Slight decrease seen in US, European flu activity

Patient on oxygen
Patient on oxygen

Katarzyna Bialasiewicz / iStock

The US Centers for Disease Control and Prevention (CDC), in its weekly FluView report, said that influenza activity decreased slightly this past week while reporting an additional five pediatric deaths. Officials have now confirmed 34 flu-related deaths in children this season.

And in related news, UK scientists yesterday reported a novel subclade of the H3N2 strain of influenza.

H3N2 dominates across states

As in last week's report, there was a higher proportion of influenza B in lab specimens this week (16.3%), but influenza A is still the predominant type, showing up in 83.7% of samples. Increasing influenza B cases is common for the second half of the flu season. 

H3N2 is still the dominant subtype (93.3%) of influenza A specimens. Last week, the CDC reported that only about half of all flu vaccine recipients were protected against the flu this season in part because H3N2 was circulating so widely. Even in years when the circulating viruses antigenically match the vaccine strains, H3N2 causes more severe cases.

Today's report said flu is now considered widespread in only 44 states and Puerto Rico. That’s down 2 states from last week's season high of widespread activity in 46 states.

Hospitalizations rise

The overall rate of hospitalizations from influenza is now 33.7 laboratory-confirmed influenza-associated hospitalizations per 100,000 population. Last week, the rate was 29.4 per 100,000 population. Patients over the age of 65 have by far the highest rate of hospitalizations (155.2 per 100,000).

The proportion of clinic visits for influenza-like illness (ILI) was 4.8%, which remains well above the national baseline of 2.2%. Nine of the ten geographic regions in the country reported elevated ILI activity, with 27 states experiencing high ILI activity, New York City and 5 states having moderate ILI activity, Puerto Rico and 10 states reporting low ILI activity, and 8 states experiencing minimal ILI activity.

The overall deaths from pneumonia and flu held at 7.8%, the same percent reported last week, keeping the level above the epidemic threshold of 7.5% for this point in the flu season.

Novel H3N2 subclade in London

A study published yesterday in Eurosurveillance said that a novel subclade of H3N2 virus was the cause of several cases of influenza documented in December in London. The clade, proposed as 3C.2a2, was co-circulating with other strains of influenza A and could be implicated in this season's  unusually high circulation of respiratory syncytial virus (RSV), the authors said.

Rather than demonstrating antigenic drift, the researchers believe the novel clade occurred because of co-circulating strains (often in healthcare environments) that led to complex alterations of glycosylation sites.

"Our findings in London of the rapid emergence of genetically drifted influenza A(H3N2) viruses underscore the potential for such strains to spread rapidly in hospital environments among patients and staff," the authors wrote.

According to the European Centre for Disease Prevention and Control (EDCD), influenza activity remained elevated on the continent but was lower than last week. Influenza A viruses predominated, accounting for 96% of all sentinel detections and 99% of subtyped influenza A.

Vaccine effectiveness is estimated to be 38% in Europe, which has led to severe outcomes for elderly patients, the ECDC said.

See also:

Feb 24 CDC FluView

Feb 23 Eurosurveillance study

Feb 24 ECDC Flu update

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