Early start of human influenza activity in the northern hemisphere
21 November 2003
Since the beginning of October, outbreaks of influenza in a number of countries in Europe and North America have been due largely to A(H3N2) viruses; whereas, A(H1) viruses were responsible for outbreaks in Iceland. Influenza activity has continued to increase during November in Canada, Finland, France, Israel, Norway, Portugal, Spain, the United Kingdom (UK) and the United States of America (USA).
In Europe (Ireland, Norway, Portugal, Spain, Switzerland), most H3N2 viruses that have been characterized antigenically so far this season were A/Fujian/411/2002-like; in the UK, 2/3 of isolates characterized so far were A/Fujian/411/2002-like and one third were A/Panama/2007/99-like. In Canada, more than a half of the A(H3N2) isolates characterized so far are A/Fujian/411/2002-like, while the rest were A/Panama/2007/99-like. In the US, more than ¾ of the A(H3N2) isolates were A/Fujian/411/2002-like; the rest are A/Panama/2007/99-like.
In the last eight weeks, five deaths in children, two in England and three in Scotland, have been confirmed as due to A/Fujian/411/2002-like virus.
A/Fujian/411/2002-like strains are drift variants of the A/Panama/2007/99 vaccine virus. Therefore antibodies produced against the A/Panama/2007/99-like strain(see WER 2003 78(9): 58-62 and WER 2003 87(11):77 ) do cross-react with A/Fujian/411/2002-like strains, but at a lower level.
For example, in laboratory tests of sera from recipients of influenza vaccines currently used in the northern hemisphere, there were on average 41% reductions in the post-immunization antibody titres to A/Fujian/411/2002-like strains, when compared with the A/Panama/2007/99 vaccine strain. However despite such reductions, the tests demonstrated that 76% of adult and 72% of the elderly vaccinees possessed protective levels of antibody(HI titres of >40) to A/Fujian/411/2002-like strains.
For more information, please review WHO Weekly Epidemiological Record