Most Severe Influenza Seen in Young, Unvaccinated Adults

Laurie Barclay, MD

February 11, 2014

Young adults who had not been vaccinated against influenza had severe cases and needed the most intensive treatment, according to a case series published online February 10 in the American Journal of Respiratory and Critical Care Medicine. In this case series, by Jelena Catania, MD, from the Division of Infectious Diseases, Duke University Hospital, Durham, North Carolina, and colleagues, only 2 of 22 patients who required intensive care had been vaccinated before their illness.

"Our observations are important because they reinforce a growing body of evidence that the influenza vaccine provides protection from severe illness requiring hospitalizations," lead author Cameron Wolfe, MD, assistant professor of medicine, Division of Infectious Diseases, Duke University Hospital, said in a news release. "The public health implications are important, because not only could a potentially deadly infection be avoided with a $30 shot, but costly hospitalizations could also be reduced."

Of the first 55 patients treated for influenza at Duke from November 2013 through January 8, 2014, 48 (87.3%) were infected with H1N1 virus, which was responsible for the 2009 pandemic. In 2009, H1N1 influenza also tended to be particularly severe in young adults.

Most hospitalizations were in previously healthy young persons, with a median age of 28.5 years (range, 2 months to 101 years).

Less than one quarter of the patients (23.6%; 13/55) were vaccinated against influenza at least 2 weeks before the onset of their acute illness.

Only 11 (33.3%) of the 33 patients hospitalized but not admitted to the intensive care unit (ICU) had previously been vaccinated. Most of these patients were immunocompromised or immunosuppressed as a result of medication or were chronically ill.

Only 2 (9.1%) of the 22 patients who required intensive care had been vaccinated before they fell ill.

"We observed a high percentage of hospitalized patients for influenza requiring ICU-level care, which appears higher than observed in our hospital during the 2009 pandemic flu season," coauthor John W. Hollingsworth, MD, associate professor of medicine, Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Duke University Medical Center, said in the news release. "It remains unclear whether the high rate of ICU admissions represents a diagnosis bias or whether the severity of illness being caused by the current H1N1 virus is higher."

Rapid Influenza Test May Have Low Sensitivity

As noted in earlier research, the rapid test for influenza may be insufficiently sensitive. In the present case series, 31.8% (7/22) of the patients admitted to the ICU had previously negative influenza tests, including 4 patients with false-negative rapid influenza antigen tests.

The delay in diagnosis resulted in a missed window of opportunity for taking oseltamivir, which is more effective for symptom relief and for shortening influenza duration if taken within 24 hours of the time the diagnosis of influenza is considered.

The Centers for Disease Control and Prevention therefore recommends that the decision regarding antiviral treatment should not await laboratory confirmation. Independent of symptom duration, indications for antiviral treatment include hospitalization, severe complicated and progressive illness, and presence of risk factors for influenza complications.

"Together, our observations during this influenza season support a high prevalence of the H1N1 virus affecting young adults and requiring ICU care, high false-negative rates of rapid flu tests, and delay in starting antiviral treatment," Dr. Wolfe said in the release. "Added to the finding of very low vaccination rates among both hospitalized and ICU admissions, our observations support previous findings that vaccination reduces the severity of disease and vaccinations should be encouraged as recommended by the...Centers for Disease Control and Prevention."

Support for this study provided by the Clinical Microbiology Laboratory at Duke University Medical Center and the National Institutes of Health.

Am J Respir Crit Care Med. Published online February 10, 2014. Abstract

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