Kansanterveyslaitos/Folkh„lsoinstitutet/National Public Health Institute

Influenza activity and influenza viruses
during the 2001-2002 epidemic season in Finland

Updated on May 14, 2002

During the latter half of 2001 influenza A and B infections were detected as follows (all were sporadic cases in the south of Finland):

A local outbreak was recorded in January 2002 midmonth in a garrison in the south of Finland. Two H3-subtype influenza A virus strains were isolated. In the early February, a local outbreak due to influenza A/H3 was also confirmed in another garrison and sporadic cases occurred at the capital area and elsewhere in the south of Finland. Halfway through the month low intensity local oubreaks caused by A/H3 came common in the south of Finland thus indicating regional occurrence of influenza. The number of patients treated for acute respiratory infections in the health clinics in Helsinki remained low until week 7, when it increased to a level maybe indicative of an influenza outbreak in the general population. In concurrently, absenteeism at schools and nursery schools in Helsinki slightly increased and virologically verified influenza A/H3 cases became more frequent. By week 8, influenza A/H3 virus was detected in 14 localities in the south of Finland (Fig. 3). Low-intensity influenza A outbreaks continued to occur up to the second half of April, when antigen detections became less frequent. H3N2 subtype viruses predominated.

Just two cases of H1 subtype strains were isolated during the season, the first in the middle of February from a baby in the capital area and the second in the early March from a conscript in a military training centre on the southern coastal area close to Helsinki.

Influenza B activity increased from the beginning of March, but still remained at a low level during the whole season. Some small outbreaks were detected among military conscripts, mainly in the southern garrisons.

The first H3 subtype strains isolated in Finland were antigenically closely related (HI tests) to the vaccine virus A/Panama/2007/99. In the first phylogenetic comparison of nucleotide sequences coding for HA1, an epidemic virus (A/Finland/2/02) was distinct from the strains isolated during the 1998/99 and 1999/2000 epidemic seasons in Finland (Fig. 1). A/Finland/2/02 and A/Panama/2007/99 grouped with each other, but the former virus had evolved apart from the vaccine virus. A total of ten amino acid residues in HA1 differentiated these two viruses, many of them at or in the vicinity of antigenic site D (Fig. 2) . The H3N2 subtype strains isolated afterwards in Finland were more close to A/Panama/2007/99 when studied for their HA1 nucleotide sequences.

All influenza B viruses studied antigenically (n= 19) and for their HA1 sequences (n= 4 for the present) belonged to the B/Panama/2007/99-like viruses thus being distinct from B/Hong Kong/330/01 and other viruses in the lineage of B/Victoria/2/87-like viruses.

For weekly figures of laboratory-confirmed influenza, see Table 1 , for clinical data in the capital area, see from Table 2 onwards.

Table 1: Diagnostic laboratory findings reported by

Influenzavirus A and B isolates and antigen detections
YEAR WEEK A* A/H3N2 A/H1N1 B YEAR WEEK A* A/H3N2 A/H1N1 B
2001 38 0 0 0 0 2002 1 1 0 0 0
39 1 0 0 0 2 0 0 0 0
40 0 0 0 0 3 1 2 0 0
41 0 0 0 0 4 0 0 0 0
42 0 0 0 0 5 10 12 0 2
43 0 0 0 0 6 15 22 0 0
44 0 0 0 0 7 32 26 1*** 1
45 1 0 0 0 8 39 23 0 2
46 0 0 0 0 9 35 13 0 4
47 0 0 0 0 10 55 9 1*** 8
48 0 0 0 1 11 41 4 0 5
49 1 0 0 0 12 71 3 0 2
50 0 0 0 0 13 54 3 0 3
51 1 0 0 0 14 41 3 0 6
52 1 0 0 0 15 52 1 0 3
16 40 0 0 6
17 28 0 0 8
18 16 0 0 1

* Not studied for influenzavirus A subtype
** Incomplete data
*** Not studied for N so far

Fig. 1: Phylogenetic comparison of the HA1 domain of A/Finland/2/02 isolated in 2002 and other H3N2 subtype strains isolated during the 1999/2000 and 1998/1999 epidemic seasons in Finland.

Horizontal lines are proportional to the number of nucleotide changes.
The outgroup species in the unrooted tree is A/Beijing/363/89 isolated in 1989.
(FN = Finland, BJ and BJG = Beijing, NCH = Nanchang, SDN = Sydney, MSC = Moscow, PNM = Panama)

Fig. 2: Ten amino acid substitutions (spacefilled) in HA1 differentiated A/Finland/2/02 (isolated in February 2002) from A/Panama/2007/99.

Residues in the receptor binding site are shown in red.

Fig. 3: Virologically confirmed influenza A(H3N2) in Finland in 2002:
new weekly localities.

Table 2: Weekly absenteeism in Helsinki reported by

Means of weekly absenteeism figures in schools and nursery schools during the 1996/97 epidemic season (weeks 41/96 - 17/97) were 2.45% and 9.6%, respectively. Peak values during the 1998/99 season were 4.7% (weeks 5, 6 and 8 in 1999) and 14.8% (week 5 in 1999), respectively, and during the 1999/2000 season 4.3% (week 5 in 2000) and 14.9% (week 6 in 2000), respectively.

YEAR WEEK SCHOOLS NURSERY SCHOOLS YEAR WEEK SCHOOLS NURSERY SCHOOLS
2001 38 2.4% 9.1% 2002 1 NA NA
39 2.1% 10.0% 2 2.0% 9.8%
40 1.8% 9.7% 3 2.1% 13.7%
41 2.3% 11.0% 4 2.6% 14.0%
42 1.9% 9.2% 5 2.3% 13.8%
43 1.7% 7.0% 6 2.6% 10.3%
44 2.2% 8.0% 7 2.9% 14.7%
45 2.5% 9.7% 8 1.7% 11.6%
46 2.0% 12.3% 9 2.8% 11.0%
47 2.1% 12.1% 10 2.4% 14.0%
48 2.5% 10.7% 11 2.9% 13.7%
49 2.3% 11.9% 12 2.5% 10.1%
50 2.5% 10.6% 13 2.1% 8.1%
51 NA NA 14 2.5% 9.8%
52 NA NA 15 2.3% 9.9%
16 2.7% 9.2%
17 2.0% 9.1%

NA = closed for holiday or data not available for other reasons

Table 3: Consultations for ARI at the hospital emergency rooms in Helsinki

reported by

Total numbers of patients treated for acute respiratory infections, tonsillitis acuta, influenza-like illness and pneumonia are given. The peak of adults was 151 cases during the 1998/99 season (week 7 in 1999) and 113 cases during the 1999/2000 (week 52 in 1999). The population consists about 500 000 persons.

YEAR WEEK ADULTS CHILDREN YEAR WEEK ADULTS CHILDREN
2001 38 44 NA 2002 1 56 NA
39 36 NA 2 37 NA
40 34 NA 3 26 NA
41 36 NA 4 44 NA
42 43 NA 5 33 NA
43 33 NA 6 63 NA
44 39 NA 7 57 NA
45 33 NA 8 51 NA
46 36 NA 9 53 NA
47 46 NA 10 28 NA
48 46 NA 11 75 NA
49 NA NA 12 73 NA
50 42 NA 13 77 NA
51 31 NA 14 53 NA
52 65 NA 15 60 NA
16 49 NA
17 34 NA

NA = data not available

Table 4: Consultations for ARI at the health centres in Helsinki

reported by

Total numbers of patients treated for acute respiratory infections, tonsillitis acuta, influenza-like illness and pneumonia are given. Numbers from the seven districts are combined. The peak was 794 cases during the 1998/99 season (week 7 in 1999) and 652 cases during the 1999/2000 season (week 3 in 2000). The population consists about 500 000 persons.

YEAR WEEK CASES YEAR WEEK CASES
2001 38 213* 2002 1 204**
39 239* 2 NA
40 253** 3 203*
41 261** 4 269**
42 200** 5 291**
43 255** 6 169**
44 225** 7 375**
45 219** 8 211**
46 251** 9 NA
47 302* 10 264**
48 198** 11 138**
49 NA 12 NA
50 169** 13 208**
51 125** 14 247**
52 170** 15 175**
16 166**
17 182**

* = combined data from six of the seven areas that participated in 1999/2000 and previously
** = incomplete data; reports missing from one of the six areas
NA = data not available or even more incomplete

reijo.pyhala@ktl.fi May 14, 2002