No 44 - 2010

Whooping cough 2009

This report comprises all cases of laboratory-diagnosed whooping cough detected in Denmark in 2009 by culture and/or PCR. Diagnosis by serology was not available in 2009.

A total of 541 cases of whooping cough were detected. The number of cases and incidence per area are shown in Table 1.

This report is based on data from whooping cough cases (number in parenthesis) detected at Statens Serum Institut (188), DCM Herlev Hospital (20), DCM Hvidovre Hospital (13), DCM Odense University Hospital (167), DCM Aarhus University Hospital (76) and DCM Viborg Regional Hospital (77).

Age and gender

Distribution by age and gender are shown in Table 2.
As in previous years, the highest whooping cough incidence was seen in infants below one year of age and in children aged 10-14 years. The incidence in these two groups was 177 and 43 per 100,000, respectively.

Adolescents and adults aged 15 years and older comprised 35 % (194 cases) of the total number of detected cases. The overall 2009 incidence of 10 per 100,000 was in line with the corresponding numbers from 2007 and 2008 (when incidences of 8 and 9 per 100,000, respectively, were seen).
As in previous years, the majority in most age groups were girls/women, particularly among adults.

Seasonal variation

The occurrence of whooping cough was unevenly distributed across the year as 31 % (167) of all cases were detected in the months of May and June.

Local clusters

The highest numbers of whooping cough cases were detected on Funen, particularly in the municipalities of Faaborg-Central Funen (60 cases) and Odense (49).
In Eastern Jutland, the bulk of cases detected were from the municipalities of Silkeborg (45) and Aarhus (24), and in Southern Jutland the most heavily affected municipality was Sønderborg (50).

Commentary

Whooping cough is extremely contagious and the majority of the population is susceptible, as vaccination and previous whooping cough infection only yields 5-10 years of protection. This is confirmed by the fact that since the introduction of revaccination at 5 years of age in 2003, an increasing share of cases has been seen among adolescents and young adults.

Whooping cough typically occurs cyclically with epidemics every 3-5 years. The most recent Danish whooping cough epidemic occurred in 2002, and - not counting the increased occurrence seen in 2004 - it has thus been 8 years since the previous Danish epidemic.
(T. Dalby, DBMP) 

Whooping cough in children below 2 years

In children < 2 years of age, whooping cough is individually notifiable. Form 1515 is to be used in laboratory-confirmed cases.
2009 saw a total of 91 notified cases of whooping cough in children below 2 years of age, 48 boys and 43 girls. Reminders were sent out for 39 % of the notifications.

The age distribution was as follows: 28 (31 %) were children < 2 months, 31 (34%) were 3-4 months, 17 (19 %) were 5-11 months and 15 (16 %) were 12-23 months old.

Among the 91 notified children below 2 years, 57 (63 %) were unvaccinated, while seven (8 %) had received three vaccinations.
The share of children below six months of age who were admitted due to whooping cough was 86 %. Furthermore, three (23 %) children aged 6-11 months and three (20 %) aged 12-23 months were admitted, Figure 1.

Transmission

The source of infection was known in 51 % of the notified cases in children. Among known infection sources, siblings comprised 52 %, other family members 28 %, other known persons 11 %, whooping cough in the environment 4 %, infection at a hospital 1 % and at child-care institutions 1 %. Antibiotic prophylaxis for exposed children was described in EPI-NEWS 45/02 (pdf).

Commentary

The 2009 occurrence of whooping cough in children below 2 years of age was slightly below that observed in 2008 (106) and at par with 2007 (94).
Protection against whooping cough is only significant after two vaccinations, it is therefore essential to observe the planned vaccination times at 3 and 5 months.

However, whooping cough vaccination is not 100 % efficient, and some whooping cough should be expected among fully vaccinated children below the age of two years.
(A.H. Christiansen, P.H. Andersen, Dept. of Epidemiology) 

Individually notifiable diseases and selected laboratory diagnosed infections (pdf) 

3 November 2010