Primary care data

Influenza activity

Among the 41 countries reporting on influenza activity for week 11/2017, 37 countries reported a return to baseline levels and 4 countries reported medium intensity. Of the 39 countries reporting on geographic spread, 6 reported widespread influenza activity.

The percentage of influenza virus detections among sentinel specimens continued to decrease, from 22% in week 10/2017 to 17% for week 11/2017, with 12 of 24 countries reporting dominance of influenza B viruses.

Viruses detected in sentinel-source specimens (ILI and ARI) 

For week 11/2017, 164 of 954 (17%) sentinel specimens tested positive for influenza viruses (Table). Of these, 62% were type B and 38% type A viruses. The proportion of type B viruses commonly increases in the second half of an influenza season and this is the first week where the number of B viruses exceeded the number of influenza A viruses. The great majority (94%) of subtyped influenza A viruses were A(H3N2). The lineage of 33 influenza B viruses was determined, of which 19 (58%) fell in B/Yamagata and 14 (42%) in B/Victoria lineages, the same proportions as in recent weeks. Of 18 countries across the region that each tested at least 10 sentinel specimens, only 2 reported proportions of influenza virus detections of 30% or above.

Table. Influenza virus detections in sentinel-source specimens by type and subtype, week 11/2017 and cumulatively

 

Current Week

Season 2016-2017

Virus type and subtype

Number

%a

Number

%a

Influenza A

63

48

16 135

92

A(H1N1)pdm09

3

6

181

1

A(H3N2)

46

94

13 464

99

A not subtyped

14

-

2 490

-

Influenza B

101

62

1 356

8

B/Victoria lineage

14

42

276

44

B/Yamagata lineage

19

58

345

56

Unknown lineage

68

-

735

-

Total detections/ Total tested

164 / 954

17

17 491/ 46 380

38

aFor influenza type percentage calculations, the denominator is total detections; for subtype and lineage, it is total influenza A subtyped and total influenza B lineage determined, respectively; for total detections, it is total tested.

Since week 40/2016, similar cumulative distributions of influenza types and type A subtypes have been observed: of all typed viruses, 92% were type A, with 99% of those subtyped being A(H3N2) (Figure and Table). Of the 621 influenza B viruses that have been ascribed a lineage since week 40/2016, 345 (56%) were of the B/Yamagata lineage and 276 (44%) were of the B/Victoria lineage.

Figure. Influenza virus detections in sentinel-source specimens by type and subtype, by week

 

The data in the pie chart is cumulative.