US flu activity rises again as study highlights H1N1 severity

Elderly hospital patient
Elderly hospital patient

ponsulak / iStock

Flu activity in the United States rose again, with more states reporting widespread levels, the US Centers for Disease Control and Prevention (CDC) said on Nov 29 in its latest weekly report.

Meanwhile, a study out of Australia demonstrates more severity from the H1N1 flu strain compared with H3N2 in hospitalized patients.

South, West early US hot spots

Influenza B is still the predominant strain in the United States, an unusual pattern for the early part of the season, though all three strains are circulating, with some variation by region and age-group, the CDC said.

Overall, the percentage of clinic visits due to flulike symptoms rose to 2.9%, keeping it above the national baseline for the second week in a row. Four of the CDC's regions are above their region-specific baselines. The percentage of respiratory specimens that tested positive for flu rose to 8.0%, up from 7.3% the week before.

The CDC's flu maps show that the southern and western states are the early-season hot spots. Eight jurisdictions reported high flu activity, another clinic visit indicator. The number of states reporting widespread activity rose to 10, up from 6 the previous week.

The overall hospitalization rate rose from 1.4 per 100,000 population to 2.0 per 100,000 population, with the rates highest in seniors, followed by children younger than 5. Nearly 60% of hospitalizations were linked to influenza A, with 0.7% linked to influenza A and B coinfections.

One more pediatric flu death was reported—linked to influenza B—putting the season's total at five. Overall deaths from pneumonia and flu, typically a marker that lags other indicators, was 5.1%, well below the seasonal baseline and epidemic threshold.

H1N1 severity

In other flu developments, Australian researchers who conducted a 3-year study of adults hospitalized for respiratory illnesses found that 2009 H1N1 influenza was associated with more severe illness than H3N2 and other respiratory viruses. The team published its report Nov 28 in Epidemiology and Infection.

The investigators reviewed data from 17,332 adults who were admitted to a hospital in Perth from 2012 through 2015 with a respiratory illness. Patient were tested for three seasonal flu strains as well as metapneumovirus, respiratory syncytial virus, and parainfluenza. Of the total number of patients, 1,753 (10.1%) had test results.

When the authors compared viruses and patients' clinical courses, they found that those infected with H1N1 were more likely to experience severe outcomes such as intensive care unit admission, pneumonia, and longer hospital stays than their peers with H3N2 infections. Patients with viruses other than seasonal flu had outcomes similar to those sick with H3N2.

The researchers concluded that an awareness of circulating flu strains is a key factor in healthcare preparedness.

See also:

Nov 29 CDC FluView report

Nov 28 Epidemiol Infect abstract

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