2000 Indiana Report of Infectious Diseases

Pertussis

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Rates presented are per 100,000 population and are based on the U.S. 2000 Census.

Cases = 153
Crude Rate (per 100,000 population) = 2.52

In 2000, Indiana had 153 reported cases of pertussis, an increase (70%) from 90 cases in 1999. 41 counties had at least one case of pertussis during the year, up from 29 counties in 1999. (See Figure Per1) The four counties shown in Table Per1 accounted for 49.6% of all cases in 2000. 

As Figure Per2 shows, pertussis incidence, unlike other vaccine-preventable diseases, has not declined in recent years but has actually increased over the past 15 years. A mean of 80 cases per year was found for the period 1986-1990; a mean of 96 cases per year for 1991-1995; and a mean of 132 cases per year for 1996-2000. It is generally believed that this long-term increase in pertussis incidence is attributed to better recognition and reporting of suspected cases. Due to the cyclical nature of the disease, pertussis incidence normally peaks every three to four years. These peaks can also be seen in Figure Per2. 

Laboratory confirmation was available for 46 cases (30.1%): 40 were confirmed by culture and 6 were confirmed by polymerase chain reaction (PCR). The remainder of the cases were linked to a laboratory confirmed case (23, 15.0%) or met the case definition for pertussis (84, 54.9%).

Pertussis is the most frequently reported vaccine-preventable disease of children under age 5. In 2000, 56 cases (36.6% of all cases, case rate = 66/100,000 population) occurred in infants less than one year of age and 19 cases in the 1-4 year age group (12.4% of all cases, case rate = 5.6). Figure Per3 depicts the percentage of cases occurring in selected age groups. 

Since waning immunity following natural infection or vaccination occurs, pertussis can affect persons of any age. In 2000, Indiana saw an increase in disease in those ages 10-19, with 32 cases of the 153 cases (21%) occurring in this age group. From 1993-1999, the percent of cases in this age group ranged from 9%-12%. Increased awareness among health care providers that pertussis can occur in adolescence may account for a part of the increased percentage. 

In 2000, pertussis outbreaks occurred in the ages 10-19 population in Allen and Dearborn Counties, with nine and eight reported cases, respectively. Infection occurring in junior and senior high schools accounted for some disease transmission in these two counties.  Table Per1 shows the number of cases and incidence rate for selected counties.

Females accounted for 84 (55%) of the reported cases, and males accounted for 69 (45%).  By race, whites accounted for 137 (89.5%), blacks for 14 (9.2%), and the race was unknown for 2 cases.  Of the 118 cases where ethnicity was identified, none was classified as Hispanic (ethnicity was not identified on 35 cases). 

Unvaccinated children are at highest risk for severe disease, but fully immunized children may also develop disease. Vaccine efficacy for the currently licensed vaccines is estimated to be 71-84% for preventing serious disease. Table Per2 shows the number and percent of cases that were up-to-date for pertussis vaccination at the time of illness. 

Table Per1: Selected Counties with Number of Cases, Percent and Case Rate, Indiana, 2000

County

Number of Cases
(Percent)

Case Rate
(per 100,000 population)

Allen

32 (20.9%)

9.64

Dearborn

13 (8.5%)

28.1*

Lake

12 (7.8%)

2.48*

Marion

19 (12.4%)

2.21*

All Other Counties

77 (50.4%)

1.77

Total Indiana

153 (100%)

2.52

Table Per2: Vaccination History of Selected Age Groups, Number of Doses (Percent), Indiana, 2000

Age Group

Number of Cases

Number (Percent)
Appropriately Immunized

Unknown Vaccine
History

6-11 months

7

0 (0%)

0

1 year

8

5 (67.5%)

0

2-4 years

11

9 (82%)

0

5-6 years

2

1 (50%)

0

7-15 years

41

31 (76%)

2

 

Footnotes

* - Rate based on less than 20 cases and should be considered unstable.