Kansanterveyslaitos/Folk

Influenza activity and influenza viruses
during the 2006-2007 epidemic season in Finland

Updated on May 16, 2007

Influenza A/H3: The first cases of infection with influenza A virus were recorded at KTL (NPHI) from samples collected during week 1 (a sporadic case in the capital area) and week 2 (a local outbreak among conscripts in a garrison in the northern part of Finland). In HI tests, the latter strains were related to Wisconsin/67/2005, the H3N2 subtype strain in the recommended composition of influenza virus vaccines for the 2006-2007 season ( Table 2 ).  In a phylogenetic analysis they grouped with Wisconsin/67/2005 for their HA1 genes ( Fig 1). Fig 2 deals with their amino acid markers in HA1.

During weeks 3 and 4, H3 subtype strains were recorded in three garrisons in different parts of Finland. During weeks 5 and 6, the number of affected military units increased. During week 7 the positive laboratory findings from military conscripts turned down, while in the civilian population they turned up during week 6. The strains isolated during weeks 8-10 were with few exceptions from civilian persons. The H3 subtype strains isolated after the first local outbreak (weeks 1 and 2) were also related toWisconsin/67/2005 antigenically, but the results in HI tests were variable ( Table 2 ). In a phylogenetic analysis they formed a separate branch within the Wisconsin/67/2005 clade ( Fig 1 ). This branch spread all over Finland (municipalities where verified: Helsinki, Vantaa, Kirkkonummi, Riihimäki, Hattula, Salo, Säkylä, Jyväskylän mlk, Lappeenranta, Valkeala, Joensuu, Kajaani, Rovaniemi and Sodankylä). From week 6 onwards laboratory findings fell away and the last case diagnosed at KTL (NPHI) was a patient who was taken ill in the capital area during week 14.

Influenza A/H1: Only one H1 subtype strain was isolated during the 2006-2007 epidemic season in Finland (week 10). The patient was a 4-year-old boy in East Finland (Joensuu). The strain gave low HI titres with antisera produced against a representative A/New Caledonia/20/99 lineage virus from the 2004-2005 epidemic season ( Table 2 ). In a phylogenetic analysis the isolate belonged for its HA1 sequence to the new Solomon Islands/3/2006 sublineage characterized by amino acid changes K73R, K140E, P270S and D272G ( Fig 3 ).

Influenza B: Laboratory findings at KTL (NPHI) were from weeks 8 and 9 (civilian adults from South-West Finland). In HI tests the first strain belonged to B/Yamagata/16/88 lineage viruses not represented in the recommended composition of influenza virus vaccines for the 2006-2007 season.

For weekly figures of laboratory-detected influenza viruses, see Table 1

Table 1: Diagnostic laboratory findings of influenza made by NPHI

(data by week of sampling specimens)

 

INFLUENZA ISOLATES AND DETECTION OF VIRAL ANTIGENS AND RNA

YEAR

WEEK

A*

A

H3

A

H1

B

YEAR

WEEK

A*

A

H3

A

H1

B

2006

37-52

0

0

0

0

2007

1

1

0

0

0

 

Total

0

0

0

0

 

2

0

2

0

0

 

 

 

 

 

 

 

3

2

5

0

0

 

 

 

 

 

 

 

4

3

6

0

0

 

 

 

 

 

 

 

5

0

13

0

0

 

 

 

 

 

 

 

6

11

29

0

0

 

 

 

 

 

 

 

7

2

16

0

0

 

 

 

 

 

 

 

8

4

14

0

1

 

 

 

 

 

 

 

9

1

8

0

2

 

 

 

 

 

 

 

10

3

3

1

0

 

 

 

 

 

 

 

11

0

3

0

0

 

 

 

 

 

 

 

12

1

2

0

0

 

 

 

 

 

 

 

13

2

4

0

0

 

 

 

 

 

 

 

14

1

0

0

0

 

 

 

 

 

 

 

15-17

0

0

0

0

 

 

 

 

 

 

 

Total

31

105

1

3

* Not studied for the subtype, ** Incomplete data

 

Table 2: Preliminary subtyping of a collection of influenza A viruses isolated in January - March 2007 in Finland.

 

 

HI titre with antisera against

Fin/635/06 (H3N2)

Fin/690/06* (H3N2)

Fin/552/05** (H1N1)

Fin/635/06

640

160

<20

Fin/690/06

640

1280

<20

Fin/552/05

<20

<20

1280-2560

2006-07

Fin/804/07

 

1280

 

640

 

<20

Fin/805/07

320

160

<20

Fin/806/07

1280

640

<20

Fin/807/07

640

1280

<20

Fin/808/07

320

640

<20

Fin/810/07

320

80

<20

Fin/811/07

640

320

<20

Fin/812/07

320

160

<20

Fin/813/07

320

320

<20

Fin/814/07

640

640

<20

Fin/887/07

<20

<20

160

* A/Wisconsin/67/05-lineage,        **A/New Caledonia/20/99-lineage

 

Fig 1: Phylogenetic tree of the HA1 sequences of a small collection of H3N2 subtype field strains isolated in recent years in Finland (F) and Estonia (Est).

The tree grows horizontally to the right. Reference strains are in green and the 2006-2007 isolates in red. Epidemic seasons when the field strains were isolated are indicated. The first 2006-2007 isolates grouped with A/Wisconsin/67/2005 and some of the field strains that were isolated during the 2005-2006 season. The strains isolated later during the season in different parts of Finland formed a separate branch in this clade.

 

Fig 2: Amino acid residues in HA1 characteristic of the first H3N2 subtype field strains isolated in Finland in January 2007.

The first two strains, A/Finland/797/07 and A/Finland/798/07 (Fig. 1), shared two of the amino acid markers of the “A/Wisconsin/67/2005 subgroup” (in red, space-filled), S193F (antibody combining site B) and D225N (receptor binding site). They differed from the Wisconsin subgroup by 6 amino acid changes in HA1 (in blue, space-filled): N6I, S45N, R142G (site A, a typical residue for the strains that circulated during the first half of the 1990’s), L157S (site B), K173E (characteristic of many strains that circulated during the 2004-2005 and 2005-2006 seasons in Finland but not of the Wisconsin subgroup) and A304T.

 

Fig. 3: Phylogenetic tree of the HA1 sequences of a collection of H1 subtype field strains isolated in Finland (F) since 1977.

The tree grows to the right. Reference strains in green and the first isolate from the 2006 to 2007 epidemic season in red. The seasons are indicated.

 

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(reijo.pyhala@ktl.fi)