Kansanterveyslaitos/Folkh„lsoinstitutet/National Public Health Institute

Influenza activity during the 2000-2001 season in Finland

Updated on July 2, 2001

The first influenza B virus for the season was detected in week 41 and the first influenza A virus in week 44. Both patients were young children in Turku, and the laboratory findings were done at the Department of Virology, University of Turku, where virological screening has been enhanced for scientific purposes. During weeks 44-49, a total of 29 cases of influenza A were confirmed among children in Turku.

Three of the 29 influenza A virus strains were further studied at the NPHI and identified as H1N1 subtype viruses closely related to A/New Caledonia/20/99, one of the three virus components of the vaccine in use in autumn 2000. When studied for the HA1 nucleotide sequence, seven nucleotide substitutions and one amino acid substitution differentiated the first virus isolate from A/New Caledonia/20/99. This close resemblance (Fig. 1) may be indicative of good protective efficacy of the vaccination against the H1N1 viruses of this epidemic season. In accordance with this, post-vaccination HI antibodies at titres of 40 or higher against a recent H1N1-isolate (A/Finland/768/00) were detected in the sera of 81% of young adult vaccinees (n=47) as compared to 4% in the pre-immunization phase.

First infection with influenza A(H1N1) outside Turku was detected from a military conscript who was taken ill in Helsinki in week 49. With exception of the local outbreak in Turku, only sporadic cases of influenza were virologically confirmed in Finland up to the end of 2000.

First local outbreak caused by influenza B was recorded among conscripts in a garrison in north of Finland during the second week of January, when H1N1 subtype influenza A viruses also spread in some garrisons in north and southwest of Finland. Eight amino acid substitutions in HA1 differentiated the first influenza B virus isolate (B/Finland/772/01) from the vaccine strain of autumn 2000 (B/Yamanashi/166/98). A difference of five substitutions was recorded in comparison with B/Sichuan/379/99, the vaccine strain of the southern hemisphere in spring 2001. Fig. 2 illustrates genetic relationships of HA1 in a collection of influenza B virus strains. In accordance with these evolutionary changes, 77% of volunteers vaccinated in autumn 2000 (n=47) exhibited in their postvaccination sera a protective antibody titre (40) against the recent virus isolate B/Finland/772/01, while a rate of 94% was recorded against the virus variant from the 1999/2000 season (B/Finland/767/00).

By week 2/2001, influenza A virus was detected in nine localities and influenza B virus in two localities (Fig. 3) . Towards the end of January, findings of influenza B became more prevalent, but influenza A still predominated. By week 4/2001, influenza A virus was detected in seventeen localities and influenza B virus in four localities (Fig. 4).

At the beginning of February, influenza activity increased markedly in the general community of Finland. In week 5/2001, absenteeism from schools and nursery schools and consultations for ARI at the Health Centres (general practitioners) in Helsinki reached a level indicative of moderate epidemic activity. Laboratory findings still indicated circulation of influenza A/H1N1 with sporadic occurrence of influenza B. In the middle of February, the emphasis of the epidemic activity had switched from garrisons to schools. In the end of February, the incidence of influenza-like illness was assumed to be decreasing in the general population. In Helsinki the absenteeism and consultation rates turned to a decrease in week 8.

The epidemic activity of A/H1N1 viruses decreased further during March and the outbreaks were practically over by the end of the month; on the other hand, findings of influenza B increased.

At the beginning of April, sporadic cases of influenza A and B were still detected, most in the southwest of Finland. Influenza B viruses continued to be isolated up to May.

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
2000 38-40 0 0 0 0 2001 1 3 0 0 3
41 0 0 0 1 2 10 0 12 5
42 0 0 0 0 3 25 0 7 7
43 0 0 0 0 4 64 0 27 5
44 0 0 1 0 5 70 0 12 3
45 1 0 1 0 6 114 0 7 7
46 6 0 1 0 7 100 0 12 5
47 3 0 0 0 8 65 0 2 4
48 5 0 0 0 9 57 0 7 11
49 11 0 1 0 10 41 0 0 7
50 8 0 0 0 11 15 0 0 11
51 10 0 0 0 12 16 0 0 24
52 12 0 0 0 13 18 0 0 12
14 11 0 0 20
15 0 0 0 18
16 0 0 0 17
17 0 0 0 24
18 0 0 0 14
19 0 0 0 11
20 0 0 0 6
21 0 0 0 2
22 0 0 0 3
24 1 0 0 0
26 1 0 0 1

* Not studied for influenzavirus A subtype
** Incomplete data

Fig. 1: Phylogenetic relationships of the HA1 domain of some
influenza A (H1N1) virus strains isolated in 1977-2000.

All lines are proportional to the number of nucleotide changes.
1) Finland/768/00 (epidemic virus in the current season),
2) New Caledonia/20/99 (vaccine-like virus in autumn 2000),
3) Beijing/262/95, 4) Finland/603/98, 5) Bayern/7/95,
6) Finland/75/88, 7) Finland/30/77.

Fig. 2: Phylogenetic relationships of the HA1 domain of some
influenza B virus strains isolated in 1983-2001.

All lines are proportional to the number of nucleotide changes.
1) Finland/772/01 (epidemic virus in the current season),
2) Finland/670/99 (epidemic virus in the 1998-99 season),
3) Sichuan/379/99 (vaccine-like virus in 2001 in the southern hemisphere),
4) Yamanashi/166/98 (vaccine-like virus in autumn 2000 in the northern hemisphere),
5) Finland/450/97, 6) Harbin/7/94, 7) Finland/268/93, 8) USSR/100/83

Fig. 3: Localities for virologically confirmed
influenza A and B by week 2/2001.

Fig. 4: Localities for virologically confirmed
influenza A and B by week 4/2001.

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
2000 38 2.1% 6.9% 2001 1 NA NA
39 1.7% 6.8% 2 1.2% 6.8%
40 2.1% 6.2% 3 2.0% 9.2%
41 1.8% 7.4% 4 2.3% 10.6%
42 2.0% 7.7% 5 3.8% 15.8%
43 1.9% 8.7% 6 5.5% 19.0%
44 1.6% 6.4% 7 NA 18.1%
45 2.4% 8.9% 8 NA 11.3%
46 2.2% 10.3% 9 4.6% 10.0%
47 2.3% 9.5% 10 4.0% 10.5%
48 2.4% 9.5% 11 3.5% 10.2%
49 2.1% 9.8% 12 2.2% 11.6%
50 2.9% 6.2% 13 2.2% 9.8%
51 NA NA 14 2.4% 13.3%
52 NA NA 15 2.4% 9.4%
16 2.0% 9.1%
17 2.3% 9.1%
18 2.5% 6.2%
19 1.8% 5.7%
20 1.8% 6.1%
21 2.1% 4.8%
22 2.3% 5.3%

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
2000 38 53 NA 2000 1 88 NA
39 59 NA 2 69 NA
40 47 NA 3 89 NA
41 60 NA 4 67 NA
42 54 NA 5 68 NA
43 70 NA 6 90 NA
44 57 NA 7 121 NA
45 50 NA 8 79 NA
46 70 NA 9 89 NA
47 77 NA 10 57 NA
48 69 NA 11 86 NA
49 105 NA 12 103 NA
50 58 NA 13 71 NA
51 58 NA 14 82 NA
52 91 NA 15 74 NA
16 46 NA
17 115 NA
18 70 NA
19 75 NA
20 42 NA
21 87 NA
22 72 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
2000 38 324 * 2001 1 250 **
39 282 * 2 336 *
40 245 * 3 285 *
41 251 * 4 246 *
42 279 * 5 420 *
43 324 * 6 525 *
44 360* 7 499 *
45 307 * 8 392 *
46 404 * 9 416 *
47 352 * 10 273 *
48 397 * 11 163**
49 319 * 12 178 **
50 297 * 13 280*
51 328 ** 14 391*
52 207 ** 15 205*
16 273**
17 91**
18 101**
19 263**
20 168*
21 182*
22 191*

* = combined data from six of the seven areas that participated in the previous years
** = incomplete data; reports missing from one or more of the six areas

reijo.pyhala@ktl.fi July 2, 2001