Avian Flu Diary: North America: Flu Surveillance Week 3

# 5266

The weekly surveillance numbers from Canada and the United States were released yesterday and some interesting trends are beginning to emerge.

In Canada, the peak of the flu season may have been reached, although some indicators are up while others are down – while in the United States influenza activity is picking up pretty much across the board.

Interestingly, while the H3N2 virus remains the dominant influenza A strain detected in the United States, after several months of low activity, levels of H1N1 have risen 3 weeks in a row.

We’ll start with Canada’s weekly FluWatch report.

January 16 to January 22, 2011 (Week 03)

Summary of FluWatch Findings for the Week ending January 22, 2011

Overall influenza detections appear to have peaked, with most regions across the country now showing a decline in the percentage of positive influenza detections, except BC and the Atlantic provinces. Paediatric and adult hospitalizations have decreased this week, however, some indicators have increased including the number of regions reporting widespread and localized influenza/ILI activity, the number of outbreaks, and the ILI consultation rate.

Since the beginning of the season, 89.5% of the subtyped positive influenza A specimens were influenza A/H3N2.In week 03, detections of pandemic H1N1 2009 increased slightly to 16.9% of all subtyped influenza A specimens, compared to 15.5% in week 02. The overall proportion of positive tests for RSV has increased from 9.6% to 12.5% in week 03.

Percent positive influenza tests, compared to other respiratory viruses, Canada, by reporting week, 2010-2011

Moving south, the CDC’s FluView system paints a picture of a flu season still gaining speed. While it varies from year-to-year, February is usually the the height of the influenza season in the United States.

2010-2011 Influenza Season Week 3 ending January 22, 2011

All data are preliminary and may change as more reports are received.

Synopsis:

During week 3 (January 16-22, 2011), influenza activity in the United States increased.

Of the 5,823 specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories and reported to CDC/Influenza Division, 1,754 (30.1%) were positive for influenza.

The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.

Three influenza-associated pediatric deaths were reported. Two of these deaths were associated with influenza A (H3) virus infection and one was associated with an influenza B virus.

The proportion of outpatient visits for influenza-like illness (ILI) was 3.6%, which is above the national baseline of 2.5%. Six of the 10 regions (Regions 2, 3, 4, 5, 6, and 7) reported ILI above region-specific baseline levels. Nine states experienced high ILI activity, eight states experienced moderate ILI activity, New York City and nine states experienced low ILI activity, 24 states experienced minimal ILI activity, and data were insufficient from the District of Columbia.

The geographic spread of influenza in 25 states was reported as widespread; 16 states reported regional influenza activity; the District of Columbia and four states reported local influenza activity, Puerto Rico, the U.S. Virgin Islands, and four states reported sporadic influenza activity, Guam reported no influenza activity, and one state did not report.

U.S. Virologic Surveillance:

WHO and NREVSS collaborating laboratories located in all 50 states and Washington D.C. report to CDC the number of respiratory specimens tested for influenza and the number positive by influenza type and subtype. The results of tests performed during the current week are summarized in the table below.

All 50 states and the District of Columbia have reported laboratory-confirmed influenza this season.

Pneumonia and Influenza (P&I) Mortality Surveillance

During week 3, 7.5% of all deaths reported through the 122-Cities Mortality Reporting System were due to P&I. This percentage was below the epidemic threshold of 7.9% for week 3.

(click for interactive map)