CDC verdict on 2005-06 flu season: mild

Jun 15, 2006 (CIDRAP News) – The Centers for Disease Control and Prevention (CDC) reports that the 2005-06 influenza season was milder than the previous several seasons and had an unusually late peak.

From Oct 2, 2005, through May 20, 2006, US laboratories involved in official flu surveillance tested 139,647 patient specimens for flu viruses, and 17,414 (12.5%) were positive, the CDC said in today's Morbidity and Mortality Weekly Report. Of these, 14,093 (80.9%) were influenza A viruses, and 3,321 (19.1%) were influenza B viruses.

In today's report, the CDC lists the following indications of a relatively mild flu season:

• The proportion of specimens testing positive for influenza peaked at 23% in Dec 2005, compared with peak proportions of 23.2% to 41.0% in the previous five flu seasons.
• The highest weekly percentage of visits to "sentinel" healthcare providers for influenza-like illness was 3.3%, compared with a range of 3.2% to 7.6% in the previous five seasons.
• At the seasonal peak, the week ending Mar 11, 2006, 41 states reported regional or widespread influenza activity. This compares with 45 to 50 states in the previous five flu seasons.
• The percentage of weekly deaths attributed to pneumonia and influenza never exceeded the CDC's epidemic threshold, peaking twice at 7.8%. During the previous five seasons, the total number of weeks above the epidemic threshold ranged from 4 to 16, and the peak percentage ranged from 8.1% to 10.4%.

This season's peak week, in March, came substantially later than the peak periods in the previous five seasons, which ranged from early December to late February, according to the report.

However, the number of weeks during which 10% or more of specimens tested positive was particularly high this past flu season. The season saw 18 consecutive weeks at that level, compared with 11 to 15 weeks in the previous five seasons.

The CDC also reports that 35 children died of flu-related causes in 13 states during the season. This compares with 39 pediatric deaths in 17 states during the 2004-05 flu season and 152 such deaths in 40 states in 2003-04. Pediatric statistics were not given in the CDC's year-end reports immediately preceding those.

Of the 5,661 influenza A viruses that were subtyped, 5,231 (92.4%) were H3N2 strains and 430 (7.6%) were H1N1. And although type A viruses predominated overall, influenza B was more common in specimens from late April through May.

(In Europe, influenza B predominated. In Asia, influenza A (H1N1) and influenza B predominated, while Africa reported low levels of influenza A and B, according to the report.)

Also, circulating virus strains matched the 2005-06 vaccine quite well, the report said. Most influenza A subtypes matched the two vaccine strains. And in the early months of the season, the influenza B subtype matched as well.

The US Food and Drug Administration has recommended that the 2006-07 trivalent vaccine contain different influenza B and influenza A (H3N2) components, the report notes. The recommended virus subtypes for the coming season's vaccine are A/New Caledonia/20/99-like (H1N1), A/Wisconsin/67/2005-like (H3N2), and B/Malaysia/2506/2004-like.

The report also says the past flu season was notable in that H3N2 viruses developed a high level of resistance to the antiviral drugs amantadine and rimantadine (the adamantanes). Because of this, the CDC in January recommended not using these drugs for flu prevention or treatment for the rest of the season.

This high resistance to the adamantanes among circulating H3N2 viruses continued through May, the report states.

CDC. Update: influenza activity—United States and worldwide, 2005-06 season, and composition of the 2006-07 influenza vaccine. MMWR 2006 Jun 16;55(23):648-52 [Full text]

See also:

March 1 CIDRAP News story: "Two new strains to be used in 2006-07 flu vaccine"

CDC influenza activity site
http://www.cdc.gov/flu/weekly/fluactivity.htm