H3N2 flu outbreaks, cases trigger CDC alert

Aug 5, 2010 (CIDRAP News) – The Centers for Disease Control and Prevention (CDC) is urging healthcare providers to be alert for influenza cases, following the reporting of two small outbreaks of influenza A/H3N2 in Iowa and scattered H3N2 cases in 11 other states.

Noting that sporadic cases and localized outbreaks of flu are detected every summer, the CDC said clinicians should consider flu as a possible diagnosis in patients with acute respiratory illnesses, including pneumonia.

In an e-mailed health advisory issued last night, the CDC said the number of reported H3N2 viruses in late June and July increased slightly over previous months. In early July, two small outbreaks confirmed by reverse transcriptase–polymerase chain reaction (RT-PCR) were confirmed in non-neighboring counties in eastern Iowa, the notice added.

The first outbreak included 4 of 13 members of a college sports team who became ill, the CDC said. The second one included 9 of 12 children in a child-care setting and one parent. Two patients in the first outbreak and one in the second have tested positive by PCR, and samples have been sent to the CDC for further analysis.

None of the patients had traveled recently, and there were no epidemiologic links between the two outbreaks, the CDC said.

However, a Jul 30 epidemiologic update from the Iowa Department of Public Health (IDPH) said several patients in the two outbreaks have "connections to travel outside of Iowa." An earlier IDPH update, dated Jul 16, mentions two H3 flu cases in Johnson County.

In addition to the Iowa outbreaks, between Jun 20 and Jul 23 the CDC received H3-positive specimens from 11 other states and a "smaller number of sporadic samples" that tested positive for the 2009 H1N1 and influenza B viruses, the CDC notice said.

The CDC did not list the states that have reported cases, other than Iowa. However, the IDPH updates mentioned that H3 cases have been reported recently in Minnesota, Arkansas, Wisconsin, Pennsylvania, and Hawaii.

On the basis of hemagglutinin gene sequencing of four isolates so far, the CDC said, the viruses are expected to be similar to A/Perth/16/2009-like H3N2 viruses, a strain that's included in this year's seasonal flu vaccine.

"Perth-like H3N2 viruses were first identified in early 2009, but have not yet circulated widely in the United States," the notice stated. Previous flu vaccines did not include this strain, so last year's vaccine would not be likely to provide much protection against it, it said.

The CDC alert advises that treatment decisions should not be based on negative rapid flu test results, since rapid tests have only moderate sensitivity and also can yield false-positives. If laboratory confirmation is wanted, clinicians should order RT-PCR or viral culture.

"Clinicians should use empirical treatment with influenza antiviral medications for persons hospitalized with suspected influenza, and for suspected influenza infection of any severity in high-risk individuals, regardless of influenza immunization status," the CDC said.

H3N2 viruses have been circulating since they emerged in the pandemic of 1968-69. But last winter they were largely crowded out by the pandemic H1N1 virus in the United States. Flu experts say that flu seasons in which H3N2 viruses predominate tend to be more severe than those in which seasonal H1N1 or type B viruses are dominant.

See also:

IDPH page with access to Jul 30 and Jul 16 epidemiologic updates

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