Though pandemic flu is circulating at low levels in many parts of the world, Thailand and some West African nations are reporting increased activity, and the virus is being edged out by influenza B in China and other Asian regions, with signs of westward spread, the World Health Organization (WHO) said today.
Influenza B virus circulation seems to be moving westward, with the Russian Federation and Sweden now reporting that the virus is cocirculating with or dominating the pandemic virus. Iran also reported that, although flu activity is low, all recent detections have been influenza B.
The United States hasn't seen a spike in influenza B detections, though the US Centers for Disease Control and Prevention (CDC) said today that it is circulating at low levels. The nation saw a surge of influenza B cases near the end of the 2008-2009 flu season, which was problematic, because the B strain that was circulating didn't match the one contained in the seasonal flu vaccine. So far the influenza B strains identified at US labs this flu season are a good match for the current seasonal flu vaccine.
Vincent Racaniello, PhD, a virologist at Columbia University who writes Virology Blog, told CIDRAP News that the influenza B pattern the WHO is reporting is typical. He said the two main lineages, B/Victoria-like and B/Lee-like, have been cocirculating for 25 years, with changing patterns of prevalence and geographic distribution.
"There are frequent bottleneck years during which prevalence of B strains is low; this usually corresponds to high prevalence of influenza A strains," he said. "Once the bottleneck is relieved, there are usually changes in the prevalence of the two B lineages."
The current spread of influenza B may be a reflection of waning pandemic H1N1 activity that has occurred later in the Northern Hemisphere's flu season, Racaniello said.