Updated on June 24, 2003

Influenza A and B viruses were detected from a few sporadic cases in September and early October. An influenza B/HongKong/330/01-like strain, distinct from the B/Sichuan/379/99-like strains (HI test) that circulated during the 2001/2002 epidemic season in Finland, was isolated from a 13 years old girl, who was taken ill in week 46 in East Finland. In regard to the HA1 sequence, the isolate was related to the prototype vaccine strain for the 2002-03 season (Fig. 1), but was even more close to B/HongKong/1351/02. Furthermore, the Finnish isolate proved to be a reassortant virus that, as B/HongKong/1351/2002, exhibited viral neuraminidase characteristic of the recent viruses in the lineage of B/Yamagata/16/88-like viruses. In the end of December, two further sporadic cases of influenza B were detected in East Finland.

First local outbreak was recorded early in January 2003, this time among young military conscripts in North-East Finland. Several cases of influenza B and first case of influenza A in the capital area were detected during week 2. A local outbreak in the general population with more than 50 % school absenteeism, caused by influenzavirus B, was recorded during week 3 in western Central Finland. Intense outbreaks among school children became general in the latter half of January and the number of garrisons where low-intensity outbreaks due to influenza B were detected increased contemporaneously. With three exceptions the influenza B virus isolates analysed during January were closely related to B/HongKong/330/01 in HI tests. The exceptional strains isolated from two localities in South Finland belong to the B/Sichuan/379/99-like viruses that circulated during the previous year.

At the beginning of February, the influenza B virus outbreaks in garrisons and at schools still extended. Among the 35 strains analysed by HI during the first half of February, there were two B/Sichuan/379/99-like strains and 33 B/HongKong/330/01-like strains. In garrisons the outbreaks were practically over and in the general community evidently ceasing in the midterm of February. Among the elderly people laboratory-confirmed infections with influenza B were rather infrequently detected. In the end of February some influenza A/H3 strains were isolated, antigenically related to A/Panama/2007/99.

In March, sporadic cases of influenza A and B continued to be detected. A local outbreak caused by H1 subtype influenza A was recorded in a garrison in the northern half of Finland (week 10) and another local outbreak due to H3 subtype virus in a garrison in Eastern Finland (week 11). During the latter half of March some new epidemic foci of H1 and H3 were recorded while infections with influenza B were infrequent.

During the first half of April, some sporadic cases of influenza A/H1 and A/H3 continued to be detected. During the latter half only H3-subtype strains were recorded. Some A/H1-subtype strains isolated previously were studied for the NA sequence and found to exhibit N2. A number of A/H3 strains gave reduced HI titres with antisera to A/Panama/2007/99 and probably were more closely related to A/Fujian/411/02 antigenically.

In May and June, sporadic cases of influenza A were still recorded in Finland.

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

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

Horizontal lines are proportional to the number of nucleotide changes. The vaccine virus in the 2002-03 season is HK/330/01 (blue). FN/159/02 is a strain isolated in Finland in December 2002 (red).

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.

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

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.

NA = data not available

Total numbers are given for patients treated for acute respiratory infections, tonsillitis acuta, influenza-like illness and pneumonia. Health Centres of four districts in Helsinki (western, central, north-eastern and eastern) are engaging.

NA = data not available

* = incomplete data (reports from three health centres only)