Influenza virus activity in the world
13 April 2012
Based on FluNet reporting (as of 12 April 2012, 6.15 UTC) during weeks 12 to 13 (18/03/2012 to 31/03/2012), National Influenza Centres (NICs) and other national influenza laboratories from 80 countries, areas or territories reported data. The WHO GISRS laboratories tested more than 36 319 specimens. 9 814 were positive for influenza viruses, of which 7 575 (77.2%) were typed as influenza A and 2 239 (22.8%) as influenza B. Of the sub-typed influenza A viruses, 754 (16.2%) were influenza A(H1N1)pdm09 and 3 907 (83.8%) were influenza A(H3N2). Of the characterized B viruses, 364 (57.1%) belong to the B-Yamagata lineage and 273 (42.9%) to the B-Victoria lineage.
During weeks 12 and 13 in 2012, laboratory confirmed influenza activity decreased in a number of countries in the northern hemisphere while still remained at high levels in some others.
Influenza A(H3N2) continued to be the predominant virus subtype detected globally while influenza B was the main virus circulating in Canada and the Republic of Korea. Influenza A(H1N1)pdm09 remained low in general. In Europe, the number of virus detections decreased in many countries. Influenza A(H3N2) continued to predominate with a slight increase in the co-circulation of influenza B virus in some countries and sporadic detections of A(H1N1)pdm09 viruses. In northern Africa, influenza A(H3N2) activity was sporadic.
In North America , influenza A(H3N2) remained the predominant virus subtype in the United States of America. Increased influenza B virus detections in Canada were reported compared to previous weeks and was the main virus detected in the country. A(H1N1)pdm09 viruses continued to circulate in some regions in the United States of America. In Central America, A(H1N1)pdm09 was the main circulating virus subtype in Guatemala and was also detected at low levels in Mexico.
In Asia, influenza B virus activity declined in China and influenza A(H3N2) became the predominant virus subtype. Local outbreaks of influenza A(H3N2) and B virus were reported in China Hong Kong Special Administrative Region. Influenza A(H3N2) and influenza B viruses co-circulated in Japan. In the Middle East, Qatar continued to report A(H1N1)pdm09 as the predominant virus subtype.
In the southern hemisphere, influenza activity remained low with the majority of the sporadic virus detections identified as influenza A(H3N2).
Characterization of recent A(H1N1)pdm09 viruses has shown that these are still similar to the vaccine A/California/7/2009 virus. Antigenic and genetic analysis of the A(H3N2) viruses on the other hand has revealed significant drift from the A/Perth/16/2009 vaccine virus. The recent viruses are more closely related to the A/Victoria/361/2011-like virus, the recommended A(H3N2) virus component for the 2012-2013 northern hemisphere influenza vaccine. With regard to the B viruses reported world-wide, some countries detected a higher percentage of viruses from the B/Victoria/2/87 lineage while in others, a greater number of viruses from the B/Yamagata/16/88 lineage were reported.
A 13th human case of infection with the influenza A(H3N2)v virus has recently been reported in the United States of America. This is the first variant A(H3N2) virus to be detected in 2012 and is similar to the previous ones which have 7 genes from the triple reassortant A(H3N2) viruses known to have been circulating in pigs in the North America and the M gene from the A(H1N1)pdm09 virus.