No 42/43 - 2011

Whooping cough 2010

This report comprises all cases of laboratory-diagnosed whooping cough detected by culture, PCR or serology in Denmark in 2010.

A total of 371 cases of whooping cough were detected. The number of cases and incidence per area are shown in Table 1.

Age and sex

The distribution by age and sex is shown in Table 2.

The overall incidence for 2010 was 7 per 10⁵ which represents a decrease relative to 2009 (10 per 10⁵), EPI-NEWS 44/10.

The 2010 incidence was lower than that observed in 2009 in most age-groups, and particularly among the 10-14-year-olds, a marked decrease in incidence was seen: from 43 per 10⁵ in 2009 to 17 per 10⁵ in 2010.

The highest incidence was still seen in children less than one year old (110 per 10⁵ ). Adolescents and adults aged 15 years and older comprised 45% (166) of all cases. Among adults, the previously observed predominance of female cases continued, whereas the cases observed among children were equally distributed among genders.

Diagnostic method

Serological diagnostics of whooping cough in persons above the age of seven years was introduced at Statens Serum Institut in January 2010, and the method has already demonstrated its considerable importance to the number of detected whooping cough cases among adults.

Thus, 27% of all cases in 2010 among adolescents and adults aged 15 years or above were detected by serology. For all age-groups as a whole, 81% were detected by PCR, 14% by serology, 3% by culture and 1.3% by simultaneous positive tests from two methods.

Commentary

Despite the introduction of serology as a diagnostic method, the 2010 occurrence was the lowest observed since 2006.

After the introduction of revaccination against whooping cough at 5 years of age in 2003/4, the number of detected whooping cough cases in non-epidemic years has decreased from an average of 900-1,000 cases to 400-500 annual cases.

The proportion of cases in children below the age of 10 years has been decreasing since 2006, but increasing among the 10-14-year-olds. The cause of the considerable decrease seen in 2010 remains unknown, but may be associated with outbreaks among school classes in the preceding years, which has increased immunity.

The proportion of whooping cough cases among adolescents and adults aged 15 years or above has also been increasing over the past five years and now comprises nearly half of all cases. Other countries have witnessed similar shifts in age distribution, and several have introduced whooping cough boosters for children aged 10-16 years.

(T. Dalby, DBMP) 

Whooping cough in children < 2 years

In children < 2 years of age, whooping cough is individually notifiable. Form 1515 is to be used in laboratory-confirmed cases.

2010 saw a total of 77 notified cases of children < 2 years with whooping cough: 42 boys and 35 girls. Reminders were sent out for 38% of the notifications.

The age distribution was as follows: 28 children (36%) were aged < 3 months, 16 (21%) were 3-4 months, 24 (31%) were 5-11 months and nine (12%) were 12-23 months. Among the 77 notified children < 2 years, 44 (57%) were unvaccinated, while two (3%) had received three whooping cough vaccinations.

Hospital admission and sequelae

Among the 77 notified children, a total of 51 (66%) children were admitted to hospital for one or more days. Among children below three years of age, 89% were admitted, Figure 1.
A four-month-old died due to whooping cough infection.

Transmission

The source of infection was known in 53% of the notified cases in children. Among known infection sources, siblings comprised 25%, other family members 19%, other known persons 2%, whooping cough in institutions of siblings 4% and infection in child-care institutions 3%.

Commentary

In 2010, the whooping cough incidence in children < 2 years was lower than in 2009 (91 cases) and this trend continues, as 2011 has, at present, only seen the whooping cough notification of 25 children.

Protection against whooping cough is insignificant until two vaccinations have been received. It is therefore essential to observe the planned vaccination times at three and five months.
Death due to whooping cough is rare in Denmark and prior to 2010 has not been recorded since 2005, EPI-NEWS 33/05 (pdf).

(L.K. Knudsen, P.H. Andersen, Dept. of Epidemiology)

Continued increase in Mycoplasma pneumoniae cases

In week 41, a description was published at ssi.dk of the increase seen during the summer in the number of samples which tested positive to Mycoplasma pneumoniae. This increase has continued during the past two weeks and the same trend is observed across Denmark.

Reports from selected clinical microbiology departments in the Capital Region (Hillerød), Region South Denmark (Odense) and North Jutland (Ålborg) show that between 20 and 25% of all samples tested for M. pneumoniae by PCR are positive. This indicates that we are on the brink of witnessing an actual epidemic.
Statens Serum Institut follows developments closely.

(S. Uldum, DBMP)

26 October 2011