Seroprevalence study of B. pertussis infection in health care workers in Catalonia, Spain

Abstract

Pertussis is a re-emerging infection in countries with high infant immunization coverage. Healthcare workers (HCW) are exposed and can transmit the infection to especially-vulnerable patients. Therefore, pertussis vaccination of HCW is recommended.

Between June 2008 and December 2010, 460 HCW from hospital and primary healthcare centers were recruited to determine susceptibility to pertussis. IgG antibodies against pertussis (anti-pertussis ab) were measured, using a routine technique that detects antibodies against pertussis including pertussis toxin (PT) and filamentous hemagglutinin (FHA). Positive results were confirmed with a more-specific technique that only assesses anti-PT IgG antibodies.

The median age was 42 years (range, 21–65), 77.3% were female. 172 were nurses, 133 physicians, 60 other clinical workers and 95 non-clinical workers. None had received pertussis vaccination since childhood. The overall prevalence of anti-pertussis antibodies was 51.7%, (95% CI 47.1–56.4).

Anti-PT antibodies were determined in the 220 HCW with positive anti-pertussis antibodies: 4 (1.8%) were negative and 33 (15%) had a high titer (≥ 45 IU/mL).

No significant differences between the prevalence of anti-pertussis antibodies or anti-TP antibodies were found according to age, type of occupation or type of center.

Our study confirms the need for vaccination of HCW because at least half are susceptible to pertussis. High anti-PT titers found in 15% of seropositive HCW showed that they had had recent contact with B. pertussis.

Keywords: :

• Pertussis
• whooping cough
• pertussis antibodies
• anti-pertussis toxin antibodies
• health care workers
• nosocomial pertussis outbreaks

Introduction

Pertussis, or whooping cough, is a bacterial infection caused by Bordetella pertussis, an exclusively-human pathogen that can affect individuals of all ages. Infants younger than 4 mo are the most vulnerable group, with high rates of complications and mortality.¹ Neither infection nor vaccination induce life-long immunity and reinfections are frequent and occur throughout life. Pertussis is a worldwide disease, including countries with high vaccination coverage.²

In Catalonia, pertussis vaccination in children began in 1965. Initially, whole-cell vaccine (DTwP) was used but was replaced by acellular pertussis vaccine (DTaP) in 2002. The incidence of pertussis decreased with vaccination, reaching the lowest level in the early 1990s. As observed in other countries with high vaccination coverages,³ in recent years the incidence of pertussis has increased in Catalonia, with major outbreaks occurring.^4,5 Infants aged <1 y are the most affected age group, with most cases occurring in children aged <6 mo who have not initiated vaccination or have not completed primary vaccination.

Nosocomial outbreaks of pertussis are not uncommon and healthcare workers (HCW) may be a source of infection in infants whose poor health status (prematurity, etc) makes them particularly vulnerable.^1,6,7 Numerous nosocomial outbreaks of pertussis with a large number of exposed HCW have been reported.^.8–13 This highlights the importance of vaccination of HCW, especially in maternity and pediatric units, a strategy recommended in various countries.

In order to reduce pertussis transmission in children in whom the disease may be associated with serious complications, since 2004 in Spain vaccination against pertussis is recommended for HCW who care for preterm children requiring hospitalization¹⁴ and since 2011 for HCW working in pediatrics and obstetrics.¹⁵

However, pertussis vaccination coverages in HCW are still very low and it is assumed that many HCW are susceptible. The objective of this study was to determine the seroprevalence of antibodies against pertussis in Catalan HCW.

Results

Five of the 10 primary healthcare Occupational Risk Prevention (ORP) services and 6 of the 9 hospital ORP invited to participate in the study accepted. The participating centers were located in 5 of the 7 Catalan health regions, representing 87.6% of the population.

Between June 2008 and December 2010, 460 HCW were recruited: 172 nurses, 133 physicians, 60 other clinical workers and 95 non-clinical workers. The median age was 42 y (range, 21–65) and 77.3% were female (Table 1).

Table 1. Results of the bivariate analyses of pertussis antibodies in healthcare workers

Variable	Positive anti-pertussis ab	n	Prevalence(%) (95%CI)	OR (95%CI)	P
Year of birth
1981 and after	20	44	45.5 (30.4–61.2)	1
1965–1980	102	208	49.0 (42.1–56.0)	1.16 (0.60–2.22)	0.666
1964–1955	66	123	53.7 (44.4–62.7)	1.40 (0.70–2.77)	0.351
1954 and before	50	85	58.8 (47.6–69.4)	1.71 (0.82–3.57)	0.150
All	238	460	51.7 (47.1–56.4)
Sex
Male	57	104	54.8 (44.7–64.6)	1.17 (0.76–1.82)	0.477
Female	181	356	50.8 (45.5–56.2)	1
Professional group
Physician	73	133	54.9 (46.0–63.5)	1.22 (0.66–2.24)	0.529
Nurse	87	172	50.6 (42.9–58.3)	1.02 (0.57–1.84)	0.938
Other clinical workers	30	60	50.0 (36.8–63.2)	1
Non-clinical workers	48	95	50.5 (40.1–60.9)	1.02 (0.54–1.95)	0.949
Type of care
Pediatric care	54	95	56.8 (46.3–70.0)	1
No pediatric care	184	365	50.4 (45.2–55.7)	1.30 (0.82–2.04)	0.265
Type of center
Primary healthcare	140	277	50.5 (44.5–56.6)	1
Hospital	98	183	53.6 (46.0–60.9)	1.13 (0.78–1.64)	0.527

Anti-pertussis ab, pertussis antibodies; OR, odds ratio; CI, confidence interval.

A total of 238 HCW had positive anti-pertussis antibodies. Overall prevalence was 51.7%, (95% CI 47.1–56.4).

Anti-PT antibodies were determined in 220 of the HCW with positive anti-pertussis antibodies (in 18 subjects no sample was available), of whom 4 (1.8%) were negative and 33 (15%) had a titer ≥ 45 IU/mL (Table 2).

Table 2. Anti-PT antibodies level in HCW with positive anti-pertussis antibodies

Anti-PT ab	Meaning	n	%
<5 UI/mL	N	4	1.8
5–35 UI/mL	NCR	161	73.2
36–44 UI/mL	I	22	10.0
45–99 UI/mL	P	23	10.5
≥100 UI/mL	PI	10	4.5
All		220	100

Anti-PT ab, anti- pertussis toxin antibodies; N, Negative or antibodies not detected; NRC, No recent contact, positive titer with low antibodies; I, Positive indeterminate, possible infection or recent vaccination but not sure; P, Positive, recent infection or vaccination; PI, Positive infection, current infection or vaccination in last 12 mo.

No significant differences between the prevalence of anti-pertussis ab or anti-PT antibodies were found according to age, type of occupation or type of center. No HCW studied had been vaccinated with the adult type pertussis vaccine (Tdap)

Discussion

Our results show that around half the HCW studied had no antibodies against pertussis, similar to the finding of studies in HCW conducted in Spain and other countries.^16-18 De Juanes et al.¹⁶ found a presence of pertussis antibodies in 51.7% of HCW tested in routine health exams at a university hospital in Madrid (Spain), with no significant differences according to age or type of occupation, Higa et al.¹⁷ found 43.8% of IgG-PT in medical staff at a Japanese university hospital, and Hashemi et al.¹⁸ found a prevalence of IgG-PT antibodies of 47.6% in Iranian medical students.

Of HCW in whom antibodies were measured, 15% (7.8% of all participants) had a titer ≥45 IU/mL, which is an indicator of recent B. pertussis infection,¹⁹ highlighting the persistence of circulation among HCW. This may be due to a higher risk of infection in the healthcare environment or simply a result of the circulation of B. pertussis in the general population. A Dutch study found that 9% of people aged > 9 y had high anti-PT titles, indicating recent infection.²⁰ In a Chinese study of seroprevalence in the general population, 8.9% of subjects studied had anti-PT titers > 30 IU/ml, showing a high incidence of infection in adolescents and adults, which supports the appropriateness of revaccinating adolescents and adults.²¹ In Europe, recent infection was significantly more likely in adolescents aged 10–19 y and adults in high-coverage countries (Finland, the Netherlands, France, East Germany), whereas infection was more likely in children aged 3–9 y than adolescents in low-coverage (<90%; Italy, West Germany, United Kingdom) countries.²²

Whether or not pertussis circulation is similar in healthcare environments and the general population, infection in the healthcare setting is particularly serious because it may affect especially vulnerable subjects, as newborns or premature infants. Nosocomial pertussis outbreaks have been well documented, being a health-care worker with pertussis the source of infection many times.^8-13,23 Our results also suggest the importance of complying Tdap vaccination recommendations for HCW in order to prevent transmission of pertussis to patients.^2,24-26

Moreover, this strategy for the control of nosocomial outbreaks of pertussis may also have a better cost-effectiveness ratio, since nosocomial outbreaks are difficult to manage and result in significant associated expenditure.^27-29 Begget el al.²⁷ in an outbreak in a general hospital in Washington State estimated that the cost of a case of pertussis was 43,893 US$ and Yasmin et al.²⁹ in an outbreak affecting a neonatal intensive care unit estimated the cost per case as 6,500 US$.

Exposure of HCW to pertussis during contact with children who have the disease is largely unavoidable, and management of this exposure results in substantial costs to hospitals, even when the number of pertussis cases is low. Universal pre-exposure pertussis vaccination of HCW is a better utilization of resources than a case-based post-exposure strategy.³⁰

One limitation of this study is that, as a convenience sample was used, the results may not be generalizable to all Catalan HCW. Likewise, the serological study was made in HCW who voluntarily attended ORP health examinations. Therefore, the prevalence of pertussis antibodies in the study subjects may differ from that of HCW who did not attend these health examinations. However, the large sample size, including hospital and primary healthcare workers from 5 of the 7 Catalan health regions, and the fact that less than 5% of HCW invited to participate refused, suggest that our results may reflect the true situation in many Catalan healthcare centers.

In conclusion, HCW with high titer of anti-PT antibodies show evidence of the circulation of B. pertussis in Catalonia, nevertheless a high percentage of susceptible HCW were detected. Taking into account the risk for patients attended by these HCW, Catalan HCW should receive Tdap vaccination, especially in HCW working in pediatric and obstetric settings.

Materials and Methods

The study was performed using a convenience sample. ORP services from the 10 primary healthcare areas and 9 of the leading tertiary hospitals in Catalonia were asked to recruit subjects participating in the study. We considered as HCW all persons working in healthcare settings who had the potential for exposure to patients and/or to infectious materials. HCW included physicians, nurses, other clinical workers (nursing assistants, therapists, technicians, emergency medical service personnel, pharmacists, laboratory personnel, students and trainees) and non-clinical workers (house-keeping, laundry, security, maintenance, administrative staff and billing).

HCW attending voluntary periodic health examinations between June 2008 and December 2010 were informed of the study and were recruited after written informed consent was obtained. The study was approved by the Ethics Committee of the University of Barcelona. Blood samples were obtained and were centrifuged (10 min at 2500 rpm) and sera were stored at −20 °C until analysis. Demographic and epidemiological variables were collected using a questionnaire (age, sex, type of HCW, type of center, and vaccination history) that was completed under the supervision of ORP physicians and nurses. If available, the vaccination card was also reviewed.

The seroprevalence of anti-pertussis IgG antibodies (anti-pertussis ab), which includes the anti-pertussis toxin (anti-PT) and anti-filamentous hemagglutinin (anti-FHA) antibodies, was determined using a screening test (Bordetella pertussis ELISA test. Sekisui Virotech GmbH. Germany), which has a sensitivity of >99.8% and a specificity of 78.1% for IgG antibodies against PT and FHA. Results were considered positive if anti-pertussis antibodies were detected and negative if they were not.

Filamentous hemagglutinin is an antigen group also found in other agents of the species Bordetella (e.g., Bordetella parapertussis, Bordetella bronchiseptica) and cross reactivity is therefore to be expected. For this reason, in people with a positive result, IgG anti-PT antibodies were determined by a quantitative ELISA test (Pertussis Toxin ELISA test. Sekisui Virotech GmbH. Germany) that can detect acute or recent infection. Titers >40 IU / mL indicate recent contact with B. pertussis, with a sensitivity of 80% and a specificity of 95%.¹⁹

The results were grouped into the following categories: N (anti-PT < 5 IU / mL): Negative or antibodies not detected; NRC (anti-PT = 5–35 IU / mL): No recent contact, positive titer with low levels of antibodies; I (anti-PT = 36–44 IU / mL): Positive indeterminate, possible infection or recent vaccination but not sure; P (anti-PT = 45–99 IU / mL): Positive, recent infection or vaccination; and PI (anti-PT ≥ 100 IU / mL): Positive infection, current infection or vaccination in last 12 mo.

All determinations were performed following the manufacturer's recommended instructions.

Antibody prevalence and their 95% confidence intervals (CI) were calculated. To determine which variables were independently associated with antibody prevalence, odds ratios (OR) and 95% CI were calculated for different variables. Statistical significance was established assuming an α error of 0.05. The data and statistical analyses were processed using the Statistical Package for Social Sciences (SPSS 19.0 for Windows) and R 2.13.0 (R Development Core Team 2011).

Abbreviations:
Anti-FHA	=	Anti-filamentous hemagglutinin antibodies
Anti-pertussis ab	=	Pertussis antibodies
Anti-PT ab	=	Anti-pertussis toxin antibodies
Tdap	=	Adult type diphtheria-tetanus acellular pertussis (vaccine)
DTaP	=	Diphtheria-tetanus acellular pertussis (vaccine)
DTwP	=	Diphtheria-tetanus whole-cell pertussis (vaccine) ELISA, Enzyme-linked immunosorbent assay
FHA	=	Filamentous hemagglutinin
HCW	=	Health care workers
ORP	=	Occupational Risk Prevention
PT	=	Pertussis toxin

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgements

We thank the workers of the Basic Preventive Units of the Catalan Health Institute and the hospitals that participated in the study.

The members of the Working Group for the Study of the Immune Status in Healthcare Workers in Catalonia are: J Amores, T Catalán, A Company, M Cortés, C Diez, E Domenech, X Esteva, J Funes, R Llobet, A Marinetto, M Plana, J Sol, A Tarragó, M Vendrell (Unitats Bàsiques de Prevenció de Riscos Laborals d’Atenció Primària del Institut Català de la Salut); E Barbé, J Peña, M L Villarte (Hospital Arnau de Vilanova, Unitat Bàsica de Prevenció Lleida Alt Pirineu); MT Mestre, P Carbajo (Hospital U. de Tarragona Joan XXIII); I Casas, R Guerola (Hospital Germans Trias, Badalona); A Gascó (H. Viladecans); A Ayora, I Bravo (Hospital Vall d’Hebron, Barcelona); RM Labarta, J Matllo (Servei Propi Mancomunat de Prevenció de Riscos Laborals, Barcelona); P Varela (Hospital Clínic, Barcelona); J Espuñes, R Moreno, M Martínez (Public Health Agency of Catalonia).

Funding

This study was supported by the Direcció General de Salut Pública, Generalitat of Catalonia, Fundación Bosch i Gimpera (Project 305232), AGAUR (Project 2009 SGR42) and CIBERESP.