Novel H1N1 Infection in Chicago: Children Preferentially Vulnerable

Children are considerably more vulnerable to infection with the novel H1N1 virus than adults, according to epidemiologic data from the Chicago Department of Public Health. Results of the surveillance of laboratory-confirmed cases of H1N1 infection were published in the latest issue of the CDC's MMWR.

Data from April 24 to July 25 indicate the following overall attack and hospitalization rates (per 100,000) by age group within the city's 77 communities.

Age Group, years	Hospitalization Rate (n = 205)	Overall Attack Rate (N = 1557)
0-4	25	113
5-14	11	147
15-29	4	48
30-59	5	25
≥60	4	10

Consequently the overall attack rate among children aged 5-14 years is more than 14 times that among adults aged 60 years or older. Hospitalization rates are also considerably higher among the younger pediatric populations. These data support current guidelines for preferentially vaccinating younger age groups against the novel H1N1 virus.

Reasons for the higher attack and hospitalization rates among children may be related to the existence of partial (ie, cross-reactive) immunity to the novel H1N1 virus among the elderly and/or higher virus-transmission/contact rates among children.

Overall attack and hospitalization rates are also higher among minority ethnic groups, for unclear reasons; although the relatively higher prevalence of underlying conditions, like asthma or diabetes, in minority ethnic groups may explain their increased vulnerability to H1N1 infection.

Ethnic Group	Hospitalization Rate (n = 205)	Overall Attack Rate (N = 1557)
Black, non-Hispanic	9	29
White, non-Hispanic	2	11
Hispanic	8	36
Asian/Pacific Islander	8	37

Among those hospitalized, 40 (19.5%) were admitted to an intensive-care unit; 9 (4%) required mechanical ventilation. The rates of preexisting asthma and diabetes in hospitalized patients were 21% and 7%, respectively. Also notable is the fact that 14 (7%) hospitalized patients were pregnant. Among those surviving patients with admission and discharge data (n = 97), the median hospital stay was 2 days (range, 1-11 days).

As of August 24, there were 7 H1N1-related deaths in Chicago (crude mortality rate, 0.45%); all were associated with respiratory compromise.

1. 20-year-old pregnant woman
2. 54-year-old women with acute myeloid leukemia
3. 22-year-old man receiving long-term hemodialysis
4. 32-year-old obese* man with asthma
5. 52-year-old man with lymphoma
6. 26-year-old woman with no reported chronic health conditions
7. 47-year-old woman with no reported chronic health conditions

The CDC editors note that the true hospitalization rate and, particularly, the attack rate associated with the novel H1N1 virus in Chicago might be overestimated, because the number of confirmed cases is underestimated. To alleviate laboratory workloads, H1N1 testing was discouraged for outpatients by the city's Department of Public Health after April 30th.

The most common clinical symptoms among affected Chicagoans are fever (73%), cough (68%), sore throat (29%), and dyspnea (15%).

* Another recent MMWR report from the Michigan Department of Community Health suggests that obese and extremely obese patients may be particularly vulnerable to complications associated with novel H1N1 infection.