1. Background
Pertussis is caused by Bordetellapertussis, and is a highly communicable, acute bacterial infection of the respiratory tract (1, 2). This organism is a gram-negative, fastidious, pleomorphic bacillus that continues to cause a global public health problem and can lead to a fatal illness in young infants (3-7). Neither its infection nor its vaccination provides permanent immunity (8-10). Therefore, In spite of high vaccination coverage in childhood, this disease remains a cause of substantial morbidity in the world (8-15). It is the only disease with universal childhood vaccination but with an escalating trend in the US (16, 17).
2. Objectives
Adults and adolescents with mild or subclinical infections are important sources of Pertussis for Infants and young children (6, 7, 17-19). Most related reports are from high-income communities, and limited experience exists from low- and middle-income countries. The current study aimed to report the prevalence of Pertussis in a sample of Iranian young adults.
3. Patients and Methods
3.1. Setting
This multicenter study was conducted in 2007 in four universities located in four provinces in Iran. The universities were Isfahan University of Medical Sciences in Isfahan, Shahid Beheshti University of Medical Sciences in Tehran, Kermanshah University of Medical Sciences in Kermanshah, and Hamadan University of Medical Sciences in Hamadan. Students from different parts of the country are accepted in universities after passing a national entrance examination, thus our study populations were considered to be nationally representative.
3.2. Study Population
At the beginning of the academic year of 2007, all newly accepted students to the aforementioned universities were enrolled in this study on the first day of their entrance to the university. Those students with history of receiving intravenous immunoglobulin (IVIG) infusion, respiratory illnesses during the previous month or with immunodeficiencies were not included into the study. In the case of lysis or lipemic status in obtained serum samples, the samples were excluded from study. The study was approved by the Research & Ethics Committees of the four collaborating universities. All students signed informed consent.
3.3. Laboratory Tests
All serum samples were screened for immunoglobulin G (IgG) antibodies against Bordetellapertussis toxin (PT) by enzyme-linked immunosorbant assay (ELISA) with using IBL-AMERICA Bordetellapertussis IgG Antibody ELISA Test Kit (Minnesota, USA). All tests were performed in the laboratory of the Infectious Diseases Research Center, affiliated to Isfahan University of Medical Sciences, Isfahan, Iran.
3.4. Statistical Analysis
We considered various cut-off points of anti-Pertussis IgG (anti-PT IgG) to determine the prevalence of Pertussis in the study samples. Two-tailed Chi-square test was used to find statistically significant differences based on various anti-PT IgG cut-off points. We also employed Chi-square for trend to investigate the linear incremental trend in these cut-off points. Data analysis was conducted by SPSS software package version 13.0 (SPSS Inc., Chicago, Illinois, USA). The significance level was set at P < 0.05.
4. Results
Overall, 1617 freshman students of four universities of medical sciences in Iran were studied. The mean age was 19.64 ± 2.1 years with a range of 17 to 38 years. Distribution of age subgroups was as follows: 1079 (66.9%) cases were aged under 20 years, 470 (29.1%) cases were 20-24 years, 61 (3.8%) cases were 25-29 years and 4 (0.2%) cases were more than 30 years. Overall, 980 (60.7%) students were female and 635 (39.3%) were male. The mean number of family members was 5.5 ± 1.8 (median = 5) with the minimum of 2 and the maximum of 14. Distribution of family member subgroups was: 498 (31%) cases ≤ 4 members, 434 (27%) cases 4-5 members and 676 (42%) cases > 6 members. The mean serum IgG titer was 66.19 ± 51.8 U/mL with the minimum 2 U/mL and maximum 250 U/mL. The mean of serum IgG titer was 61.97 ± 52 U/mL in females and 72.71 ± 50 U/mL in males. Table 1 presents the frequency of sero-positivity of Pertussis according to gender, and Table 2 presents it according to age groups. The prevalence of Pertussis was 37.7% (609 cases) by considering cut-off point of anti-PT IgG of 80U/mL, i.e. IgG titer ≥ 80 U/mL, 31.6% (511 Cases) by cut-off point of 94 U/mL (14), and 28.4% (460 Cases) by cut-off point of 100 U/mL (Table 3).
Table 1. The Frequency of Anti-Pertussis Sero-Positivity Based on Various Cut Points According to Gender
| Female, No. (%) | Male, No. (%) | Total, No. (%) | |
|---|---|---|---|
| Cut point 5 U/mL | |||
| Positive | 970 (60) | 626 (38.7) | 1596 (98.7) |
| Negative | 12 (0.7) | 9 (0.6) | 21 (1.3) |
| Total | 982 (60.7) | 635 (39.3) | 1617 (100.0) |
| Cut point 94 U/mL | |||
| Positive | 287 (17.7) | 224 (13.9) | 511 (31.6) |
| Negative | 695 (43.0) | 411 (25.4) | 1106 (68.4) |
| Total | 982 (60.7) | 635 (39.3) | 1617 (100.0) |
| Cut point 100 U/mL | |||
| Positive | 260 (16.1) | 200 (12.4) | 460 (28.4) |
| Negative | 722 (44.7) | 435 (26.9) | 1157 (71.6) |
| Total | 982 (60.7) | 635 (39.3) | 1617 (100.0) |
Table 2. The Frequency of Anti-Pertussis Sero-Positivity Based on Various Cut Points According to Age Group
| Age Groups, No. (%), y | |||||
|---|---|---|---|---|---|
| < 20 | 20 - 24 | 25 - 29 | > 30 | Total | |
| Cut Point 5 U/mL | |||||
| Positive | 1067 (66.1) | 463 (28.7) | 59 (3.7) | 4 (0.2) | 1593 (98.7) |
| Negative | 12 (0.7) | 7 (0.4) | 2 (0.1) | 0 (0) | 21 (1.3) |
| Total | 1079 (66.9) | 470 (29.1) | 61 (3.8) | 4 (0.2) | 1614 (100) |
| Cut point 94 U/mL | |||||
| Positive | 335 (20.8) | 154 (9.5) | 21 (1.3) | 1 (0.1) | 511 (31.7) |
| Negative | 774 (46.1) | 316 (19.6) | 40 (2.5) | 3 (0.2) | 1103 (68.3) |
| Total | 1079 (66.9) | 470 (29.1) | 61 (3.8) | 4 (0.2) | 1614 (100) |
| Cut point 100 U/mL | |||||
| Positive | 299 (18.5) | 141 (8.7) | 19 (1.2) | 1 (0.1) | 460 (28.5) |
| Negative | 780 (48.3) | 329 (20.4) | 42 (2.6) | 3 (0.2) | 1154 (71.5) |
| Total | 1079 (66.9) | 470 (29.1) | 61 (3.8) | 4 (0.2) | 1614 (100) |
Table 3. Age, Gender and Number of Family Members in Seropositive Cases According to Different Cut Points
| Cut Point, U/mL | Seropositive, No. (%) | Female, No. (%) | Male, No. (%) | P value | Number of Family Members, Mean | P value | Age, Mean, y |
|---|---|---|---|---|---|---|---|
| Cut Point 5 | 1596 (98.7) | 970 | 625 | 0.823 | 5.50 | 0.138 | 19.63 |
| Cut Point 9 | 1507 (93.2) | 905 | 602 | 0.043 | 5.50 | 0.539 | 19.64 |
| Cut Point 11 | 1448 (89.5) | 863 | 585 | 0.008 | 5.50 | 0.301 | 19.66 |
| Cut Point 40 | 896 (55.4) | 491 | 405 | < 0.0001 | 5.25 | < 0.0001 | 19.87 |
| Cut Point 50 | 824 (51) | 456 | 368 | < 0.0001 | 5.25 | < 0.0001 | 19.79 |
| Cut Point 80 | 609 (37.7) | 339 | 270 | 0.001 | 5.35 | 0.004 | 19.72 |
| Cut Point 94 | 511 (31.6) | 287 | 224 | 0.012 | 5.43 | 0.214 | 19.72 |
| Cut Point 100 | 460 (28.4) | 260 | 200 | 0.032 | 5.46 | 0.457 | 19.76 |
5. Discussion
We found a considerably high prevalence of university students with positive anti-PT IgG. Given that vaccination against Pertussis leads to short-term increase in antibody level (7, 10, 11, 20), the possibility of false positive titers due to previous vaccination seems to be unlikely. As the age of our cases ranged between 17 to 38 years and according to the national immunization schedule of Iran, they had taken the last booster dose of DTP at the age of 4 to 6 years, the elevated anti PT IgG levels were attributed to recent infection with Bordetellapertussis. Similar to many previous studies (14, 18, 21-23), we used different cut-off points of anti-PT IgG levels for estimating the prevalence of Pertussis infection. A study in Poland (23) reported the prevalence of Bordetellapertussis infection according to three cut-off points of 40, 80 and 100 U/mL, reflecting contact with this bacteria in the previous two, three, and 12 months. In this study, the prevalence was reported 7.2%, 6.6% and 5.2%, respectively; whereas in our study, the corresponding figures were 55.4%, 37.7% and 28.4%, respectively. The higher prevalence of infection in our study than that of Poland is suggested to be because our study was conducted in October, the beginning of the academic year in Iran, with its previous 5 months being in spring and summer time, which have the highest prevalence of Bordetellapertussis. In a survey in Singapore (24) by using the cut-off point of 5 U/mL, the prevalence of Pertussis infection was 97% among 18-45 years old persons. It is similar to the prevalence of 98.7% documented in our study. In a study in Turkey, seropositivity in cut-off point of 9 U/mL is reported as 30%, but interestingly at the same cut-off point, 93.2% of our study population were seropositive. This difference probably comes from the routine use of a cellular Pertussis vaccine in the national immunization program in Turkey instead of the cellular vaccine, which has more efficacy and latency (25). While the specificity of cut-off point of 94 U/mL for acute infection was estimated 99% in a study in United States (14), The incidence of acute infection in same cut-off point was 31.6% in our study. However, by using all cut-off points - except than 5 U/mL - the prevalence of Pertussis infection was significantly different in terms of gender. In a study among Turkish children (26), IgG titer had no significant correlation with the number of family members, but in another study in Cameroon, (27) significant differences are reported in IgG titer of children according to the number of family members. In our study, we found significant differences in anti-PT IgG levels in cut-points of 40, 50, and 80 U/mL. We found clear differences in the prevalence of Bordetellapertussis infection in our study with other reports. A previous study in Iranian young adults was conducted among 424 randomly selected military garrisons, aged 18 to 21 years, in Tehran. By considering a cut point of 20 U/mL, the seropositivity is reported to be 60.6% (28). The corresponding figure in our study was 74%. Furthermore, a study in Iranian university students was conducted in Hamedan. A number of 163 students were studied, and by considering a cut point of 24 U/mL, the sero-prevalence rate of IgG-PT was 47.67% (29). It is noteworthy to mention that to have an ideal group for studying the sero-prevalence of IGG-PT, we recruited our study participants at the first day of their entry to university, and before their stay in the crowded environment of dormitories, whereas this point has not been considered in previous studies in Iran. Different cut-off points have been used to determine the seropositivity for Bordeellapertussis; one of the recent recommendations states that a single serum sample showing anti-PT IgG above 100 U/mL may indicate a recent infection (30). Regardless to the cut-off point used, studies of various countries revealed a substantially high prevalence of infection with Bordetellapertussis among adolescents and young adults.
Although the incidence of Pertussis is reduced in infants and young children - because of the introduction of universal childhood immunization - in countries with high vaccination rates it has been increasing in adolescents and young adults in recent years especially in ages ranging between 35-39 years. An accumulating body of evidence proposes that the current childhood immunization schedule is not efficient and long lasting for eradication of Pertussis, thus a booster dose of a cellular vaccine is recommended in later life. Several scientific consensus statements, as the American Academy of Pediatrics (31, 32) have recommended to consider a booster dose in adolescence. The recent recommendations in immunization schedule suggest that adolescents should receive a single dose of Tdap vaccine instead of Td for booster immunization against tetanus, diphtheria, and Pertussis and the preferred age for Tdap immunizations is 11 through 12 years of age (31-38).
Pertussis continues to challenge medical and public health professionals; recent increases in the prevalence and incidence may be because of the limited efficacy and latency of childhood vaccination. Moreover, age-cohort effects, vaccine effectiveness, or changes in testing patterns should be considered. Our study can serve as one of the scarce population-based reports from developing countries. A universal cut-off point should be determined for diagnosis of seropositivity, and a booster of a cellular vaccine is recommended in adolescence.
