In the first 4 months since the initial case of H1N1 influenza was diagnosed in California, the epidemiology of the infection has diverged from what is typically seen in seasonal flu epidemics, according to a report in the November 4 issue of JAMA.

At 27 years, the median age of fatal or hospitalized cases is much younger than usual, and the youngest affected patients (aged 2 months or less) rather than the elderly have the highest hospitalization rates. Moreover, “a striking percentage” of patients became extremely ill and required intensive care, wrote Dr. Janice K. Louie and her associates in the California Department of Public Health, Richmond.

The investigators described the epidemiology of the first 1,088 hospitalized and fatal cases of influenza A(H1N1) reported in their state.

Although the age range of these cases was infancy through 92 years, the median age was skewed young--27 years – because so many of the cases (approximately one-third) were in children and younger adults, and relatively few were in the elderly. The rate of hospitalization and/or fatality was 2.8/100,000 cases overall, ranging from a high of 11.9/100,000 cases among infants younger than 1 year to a low of 1.5/100,000 cases in those aged 70 years and older.

“These findings support the recommendation for priority vaccination of close contacts of young infants with the monovalent influenza A(H1N1) vaccine,” the researchers wrote.

The reason why older adults have been largely spared in this pandemic while younger adults have not is still unclear. “Serologic studies suggest that a higher proportion of adults aged 60 years and older may have preexisting immunity to the pandemic 2009 influenza A(H1N1) virus, possibly from previous exposure due to infection or vaccination,” Dr. Louie and her associates said (JAMA 2009;302;1896-902).

Nevertheless, Californians over age 50 years who did contract H1N1 were among the most likely to die from the infection. Comorbidity might be a contributing factor in these cases, since 80% of patients over age 50 years had comorbid cardiac, pulmonary, or other medical conditions.

“Despite reports that elderly persons may be ‘protected’ by preexisting immunity, clinicians should closely monitor and promptly treat older hospitalized patients with pandemic 2009 influenza A(H1N1) infection,” the investigators noted.

Many patients became severely ill with this flu. More than 30% of hospitalized patients required intensive care. Most adults and more than one-third of children required mechanical ventilation.

As expected, most cases presented with features of acute respiratory illness, and two-thirds had radiographic confirmation of pneumonia. Gastrointestinal symptoms, which usually don’t figure in seasonal respiratory flu, affected a significant portion of H1N1 cases: more than one-third reported nausea or vomiting, and one-fifth reported diarrhea.

Some patients also presented with altered mental status from hypoxia and respiratory distress.

Another epidemiologic anomaly was the large number of cases among obese patients. “Of adults with BMI [body mass index] data available, [58%] were obese and one-quarter were morbidly obese. As a point of reference, the percentage of adults who are morbidity obese in the United States is 4.8%,” Dr. Louie and her colleagues noted.

Anecdotal reports from other areas of the world, including Mexico, Chile, and Canada, also cite a high prevalence of obesity in severe and fatal cases of H1N1 infection. “Even if obesity is only a proxy measure for other underlying conditions” that predispose to severe infection, such as diabetes, cardiovascular disease, and pulmonary diseases like obstructive sleep apnea, “BMI data are an easily obtainable measurement that may be useful for quickly identifying patients to target for treatment and prevention measures, similar to age groupings,” they added.

In California hospitals with rapid antigen tests, results were falsely negative in 34% of those tested. Researchers in other areas also have found similar low sensitivity rates with point of care testing for H1N1. “Clinicians should be wary of excluding a diagnosis of pandemic 2009 influenza A(H1N1) infection based solely on nonmolecular testing,” the researchers said.

In their study, approximately 20% of hospitalized cases never received antiviral treatment, and half did not commence treatment until more than 48 hours after symptom onset.

Finally, a high percentage of hospitalized or fatal cases in both children and adults had underlying immunosuppressive conditions. “These patients require strict adherence to infection control measures, as well as serial testing of respiratory specimens; multiple negative PCR [polymerase chain reaction] results should be documented before removal of infection control precautions,” Dr. Louie and her associates said.

No financial conflicts of interest were reported for this study.

Copyright (c) 2009 Elsevier Global Medical News. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

根据11月4日出版的《美国医学会杂志》(JAMA)上的一篇研究报告，自从加利福尼亚州诊断出首例H1N1流感病例起之后4个月的观察显示，此次感染的流行病学状况不同于季节性流感流行的典型特征。

 

重症或住院病例的中位年龄为27岁，这比普通流感病例的中位年龄要小很多，并且年龄最小的感染者(2个月龄或更低)住院率最高，而非年长者。此外，患者中有相当比例病情严重，需要加强监护措施，里士满加州公共卫生部的Janice K. Louie博士及其同事在报告中写道。

 

研究者在其报告中描述了这首批1,088例住院及重症的甲型H1N1流感病例的流行病学特征。

 

虽然从婴儿至92岁的各年龄段都有发病，但发病的中位年龄更偏向于低龄——27岁，因为有很多患者(大约为1/3)是儿童和青年，而年龄较大的患者则相对较少。住院和(或)重症患者的比率为全部病例的2.8/100,000，范围从1岁以下婴儿的11.9/100,000至70岁以上患者的1.5/100,000。

 

“根据这些发现，建议为婴儿的密切接触者优先接种单价甲型H1N1流感疫苗”，研究者写道。

 

此次流感大流行期间，年长的成人不易患病，而年纪轻的成人容易患病，其原因目前尚不清楚。“血清学研究提示，60岁以上人群中很多人可能已经对此次大流行的甲型H1N1流感病毒产生免疫力，这可能是由于既往对感染因素的暴露或免疫接种”，Louie博士及其同事说(JAMA 2009;302;1896-902)。

 

然而，与H1N1流感病毒有密切接触史的50岁以上加利福尼亚人最易死于此次流感。合并症可能是这种情况的影响因素，因为50岁以上患者中80%合并有心脏、肺或其他疾病。

 

研究者强调“尽管报道称年长的人群可能受保护于已有的免疫力，临床医师仍应对其进行密切监测，并即时对已感染大流行性2009 H1N1流感的老年住院患者进行治疗”。

 

很多患者患此次流感后为重症病例，超过30%的住院患者接受了密切监护。大多数成年患者和超过1/3的儿童患者需要机械通气治疗。

 

与预想的一样，多数病例以急性呼吸道疾病为特征，2/3的患者有肺炎的影像学改变。并不常见于季节性流感的胃肠道症状，在H1N1流感中却是一种典型症状：超过1/3的患者有恶心或者呕吐，1/5的患者有腹泻。一些患者还出现了因低氧血症和呼吸道受损而产生的精神状态改变。

 

另一个流行病学异常是大量肥胖患者患病。“在可以获得成人体质指数(BMI)的患者中，58%是肥胖患者，1/4是病态肥胖。作为正常参考值，美国成人患有肥胖的比率是4.8%。” Louie博士及其同事说。

 

来自世界其他地区(包括墨西哥、智利、加拿大)的轶事性报告均提到了严重和致死性H1N1感染病例中有较高的肥胖患病率。即使肥胖只是其他基础疾病容易并发感染的一个指标，例如糖尿病、心血管疾病、肺部疾病如阻塞性睡眠呼吸暂停，“但BMI数值是一个容易获得的测量指标，可用于快速鉴别那些需治疗和预防的目标患者，类似于年龄分组”，他们补充道。

 

在加利福尼亚医院通过对患者进行快速抗原检测，结果发现，患病者中有34%呈假阴性。其他地区的研究者也发现了H1N1检测具有相似的低敏感性。“仅凭非分子学检测，临床医生应小心不要轻易地除外大流行性2009 H1N1流感的诊断。”研究者称。

 

在他们的研究中，大约20%的住院患者从未接受过抗病毒治疗，一半患者直到症状出现后48小时才接受治疗。

 

最后，儿童和成人中较高比例的住院或致死病例具有潜在的免疫抑制疾病。“这些患者需要给予严格的多重感染控制措施，以及呼吸道标本的连续检测。在终止感染控制预防措施之前应获得多次阴性PCR(聚合酶链反应)结果”，Louie博士及其同事说。

 

无本研究相关经济利益冲突的报告。

 

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