# 6135
Most years, north of the equator we find ourselves deep in the heart of flu season by the first week in February. Sometimes, we are even post-peak by now, and beginning to see a decline in cases.
But this year, influenza has been unusually slow to arrive in North America and Europe (although it is beginning to pick up).
The reasons for this late arrival are not abundantly clear, although I suspect the mild winter and growing community immunity after 3 years of having basically the same flu strains in circulation, probably have something to do with it.
While unusual, it is not unprecedented to have a late start to the flu season, as this latest entry from the CDC’s Have You Heard? website explains.
CDC Says U.S. Seeing Latest Flu Season in Nearly Three Decades
Still Not too Late to Vaccinate
February 10, 2012 --According to the CDC influenza surveillance report “FluView,” flu activity in the United States increased for the week ending February 4, 2012. The percentage of respiratory specimens testing positive for influenza nationally rose to 10.5 percent from 7.6 percent the prior week. This is the first time this season that the percent of respiratory specimens testing positive for influenza has surpassed 10 percent, which is generally a marker to indicate that flu season is beginning.
By this measure, flu is off to a late start this season. In the past 29 years, the percent of respiratory samples testing positive for flu has remained below the 10 percent mark until February only once before (1987-1988).
CDC tracks influenza activity through 8 surveillance categories. At the moment, other influenza activity indicators are lagging as well. Influenza-like-illness (ILI) remains below baseline nationally, which is again late for this time of year. However, this week, two regions of the country – the central and northwest regions – are reporting ILI activity above baseline for the first time this season, and one state – California – is the first to report widespread geographic influenza activity this season. Additionally, the second pediatric death of the season was reported in this week’s FluView, which was associated with an unsubtyped influenza A virus.
These key flu indicators are expected to increase in the coming weeks. “The increases we are seeing in the number of respiratory samples testing positive for flu should forecast increases in other flu activity indicators in the coming weeks,” says Lyn Finelli, chief of domestic surveillance for CDC’s Influenza Division.
Although influenza may be down this year, you shouldn’t count it out. We could still see a late season surge of Influenza A, and Influenza B typically doesn’t peak until March or April.
Complicating matters, yesterday’s FluView report indicates roughly half of the B viruses tested are of the Yamagata lineage, which has not been included in the seasonal vaccine for several years.
Influenza B (B/Victoria/02/87 and B/Yamagata/16/88 lineages) [37]:
- Victoria Lineage [17]: Seventeen (45.9%) of the 37 influenza B viruses tested belong to the B/Victoria lineage of viruses and were characterized as B/Brisbane/60/2008-like, the influenza B component of the 2011-2012 Northern Hemisphere influenza vaccine.
- Yamagata Lineage [20]: Twenty (54.1%) of the 37 influenza B viruses tested belong to the B/Yamagata lineage of viruses.
Only a small number of influenza B viruses from the United States have been available for testing so far this season. While less than 50% of these viruses are similar to the influenza B component in the 2011-2012 influenza vaccine, the majority of influenza B viruses circulating worldwide have been similar to the influenza vaccine strain.
It is too early in the influenza season to determine how well the seasonal influenza vaccine strains and circulating strains will match.
Ample enough reason to make good flu hygiene (covering coughs & sneezes, washing hands, staying home when sick) a priority all year round.
Because the only thing truly predictable about influenza is its unpredictability.
