EISS:Clinical influenza activity in Europe is still low, with influenza B virus being dominant

Up to week 5 of 2006, the numbers of consultations for influenza-like illness (ILI) or acute respiratory infection (ARI) have been low in all European countries reporting to the European Influenza Surveillance Scheme (http://www.eiss.org). Only slight increases of influenza activity have been reported in recent weeks by France, the Netherlands, Norway and the United Kingdom [1]. Experience of previous epidemics suggests that clinical influenza activity may still increase in the coming weeks (Figure 1).

Figure 1. Analysis of timing of peak consultations for ILI or ARI by week in the winter season for the nine countries that reported data to EISS since 1996.*

*England, Wales, Scotland, Spain, Portugal and the Netherlands reported consultation rates for ILI per 100 000 population. Belgium reported in the first years of contribution percentage consultations for ILI and later on the consultation rate for ILI. France and Germany reported in the first years of contribution percentage consultation for ARI and later on the consultation rate for ARI.

When compared with previous winters, especially those of 1997-1998 and 2000-2001, it appears that the present consultation levels are not exceptionally low (Figure 2). Remarkably, the United Kingdom experienced only very mild influenza seasons after the 2000-2001 winter, in contrast to the other countries analysed.

Figure 2. Analysis of peak influenza-like illness (ILI) consultation rates by winter season for the six countries that reported consultation rates to EISS since 1996.*

*Preliminary maximum levels till week 5 of 2006 for winter 2005-2006 were included for comparison.

Since week 40 of 2005 (the start of the current surveillance period), 1345 influenza viruses have been detected, 937 (70%) being type B and 408 (30%) type A (of which 53% were subtype H1 and 47% subtype H3). This distribution is remarkable. The last winter in which influenza A/H1 and B viruses formed a sizable proportion of the influenza virus detections was in 2000-2001. However, during the 2002-2003 winter, in which influenza activity also started late, a similar distribution of influenza A (although being of the H3 subtype) and B viruses was found in week 5 of 2003 [2], whereas later in that winter influenza A viruses dominated [3].

Of the 132 virus isolates which have been antigenically and/or genetically characterised up to week 5 of 2006, 20 were A/New Caledonia/20/99 (H1N1)-like, 23 were A/California/7/2004 (H3N2)-like, 69 were B/Malaysia/2506/2004-like (B/Victoria/2/87-lineage) and 20 were B/Jiangsu/10/2003-like (B/Jiangsu/10/2003 is a B/Shanghai/361/2002-like virus from the B/Yamagata/16/88-lineage and is currently used in the vaccine) [1]. The A/H1 and A/H3 viruses were not antigenically distinguishable from the A virus strains included in the vaccine, whereas 69 of the 89 B viruses were antigenically distinguishable from the B virus strain included in the vaccine.

Also in the United States (US) and Canada there is little influenza activity at present [4,5]. However, the (sub)type distribution of the causative viruses is different from that in Europe. In the US, 97% of virus detections were influenza A and only 3% influenza B (n=4,466). In Canada, influenza A and B viruses were detected in similar proportions (51% A, 49% B). The vast majority (>90%) of subtyped influenza A viruses in the US and Canada have been A/H3. In the US and Canada, specimens are mainly collected from patients visiting a hospital, whereas in Europe a substantial proportion of specimens are collected by general practitioners [6,7]. This might partly explain the differences in the virus (sub)types detected. In Canada 97% of the 87 characterised B viruses belonged to the B/Victoria/2/87 lineage and the rest to the B/Yamagata/16/88 lineage, comparable to Europe where 78% of the characterized B viruses belonged to the B/Victoria/2/87-lineage. In contrast, in the US only 27% of the 11 characterized B viruses belonged to the B/Victoria/2/87-lineage.

In conclusion, the low level of influenza activity in Europe during the 2005-2006 winter is not exceptional when compared with historical data. Based on observations during previous epidemics, Europe may still experience increased influenza activity in the coming weeks, but the chances are decreasing with time.

References:
  1. European Influenza Surveillance Scheme. EISS Weekly Electronic Bulletin 2006; Flu activity remains low in most countries although a sharp increase was found in England, caused mainly by influenza B virus, 10 February 2006: 174. (http://www.eiss.org). Accessed 12 February 2006.
  2. European Influenza Surveillance Scheme. EISS Weekly Electronic Bulletin 2003; Influenza activity remains low to moderate in Europe, 7 February 2003: 82. (http://www.eiss.org). Accessed 13 February 2006.
  3. European Influenza Surveillance Scheme. EISS Weekly Electronic Bulletin 2003; Declining influenza activity in Europe, 18 April 2003: 92. (http://www.eiss.org) Accessed 13 February 2006.
  4. Centers for Disease Control and Prevention. Weekly report: influenza summary update. Week ending February 4, 2006-Week 5. (http://www.cdc.gov/flu/weekly/). Accessed 12 February 2006.
  5. Public Health Agency of Canada. FluWatch, January 29, 2006 to February 4, 2006 (Week 5). (http://www.phac-aspc.gc.ca/fluwatch/05-06/w05_06/index.html). Accessed 12 February 2006.
  6. Centers for Disease Control and Prevention. Overview of Influenza Surveillance in the United States. (http://www.cdc.gov/flu/weekly/pdf/flu-surveillance-overview.pdf). Accessed 13 February 2006.
  7. Public Health Agency of Canada. FluWatch. (http://www.phac-aspc.gc.ca/fluwatch/index.html). Accessed 13 February 2006.