Whooping cough - 2018 report on disease occurrence
Laboratory-confirmed whooping cough 2018
This report comprises all cases of laboratory-confirmed whooping cough in Denmark in 2018, detected by culture, PCR or serology at a department of clinical microbiology or at Statens Serum Institut (SSI). The data analysis is based on automatic registrations in the Danish Microbiology Database (MiBa). A separate report on notified cases of whooping cough in children below 2 years of age is shown below.
The report covers only whooping cough cases verified by laboratory diagnostics. Whooping cough diagnosed on the basis of the clinical picture is thus not recorded. The detected regional differences in the report may therefore, in part, be due to differences in the use of clinical and laboratory diagnostics.
A total of 1,023 whooping cough cases were detected in 2018, corresponding to an incidence of 17.7 per 105. The highest incidence was observed in August-September and in November-December, and the level was in line with the incidence recorded in 2017. The number of cases and incidence per area in the 2015-2018 period are presented in Table 1. Data from previous years are presented under Monitoring in Numbers, Figures and Maps (In Danish language: Overvågning i tal, grafer og kort).
In 2018, the highest whooping cough incidence per area was recorded on Funen with 191 cases, corresponding to an incidence of 38.5 per 105. The incidence in the remaining areas ranged from 7 to 28 per 105. The incidence per 105 in the four largest cities of Denmark was; 18 for Copenhagen (108 cases), 16 for Aarhus (56 cases), 60 for Odense (122 cases) and 5 for Aalborg (11 cases). At the municipal level, the highest incidence was recorded in North Funen (64 per 105, 19 cases), Ballerup (64 per 105, 31 cases), Odense (60 per 105, 122 cases), Allerød (59 per 105, 15 cases) and Herlev (56 per 105, 16 cases).
The whooping cough incidences per month for the five Danish regions in the period 2016-2018 are presented in Figure 1. Generally, in 2018 the Region of South Denmark has seen many cases, particularly in the months of June and November, whereas the North Denmark Region has generally recorded few cases.
Age distribution
Distribution by age and age-specific whooping cough incidence are presented in Table 2 and Figure 2. The highest incidence was recorded among infants below 1 year of age (see “Whooping cough in children below 2 years of age below) and among older children in the 8-17-year age range. Figure 2 clearly presents a decrease at the 5-6-year age range, corresponding to the effect of the 5-year booster vaccine.
As in previous years, a female preponderance was observed as the F/M ratio was 1.4.
The number of cases observed in 2018 is presented by age and sex in Figure 2.
Figure 3 shows the age distribution by area. Some variation was observed with respect to age distribution with an excess occurrence of adults on Bornholm, in East Zealand and in South Jutland; an excess occurrence of children and adolescents was seen in Copenhagen on Funen and in West Jutland.
Diagnostic method
As in previous years, PCR was the most widely used method for detection of whooping cough (80.8%). Serology was used in 19.2% of the detected cases and no cases were detected by culture.
Serology should be used for whooping cough diagnostics only in persons aged 8 years or older as antibodies from whooping cough vaccinations may interfere with the analysis for a few years after vaccination. Serology is particularly well suited for diagnosis of adults, which is also reflected in the distribution showing diagnostic method by age, Figure 4. PCR and serology complement each other as PCR can be employed only in the initial 2-3 weeks of the disease course, whereas serology can be used only after a few weeks with symptoms.
Samples analysed in the laboratories
Thanks to the Danish Microbiology Database (MiBa), the number of samples analysed to detect whooping cough can be monitored. Thus, a strong increase is clearly seen during the 2016 epidemic,
Table 2. Furthermore, it is seen that the number of samples made in 2017 and 2018 was higher than the numbers recorded before the epidemic, in combination with a concurrent, lower share of positive findings. The number of analysed adult samples, in particular, has increased. This indicates the presence of an enhanced awareness of whooping cough in the population.
Whooping cough in children below 2 years of age
Laboratory-confirmed cases of whooping cough in children below 2 years of age are individually notifiable on Form 1515. A reminder was sent out for 35% of these notifications, as no notification had been received within one month after positive diagnostics.
2018 saw a total of 104 notified cases of whooping cough in children below 2 years of age, equally distributed among the sexes. The differences between the number of laboratory-confirmed (n=94) and notified cases below 2 years of age are caused by different registration and reporting procedures at year’s end. The age distribution in months for children below the age of two years is presented in Figure 5. The youngest affected person was just nine days old when the positive diagnostic results were known. The notification stated that the mother had been coughing for several months, but no samples had been submitted for whooping cough testing.
Among the 104 notified children below 2 years of age, 43 (41%) were unvaccinated, whereas 15 (14%) had received all three whooping cough vaccinations (DiTeKiPol/Hib® or Hexyon®).
Admission
Among the 104 cases notified, a total of 57 (55%) children were admitted for one or more days, Figure 6. Among infants below 5 months of age who had received a maximum of one vaccination, 36 of 43 (84%) were admitted. As shown, the young unvaccinated or partly vaccinated children are more affected by the condition and require admission to hospital. Conversely, only three of 21 children (14%) aged 12 months or more were admitted.
Based on the notifications, no deaths due to whooping cough were recorded in this age group in 2018. Subsequent searches in the CPR register also failed to record any whooping cough-related deaths.
Transmission
The source of infection was known in 50% of the notified children. Among the known sources of infection, siblings and other household members comprised 65%. In addition, transmission was seen in daycare or siblings’ daycare. Transmission also occurred at the parents’ place of work. Thus, several notifications stated that the parents had become infected with whooping cough at their place of work, i.e. as a nurse at a neonatal department or other hospital department, as a child-minder or as a teacher, and had then brought the infection back home to their own children. On three of the notifications, it was stated that the lacking childhood vaccination was due to conscious opt-out.
Country of infection
For the overwhelming majority of children, the country of infection stated was Denmark (96%). In the remaining cases, the possible countries of infection were Morocco, Macedonia, the Ukraine or unknown.
This report is also described in EPI-NEWS 15/19.