6.44)	0.07
6th	17 (9.0)	5 (10.9)	1.48 (0.43–5.12)	0.54	1.67 (0.40–7.05)	0.48
Medical degree years combined
3rd–4th	51 (27.0)	19 (41.3)	Ref.		Ref.
5th–6th	138 (73.0)	27 (58.7)	1.90 (0.98–3.72)	0.06	1.91 (0.94–3.91)	0.08
Vaccination status
Complete	179 (94.7)	43 (93.5)	Ref.		Ref.
Incomplete	10 (5.3)	3 (6.5)	0.80 (0.21–3.04)	0.74	0.87 (0.21–3.61)	0.84
Years since last dose
< 5	4 (2.1)	1 (2.2)	Ref.		Ref.
5–14	7 (3.7)	0 (0)	—	—	—	—
≥ 15	178 (94.2)	45 (97.8)	0.99 (0.11–9.06)	0.99	0.72 (0.07–7.22)	0.83
Years between doses
One dose only	10 (5.3)	3 (6.5)	0.83 (0.11–6.26)	0.86	0.73 (0.09–6.05)	0.77
< 1	8 (4.2)	2 (4.3)	Ref.		Ref.
1–4	118 (62.4)	38 (82.6)	0.78 (0.16–3.82)	0.76	0.67 (0.13–3.43)	0.93
> 4	53 (28.0)	3 (6.5)	4.42 (0.64–30.66)	0.13	3.52 (0.49–25.39)	0.21

Abbreviations: aOR, adjusted OR; CI, confidence interval; OR, odds ratio.

The proportion of students with positive serology decreased with increasing time since the last MMR dose (Figure 1).

Figure 1.

Mumps seropositivity over time after the last vaccination dose. * Only one participant was included in the group with less than 5 years since the last dose.

4. Discussion

A first finding of this study is that a high proportion of students had received the recommended 2‐dose MMR vaccine (94.5%) and seropositivity against the mumps virus remained 79.8% even > 15 years after the second dose. However, future healthcare workers are at risk of being infected and of transmitting the mumps virus. The gender balance of the participants (80.4% female) merely reflects the current distribution of medical students in the participating university teaching units.

The only variable found to be associated with a completed vaccination schedule in this study was being a 5th‐year student, a result similar to that found in a Japanese study [13].

Among medical students at the University of Padua (Italy), vaccination coverage with two doses of the MMR vaccine was reported to be around 90% for students born after 1995, and seropositivity against the mumps virus remained high (86.1%) even 15 years after the second dose. A more significant response was evident for females, and the question as to whether women respond better to vaccination is still open. In the same study it was concluded that the mandatory requirement of two doses of the MMR vaccine for healthcare workers would be useful to control transmission in hospital settings [14].

In a study carried out in the same years as the present study (2023 to 2024), mumps seronegativity was associated with the oldest healthcare students (aged 27–29 years) attending medical science faculties at the three largest universities in Serbia [15]. Their female:male participant ratio was 4:1, very close to the 4.1:1 of the present study, and seropositivity against mumps was 60.2%, a percentage lower than the 80.4% reported in the present study.

Similarly, an Italian study reported that 82.5% of medical students were serologically protected, although, the proportion of seropositive individuals was higher among females than males, and no differences were observed according to the number of vaccine doses received [16]. In Italy, regardless of an individual's vaccination history, accurate screening with a dose of anti‐mumps antibodies is recommended to assess serological protection before individuals start work in occupational health services [17].

Seropositivity among nursing students in Turkey was 67.2%, with no statistical differences observed between the sexes [18]. Riggenbach et al. [19] observed higher neutralizing antibody titres in females than in males, concluding that sex‐based differences to mumps immunity need to be assessed more broadly.

No differences were found in this study in serological results according to country of origin (80.4% for Spanish‐born and 81.8% for foreign‐born students), similar to other reports [20, 21]. Self‐reported vaccination against mumps was higher in international university students than in domestic students in Australia, but there was no difference in seropositivity (79.6% and 79.3%, respectively) [20]. In University of Tirana (Albania) medical students, 77% showed serological protection against mumps and no differences were observed according to country of origin [21].

In contrast to measles and rubella, no international unit measures exist to quantify anti‐mumps IgG that enable a comparison of different serological techniques [22]. Correlates of protection for mumps IgG antibodies are not well defined to accurately predict whether a target population is susceptible or protected [23].

In Spain, across the last three mumps epidemic waves, individuals in the 20–24 age group became increasingly predominant, presenting the highest incidence, while 92% of reported cases aged > 35 years had not been vaccinated. Incidence overall was higher in men than in women, although the reverse occurred in women aged > 45 years [24]. In European outbreaks in 2023, individuals aged ≥ 30 years were most affected [25].

Several studies have emphasized the important role played by waning immunity in mumps resurgence. Gokhale et al. [26] pointed out that, since robust herd immunity cannot be achieved due to the combination of waning immunity and primary vaccine failure, regular booster doses are needed to achieve and maintain this immunity. The mathematical model of mumps transmission by Lewnard et al. [27] confirms the central role of waning immunity in the re‐emergence of mumps cases, although evolving virus strains escaping immune pressure would be expected to cause a higher proportion of mumps cases in children than has been observed. In the review by Beleni and Borgmann [28], it was also concluded that the main cause of mumps endemicity was waning immunity, recommending MMR3 to adolescents to prevent mumps outbreaks, even though the benefit may be temporary. The authors recommend studies of genetic divergence between vaccine and wild‐type strains. Bianchi et al. [29] found that, after two MMR doses, circulating anti‐mumps IgG was lacking in 6% of the student study participants and that at least 1 booster dose was needed to elicit an immunity response. The authors concluded that screening as part of routine biological risk assessment of healthcare workers may be a winning strategy in preventing mumps infections. Likewise, Almansour et al. [30] pointed out that secondary vaccine failure, often > 10 years post‐vaccination, possibly explains the observed re‐emergence of mumps, acknowledging, however, that the benefits of MMR3 are not fully understood.

In an outbreak affecting US university students it was observed that mumps vaccine effectiveness was 89.4% if < 13 years passed since vaccination, but fell to just 31.8% if ≥ 13 years passed, and that the attack rate was clearly lower in students who received MMR3 compared to those who had received only 2 MMR doses (6.7 vs 14.5 cases/1000 population) [31]. Other studies have also reported reduced attack rates in individuals vaccinated with a third dose during outbreaks [9, 32]. From September 2015 to May 2016, during a large mumps outbreak among vaccinated students in Norway, MMR3 administration to 1,300 fully vaccinated close contacts led to a decline in cases. It was suggested in this study that the current genotype A vaccine provided suboptimal protection against genotype G mumps virus, and that consequently, different measures should be evaluated in outbreak settings, including behavioural factors, hygiene measures, and information on received vaccines [33]. In a highly vaccinated student population in Scotland, two distinct clusters of genotype G were identified, one circulating before the outbreak and the other after outbreak onset, suggesting that the virus that caused the outbreak was genetically different from the previously circulating virus [34].

In a US study of young adults to investigate the response to MMR3, it was observed that although antibody titres initially increased, they declined to near‐baseline levels within a year, concluding that a temporary boost at 1 month might be sufficient to control outbreaks if the appropriate population is targeted [35].

Given that IgG concentrations may not adequately represent immunological protection, authors of a study carried out in Taiwan concluded that an MMR booster vaccine, without previous testing, for healthcare personnel might be a feasible solution for waning vaccine‐derived immunity in a highly vaccinated population in the absence of endemic mumps transmission [36].

Antibodies 3 years after an MMR3 dose were reported to still be slightly higher compared to the pre‐vaccinated level, indicating that this booster dose afforded additional protection for susceptible persons during a mumps outbreak reducing the risk of virus transmission [37].

Melgar et al., [38] regarding a mathematical model to determine the vaccination effectiveness threshold required to prevent large mumps outbreaks, concluded that universal MMR3 on college entry would not maintain the vaccination effectiveness necessary to prevent large outbreaks over the course of 4 years of college and that the antibody titre decline could plateau 1–3 years post‐MMR3.

The decision to implement MMR3 vaccination in response to an outbreak needs to be considered in discussions which local health agencies [39]. In Taiwan, MMR3 vaccination is recommended during outbreaks for specific groups as determined by public authorities [40]. A study of healthcare workers in Italy concluded that pre‐vaccination screening of IgG for mumps was the most cost‐effective immunization strategy for this collective [41].

The above data indicate that several aspects of mumps prevention need to be studied further. As pointed out by Tripathy et al. [42] in their recent review of the literature on anti‐mumps vaccination, a large ocean of unexplored scientific data remains.

Regarding limitations, the study did not reach the estimated minimum sample size (255, 77.51%), and therefore may not have had sufficient statistical power to detect certain differences. In addition, despite guarantees of anonymity and confidentiality, selection bias may have resulted in some survey questions (especially those related to vaccine safety and effectiveness) being answered according to perceptions of social acceptability. Finally, the observed lack of protection may partly reflect methodological limitations rather than true susceptibility, as individuals who are seronegative after vaccination may still retain cellular immune memory [43] that can provide protection or mitigate disease severity.

Strengths of this study are that it was based on an anonymous and confidential questionnaire, that vaccination was confirmed by health documentation to avoid recall error, and that opinions on vaccine safety and effectiveness were collected before serological results were known.

In conclusion, a high proportion of the studied medical students had received the 2‐dose MMR schedule recommended in Spain and the proportion of students serologically protected is acceptable. However, in view of the well‐recognised waning immunity of the mumps component in the MMR vaccine, and since > 15 years have passed since the last vaccine dose in most of the students, incident cases may occur when they begin their professional activity in healthcare settings, particularly during outbreak situations, with the consequent risk for patients. In light of these findings, health authorities should consider prioritizing the recommendation of a MMR3 for medical students and healthcare workers in mumps outbreak settings.

Author Contributions

Conceptualization: A. Dominguez. Data curation and formal analysis: A. Dominguez D. Toledo and N. Soldevila. Visualization: E. Borràs. Funding acquisition: A. Dominguez. Investigation: I. Casas, M. Aldea and A. Prat. Methodology: A. Dominguez, P. Godoy, D. Toledo and R. Bartolomé. Writing – original draft: A. Dominguez, N. Soldevila and D. Toledo. Writing – review and editing: I. Casas, M. Aldea, A. Prat and R. Bartolomé.

Working Group on Vaccine Preventable Diseases in Medical Students

Anna Vilella (Universitat de Barcelona; Hospital Clínic de Barcelona), María Ángeles Marcos, Maria del Mar Mosquera, Juan Carlos Hurtado, Isabel Fortes (Hospital Clínic de Barcelona), Maria Esteve, Karen Colmenares, Gema Fernández, Irene Torrecilla, David Parés (Universitat Autònoma de Barcelona; Hospital Universitari German Trias i Pujol), Júlia Valera, Melissa Mena, Beatriz Blanco, Érika Pérez, Araceli Sánchez (Hospital Universitari Germans Trias i Pujol), Sonia Carrasco, Saray Mormeneo‐Bayo, Mercè Ibarz (IRB Lleida; Hospital Universitari Arnau de Vilanova).

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.