Pertussis epidemic despite high levels of vaccination coverage with acellular pertussis vaccine

Increase of pertussis cases in the Vallès region, Catalonia, Spain, September 2023 to April 2024

We describe a pertussis outbreak in the Vallès region of Catalonia, from September 2023 to April 2024. Incidence was high in children aged 10-14 years compared with previous outbreaks. Limited impact in newborns could be explained by the high vaccination coverage during pregnancy and at 11 months of age in 2022, at 85% and 94.1 %, respectively. A third booster vaccine dose during preadolescence should be considered and vaccination coverage in pregnant women be improved to control future outbreaks.

Pertussis immunisation strategies to optimise infant pertussis control: A narrative systematic review

OBJECTIVE

Countries routinely offering acellular pertussis vaccine, where long-term protection is not sustained, have the challenge of selecting an optimal schedule to minimise disease among young infants. We conducted a narrative systematic review and synthesis of information to evaluate different pertussis immunisation strategies at controlling pertussis disease, hospitalisation, deaths, and vaccine effectiveness among young infants.

METHODS

We conducted a review of the literature on studies about the primary, booster, and/or maternal vaccination series and synthesised findings narratively. Countries offering the first three doses of vaccine within six-months of life and a booster on or before the second year or life were defined as accelerated primary and booster schedules, respectively. Countries offering primary and booster doses later were defined as extended primary and booster schedules. All search results were screened, and articles reviewed and reconciled, by two authors. The Risk of Bias in Non-randomised Studies of Intervention tool was used to evaluate the risk of bias.

FINDINGS

A total of 98 studies were included in the analyses and the following recurring themes were described: timing of vaccination, vaccine coverage, waning immunity/vaccine effectiveness, direct and indirect effectiveness, switching from an accelerated to extended schedule, impact of changes in testing. The risk of bias was generally low to moderate for most studies.

CONCLUSION

Comparing schedules is challenging and there was insufficient evidence to that one schedule was superior to another. Countries must select a schedule that maintains high vaccine coverage and reduced the risk of delaying the delivery vaccines to protect infants.

The Decay of Pertussis Antibodies in Children Aged 0-14 Years in Jiangsu Province, China

The purpose of this study was to investigate possible influencing factors based on the distribution of the pertussis toxin (PT) and filamentous hemagglutinin (FHA) antibody levels in 0-14-year-old children in Jiangsu Province, China, and to analyze changes in IgG antibody levels after pertussis vaccination in children over time via a restricted cubic spline (RCS)-fitted binary logistic regression model. We collected surveillance data on pertussis through the National Notifiable Disease Reporting System (NNDRS). Serum samples were collected, and PT IgG/FHA IgG antibody levels were determined via an enzyme-linked immunosorbent assay (ELISA). A binary logistic regression model was fitted with an RCS. Peak incidence occurred in children aged 0-1 years from 2007 to 2022, and a second peak emerged in children aged 5 years and older from 2018 onwards which shifted towards older age groups. The geometric mean concentrations (GMC) of the anti-PT IgG antibody and anti-FHA IgG antibody in 1129 patients were 15.13 (13.49-16.76) IU/mL and 22.99 (21.17-24.81) IU/mL, respectively. The seropositivity rates of the anti-PT IgG and anti-FHA IgG antibodies in the group receiving a full vaccination course (four doses) were significantly higher than those of other groups (24.6% vs. 43.3%). The RCS fitting model showed a non-linear relationship between the duration after immunization and the odds ratio (OR) of having PT-IgG and FHA-IgG antibody concentrations ≥20 IU/mL in children with documented immunization histories (1-4 doses) (Poverall < 0. 001; Pnonlinear ≤ 0.001). The children with histories of immunization demonstrated antibody levels that decreased to very low levels around 17 months after the last dose of the vaccine. Therefore, it is recommended that pertussis-containing vaccines be administered as booster immunizations for older children.

Multiplexed bead-based assay for the simultaneous quantification of human serum IgG antibodies to tetanus, diphtheria, pertussis toxin, filamentous hemagglutinin, and pertactin

Background

Luminex bead-based assays offer multiplexing to test antibodies against multiple antigens simultaneously; however, this requires validation using internationally certified reference standards. Therefore, there is an urgent need to characterize existing reference standards for the standardization of multiplex immunoassays (MIAs). Here, we report the development and validation of an MIA for the simultaneous estimation of levels of human serum immunoglobulin G (IgG) antibodies for pertussis toxin (PT), filamentous hemagglutinin (FHA), pertactin (PRN), diphtheria toxoid (DT), and tetanus toxoid (TT).

Methods

The MIA was assessed using a panel of human serum samples and WHO reference standards. The WHO reference standards were also studied for suitability in the MIA. Purified antigens (PT, FHA, PRN, DT, and TT) were coupled to the spectrally unique magnetic carboxylated microspheres. The method was validated in accordance with the United States Food and Drug Administration (US FDA), European Medicines Agency (EMA), and the International Committee of Harmonization Multidisciplinary (ICH M10) guidelines, and parameters such as precision, accuracy, dilutional linearity, assay range, robustness, and stability were assessed. Method agreements with commercially available IgG enzyme-linked immunosorbent assay (ELISA) assays were also evaluated. In addition, the study assessed the level of correlation between the IgG levels estimated by the MIA and the cell-based neutralizing antibody assays for PT and DT.

Results

We identified that an equimix of WHO international standards (i.e., 06/142, 10/262, and TE-3) afforded the best dynamic range for all the antigens in the MIA. For all five antigens, we observed that the back-fitted recoveries using the four-parameter logistic (4-PL) regression fits ranged between 80% and 120% for all calibration levels, and the percentage coefficient of variation (% CV) was < 20%. In addition, the difference in mean fluorescence intensity (MFI) between the monoplex and multiplex format was < 10% for each antigen, indicating no crosstalk among the beads. The MIA also showed good agreement with conventional and commercially available assays, and a positive correlation (> 0.75) with toxin neutralization assays for PT and DT was observed.

Conclusion

The MIA that was calibrated in accordance with WHO reference standards demonstrated increased sensitivity, reproducibility, and high throughput capabilities, allowing for the design of robust studies that evaluate both natural and vaccine-induced immunity.

Preponderance of vaccine-preventable diseases hotspots in northern Ghana: a spatial and space-time clustering analysis from 2010 to 2014

BACKGROUND

Vaccine-preventable diseases (VPDs) persist globally with a disproportionately high burden in Low and Middle-Income Countries (LMICs). Although this might be partly due to the failure to sustain vaccination coverage above 90% in some WHO regions, a more nuanced understanding of VPD transmission beyond vaccination coverage may unveil other important factors in VPD transmission and control. This study identified VPDs hotspots and explored their relationships with ecology, urbanicity and land-use variations (Artisanal and Small-scale Gold Mining (ASGM) activities) in Ghana.

METHODS

District-level disease count data from 2010 to 2014 from the Ghana Health Service (GHS) and population data from the Ghana Population and Housing Census (PHC) were used to determine clustering patterns of six VPDs (Measles, Meningitis, Mumps, Otitis media, Pneumonia and Tetanus). Spatial and space-time cluster analyses were implemented in SaTScan using the discrete Poisson model. P-values were estimated using a combination of sequential Monte Carlo, standard Monte Carlo, and Gumbel approximations.

RESULTS

The study found a preponderance for VPD hotspots in the northern parts of Ghana and northernmost ecological zones (Sudan Savannah and Guinea Savannah). Incidence of meningitis was higher in the Sudan Savannah ecological zone relative to: Tropical Rain Forest (p = 0.001); Semi Deciduous Forest (p < 0.0001); Transitional Zone (p < 0.0001); Coastal Savannah (p < 0.0001) and Guinea Savannah (p = 0.033). Except for mumps, which recorded a higher incidence in urban districts (p = 0.045), incidence of the other five VPDs did not differ across the urban-rural divide. Whereas spatial analysis suggested that some VPD hotspots (tetanus and otitis media) occur more frequently in mining districts in the southern part of the country, a Mann-Whitney U test revealed a higher incidence of meningitis in non-mining districts (p = 0.019). Pneumonia and meningitis recorded the highest (722.8 per 100,000) and least (0.8 per 100,000) incidence rates respectively during the study period.

CONCLUSION

This study shows a preponderance of VPD hotspots in the northern parts of Ghana and in semi-arid ecoclimates. The relationship between ASGM activities and VPD transmission in Ghana remains blurred and requires further studies with better spatial resolution to clarify.

Evaluation of Outer Membrane Vesicles Obtained from Predominant Local Isolate of Boredetella pertussis as a Vaccine Candidate

Background:

Pertussis is a current contagious bacterial disease caused by Bp. Given the prevalence of pertussis, development of new vaccines is important. This study was attempted to evaluate the expression of main virulence factors (PTX, PRN, and FHA) from Bp predominant strains and also compare the expression of these factors in the OMVs obtained from predominant circulating Bp isolate.

Methods:

The physicochemical features of the prepared OMVs were analyzed by electron microscopy and SDS-PAGE. The presence of the mentioned virulence factors was confirmed by Western blotting. BALB/c mice (n = 21) immunized with characterized OMVs were challenged intranasally with sublethal doses of Bp, to examine their protective capacity.

Results:

Electron microscopic examination of the OMVs indicated vesicles within the range of 40 to 200 nm. SDS-PAGE and Western blotting demonstrated the expression of all three main protective immunogens (PTX, PRN, and FHA), prevalent in the predominant, challenge, and vaccine strains, and OMVs of the predominant IR37 strain and BP134 vaccine strain. Significant differences were observed in lung bacterial counts between the immunized mice with OMV (30 CFU/lung) compared to the negative control group ((6 10⁴ CFU/lung; p < 0.001). In mice immunized with OMVs (3 µg), the number of lungs recovered colonies after five days dropped at least five orders of magnitude compared to the control group.

Conclusion:

OMVs obtained from circulating isolates with the predominant profile may constitute a highly promising vaccine quality. They also can be proposed as a potential basic material for the development of new pertussis vaccine candidate.

Exploring the nasopharyngeal microbiota composition in infants with whooping cough: A test-negative case-control study

Purpose

The purpose of this study was to characterize the nasopharyngeal microbiota of infants with possible and confirmed pertussis compared to healthy controls.

Methods

This prospective study included all infants <1 year with microbiologically confirmed diagnosis of pertussis attended at a University Hospital over a 12-month period. For each confirmed case, up to 2 consecutive patients within the same age range and meeting the clinical case definition of pertussis but testing PCR-negative were included as possible cases. A third group of asymptomatic infants (healthy controls) were also included. Nasopharyngeal microbiota was characterized by sequencing the V3-V4 region of the 16S rRNA gene. Common respiratory DNA/RNA viral co-infection was tested by multiplex PCR.

Results

Twelve confirmed cases, 21 possible cases and 9 healthy controls were included. Confirmed whooping cough was primarily driven by detection of Bordetella with no other major changes on nasopharyngeal microbiota. Possible cases had limited abundance or absence of Bordetella and a distinctive microbiota with lower bacterial richness and diversity and higher rates of viral co-infection than both confirmed cases and healthy controls. Bordetella reads determined by 16S rRNA gene sequencing were found in all 12 confirmed cases (100%), 3 out of the 21 possible cases (14.3%) but in any healthy control.

Conclusion

This study supports the usefulness of 16S rRNA gene sequencing for improved sensitivity on pertussis diagnosis compared to real-time PCR and to understand other microbial changes occurring in the nasopharynx in children <1 year old with suspected whooping cough compared to healthy controls.

Bordetella pertussis in School-Age Children, Adolescents and Adults: A Systematic Review of Epidemiology and Mortality in Europe

Pertussis (whooping cough) epidemics persist globally despite high vaccine coverage among infants and young children. The resurgence of pertussis in high-income countries is partly due to waning vaccine immunity, resulting in a pool of unprotected adolescents and adults. However, pertussis is generally less severe in adolescents and adults, and this difference in presentation means it can often be unrecognised by healthcare professionals, meaning that it is largely under-diagnosed in older populations. A systematic search of MEDLINE, EMBASE and BIOSIS was undertaken to identify studies published between 1 January 1990 and 17 June 2019, with information on pertussis epidemiology and mortality in school-aged children, adolescents and adults in Europe. A formal statistical comparison (e.g. using meta-analyses) was not possible because of the mix of methodologies reported. There were 69 epidemiological studies and 19 mortality studies identified for review. Over the past decade, the reported incidence of notified pertussis cases varied widely between European countries, which is likely associated with differences in surveillance systems, diagnostic techniques and reporting regulations. However, several studies show that pertussis is circulating among adolescents and adults in Europe, and although pertussis-related morbidity and mortality are highest in infants, there is evidence that adults aged > 50 years are at increased risk. For example, in a hospital-based surveillance study in Portugal, between 2000 and 2015, 94% of hospitalised pertussis cases were infants aged < 1 year, with a case fatality rate (CFR) of 0.8%; however, among hospitalised adult cases of pertussis, the CFRs were 11.5% (aged 18–64 years) and 17.4% (aged > 65 years). Very few European countries currently include pertussis boosters for adults in the national immunisation strategy. In addition to increasing pertussis vaccination coverage in adolescents and adults, mitigation strategies in European countries should include improved diagnosis and treatment in these populations.

Azithromycin to Prevent Pertussis in Household Contacts, Catalonia and Navarre, Spain, 2012-2013

We retrospectively assessed the effectiveness of azithromycin in preventing transmission of pertussis to a patient's household contacts. We also considered the duration between symptom onset in the primary patient and azithromycin administration. We categorized contacts into 4 groups: those treated within <7 days, 8-14 days, 15-21 days, and >21 days after illness onset in the primary patient. We studied 476 primary index patients and their 1,975 household contacts, of whom 4.5% were later identified as having pertussis. When contacts started chemoprophylaxis within <21 days after the primary patient's symptom onset, the treatment was 43.9% effective. Chemoprophylaxis started >14 days after primary patient's symptom onset was less effective. We recommend that contacts of persons with pertussis begin chemoprophylaxis within <14 days after primary patient's symptom onset.

The burden of pertussis in older adults: what is the role of vaccination? A systematic literature review

INTRODUCTION

Pertussis (whooping cough) is a vaccine-preventable disease; however, neither natural- nor vaccine-induced protection is life-long. Although generally not severe in adults, pertussis can be associated with complications in patients with chronic conditions such as asthma or chronic obstructive pulmonary disease, and can be readily transmitted to more vulnerable populations, including neonates before they complete their primary vaccination. Furthermore, as the global population ages, the health and economic burden of the disease is expected to rise. Areas covered: A systematic literature review was conducted to ascertain the current epidemiological and financial burden of pertussis in older adults and to discuss the potential value of a booster vaccination in this population. Expert commentary: Our review indicates a considerable underestimation of the pertussis burden amongst older adults. Seroprevalence studies consistently demonstrate that the reported incidence may be much lower than the actual incidence. Tetanus toxoid-reduced diphtheria toxoid and acellular pertussis vaccines are immunogenic in older adults, induce high booster responses and are well-tolerated. There is therefore a good rationale for the advocacy of booster pertussis vaccination throughout life to prevent pertussis infection and its transmission, especially in adults aged ≥50 years.

Factors influencing the spread of pertussis in households: a prospective study, Catalonia and Navarre, Spain, 2012 to 2013

We aimed to investigate transmission rates of pertussis in household contacts of cases and factors associated with transmission. A prospective epidemiological study was conducted in 2012 and 2013 to determine the incidence of pertussis among household contacts of reported cases in Catalonia and Navarre, Spain. An epidemiological survey was completed for each case and contact, who were followed for 28 days to determine the source of infection (primary case) and detect the occurrence of secondary cases. Odds ratios (ORs) were used to estimate the effectiveness of vaccination and chemoprophylaxis in preventing new cases, using the formula (1 − OR) × 100. For the 688 primary cases, a total of 2,852 contacts were recorded. The household transmission rate was 16.1% (459/2,852) and rose according to the age (> 18 years) and lack of immunisation of the primary cases, and also the age (0–18 years), family relationship (siblings and children), lack of vaccination and chemoprophylaxis of contacts. Pertussis vaccine effectiveness in preventing new cases was 65.0% (95% confidence interval (CI): 11.6 to 86.2) for full vaccination (≥ 4 doses) and 59.7% (95% CI: −6.8 to 84.8) for incomplete vaccination (< 4 doses). The effectiveness of chemoprophylaxis was 62.1% (95% CI: 40.3 to 75.9). To reduce household transmission, contacts should be investigated to detect further cases and to administer chemoprophylaxis. The current vaccination status of cases and contacts can reduce household transmission.

Recommended immunization schedules for adults: Clinical practice guidelines by the Escmid Vaccine Study Group (EVASG), European Geriatric Medicine Society (EUGMS) and the World Association for Infectious Diseases and Immunological Disorders (WAidid)

Rapid population aging has become a major challenge in the industrialized world and progressive aging is a key reason for making improvement in vaccination a cornerstone of public health strategy. An increase in age-related disorders and conditions is likely to be seen in the near future, and these are risk factors for the occurrence of a number of vaccine-preventable diseases. An improvement in infectious diseases prevention specifically aimed at adults and the elderly can therefore also decrease the burden of these chronic conditions by reducing morbidity, disability, hospital admissions, health costs, mortality rates and, perhaps most importantly, by improving the quality of life. Among adults, it is necessary to identify groups at increased risk of vaccine-preventable diseases and highlight the epidemiological impact and benefits of vaccinations using an evidence-based approach. This document provides clinical practice guidance on immunization for adults in order to provide recommendations for decision makers and healthcare workers in Europe. Although immunization is considered one of the most impactful and cost-effective public health measures that can be undertaken, vaccination coverage rates among adults are largely lower than the stated goal of ≥ 95% among adults, and stronger efforts are needed to increase coverage in this population. Active surveillance of adult vaccine-preventable diseases, determining the effectiveness of the vaccines approved for marketing in the last 5 y, the efficacy and safety of vaccines in immunocompromised patients, as well as in pregnant women, represent the priorities for future research.

Highly differentiated human airway epithelial cells: a model to study host cell-parasite interactions in pertussis

BACKGROUND

Bordetella pertussis colonizes the human respiratory mucosa. Most studies on B. pertussis adherence have relied on cultured mammalian cells that lack key features present in differentiated human airway cells or on animal models that are not natural hosts of B. pertussis. The objectives of this work were to evaluate B. pertussis infection in highly differentiated human airway cells in vitro and to show the role of B. pertussis fimbriae in cell adherence.

METHODS

Primary human airway epithelial (PHAE) cells from human bronchi and a human bronchial epithelial (HBE) cell line were grown in vitro under air-liquid interface conditions.

RESULTS

PHAE and HBE cells infected with B. pertussis wild-type strain revealed bacterial adherence to the apical surface of cells, bacteria-induced cytoskeleton changes, and cell detachment. Mutations in the major fimbrial subunits Fim2/3 or in the minor fimbrial adhesin subunit FimD affected B. pertussis adherence to predominantly HBE cells. This cell model recapitulates the morphologic features of the human airway infected by B. pertussis and confirms the role of fimbriae in B. pertussis adherence. Furthermore, HBE cells show that fimbrial subunits, and specifically FimD adhesin, are critical in B. pertussis adherence to airway cells.

CONCLUSIONS

The relevance of this model to study host-parasite interaction in pertussis lies in the striking physiologic and morphologic similarity between the PHAE and HBE cells and the human airway ciliated and goblet cells in vivo. These cells can proliferate in vitro, differentiate, and express the same genetic profile as human respiratory cells in vivo.

Importance of Enhanced Surveillance for Prevention of Pertussis in Children

BACKGROUND

Bordetella pertussis infection causes considerable morbidity, even in countries with high vaccination coverage. Surveillance of pertussis is usually passive and based on mandatory reporting. We assessed the epidemiologic and clinical characteristics of pertussis cases detected by passive or enhanced surveillance.

METHODS

A prospective population-based study was carried out from January 2012 to December 2013 in Catalonia and Navarre, 2 Spanish regions with a total population of 8.2 million. For each confirmed case, variables of age, sex, clinical symptoms, laboratory results and vaccination status were investigated. Associations between the variables studied and the type of surveillance were assessed by odds ratio (OR) and its 95% confidence interval (CI). Logistic regression was used to estimate adjusted OR (aOR).

RESULTS

In children <18 years of age, passive surveillance was the most important source of detection (95.7% in <1 year). In children 5-10 years of age, passive surveillance detected 67.5% of cases. In those ≥18 years, enhanced surveillance was the most frequent source of case detection (82.5%). Inspiratory whoop (aOR: 1.94; 95% CI: 1.40-2.70), apnea (aOR: 1.92; 95% CI: 1.28-2.87) and posttussive vomiting (aOR: 1.43; 95% CI: 1.04-1.96) were symptoms included in the clinical case definition that were associated with passive surveillance.

CONCLUSIONS

Underreporting of symptomatic cases is important not only in adults, but also in children >1 year of age and suggests that providing clinics with free, fast laboratory diagnostic tests, together with enhanced surveillance of family contacts of any age is necessary to better determine existing cases. Enhanced surveillance may be helpful to better understand transmission patterns in the family and in the community.

[Acellular vaccines (DTPa/dTpa) against whooping cough, protection duration]

INTRODUCTION

An increase in whooping cough in most of the developed countries has been detected in the last decade.

OBJECTIVE

To determine whether the administration of dTpa vaccine instead of DTPa fifth dose is contributing to the appearance of these cases.

METHODS

A descriptive study based on cases of whooping cough reported during an epidemic period in the city of Alicante in the first 5 months of 2014. Only pertussis cases confirmed by PCR were included in the study, and only those vaccinated with 5 doses were included in the analysis of the period of protection.

RESULTS

A total of 104 cases of pertussis confirmed by PCR were reported, with 85 cases (82%) having had 5 doses of vaccine. The mean time and standard deviation (SD) of protection was 2.1±1.1 years with dTpa, and 5.1±1.5 years with DTPa (p<.001). In the protection, adjusted for age, it was observed that, after 3 years, only 47.6% of people vaccinated with dTpa were still protected, while people vaccinated with DTPa were 100% protected (P<.001).

CONCLUSIONS

This study found that people who were properly vaccinated against pertussis and received their last re-vaccination dose with dTpa had a shorter period of protection than those who were vaccinated with DTPa.

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

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.

Characteristics of pertussis outbreaks in Catalonia, Spain, 1997 to 2010

In Catalonia, pertussis outbreaks must be reported to the Department of Health. This study analyzed pertussis outbreaks between 1997 and 2010 in general and according to the characteristics of the index cases. The outbreak rate, hospitalization rate and incidence of associated cases and their 95%CI were calculated. Index cases were classified in two groups according to age (&lt;15 years and ≥15 years) and the vaccine type received: whole cell vaccine (DTwP) or acellular vaccine (DTaP). During the study period, 230 outbreaks were reported. The outbreak rate was 2.43 × 10(-6) persons-year, and outbreaks ranged from 2 to 32 cases, with a median duration of 18 days. There were 771 associated cases, with an incidence rate of 0.8 × 10(-5) persons-year.   After classifying outbreaks according to the age of the index case, 126 outbreaks (1.3 × 10(-6) persons-year) had an index case aged &lt;15 y and 87 (0.87 × 10(-6) person-year) had an index case aged ≥15 y (RR = 1.44, 95%CI 1.10-1.90; P = 0.007). Between 2003 and 2010, after the introduction of the acellular vaccine, the index case was vaccinated with DTwP vaccine in 25 outbreaks (0.43 × 10(-6) persons-year) and with DTaP vaccine in 32 outbreaks (0.55 × 10(-6) person-year) (RR = 0.78, 95%CI 0.46-1.31; P = 0.35). Of cases, 37.2% were correctly vaccinated, suggesting waning immunity of pertussis vaccine protection and endogenous circulation of pertussis. A greater number of outbreaks had an index case aged &lt;15 y. No changes in the disease incidence, associated cases and hospitalization rate were observed after the introduction of DTaP.