A few sporadic infections with influenza A were detected during the summer months in the northern half of Finland (reported by the Department of Microbiology, University of Oulu). No evidence of epidemic influenza activity in September, October and in the first half of November.
In the end of November, two cases of influenza A were detected at the Department of Virology, University of Turku. The patients were an adult male and a child from the southwestern part of Finland. Further nine cases of influenza A were diagnosed (antigen detection) in the beginning of December (weeks 48-49) , most in the southwestern part of the country, one in the eastern part close to Kuopio and one in Tampere. Besides, first three cases of influenza B were detected (week 49) in Turku. In week 50, scattered low-intensity local outbreaks were detected, most in the southern third of Finland. A more intense epidemic focus was recorded in Turku and surroundings. The northernmost cases of virologically verified infections with influenza A were from Kuusamo (northeast of Oulu). First cases of influenza A in the Finnish Defence Forces were diagnosed in week 51 in the south and northeast; the isolates were subtyped as H3N2 viruses similar to the A/Sydney/5/97 vaccine virus. Up to the end of December 1999, influenza A aetiology was virologically confirmed from more than 20 localities scattered all over the country. Clinical data in Helsinki for week 52 suggested increasing influenza activity.
In the beginning of January 2000, regional or widespread outbreaks with moderate intensity of H3N2 subtype influenza A viruses were assumed based mainly on the geographic distribution of laboratory-confirmed cases and in less extent to clinical data. The outbreaks peaked during the first three weeks of January and turned to fall in week 4.
Two local outbreaks due to influenza virus B were recorded among military conscripts in the southern part of the country, the first in March and the second during the first half of April.
All subtyped strains have been antigenically A/Sydney/5/97(H3N2)-like. Amino acid sequences of HA1 (the variable domain of virus haemagglutinin) of the strains analysed genetically have shown little change compared to the viruses prevalent in Finland in the previous epidemic season (Fig. 1) . In spite of the antigenic similarity, several amino acid differences were detected in HA1 between the epidemic virus and the vaccine virus of autumn 1999 (Fig. 2) .
| Influenzavirus A and B isolates and antigen detections | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| YEAR | WEEK | A* | A(H3N2) | A(H1N1) | B | YEAR | WEEK | A* | A(H3N2) | A(H1N1) | B |
| 1999 | 37 | 0 | 0 | 0 | 0 | 2000 | 1 | 141 | 4 | 0 | 1 |
| 38 | 0 | 0 | 0 | 0 | 2 | 109 | 15 | 0 | 4 | ||
| 39 | 0 | 0 | 0 | 0 | 3 | 104 | 1 | 0 | 1 | ||
| 40 | 0 | 0 | 0 | 0 | 4 | 67 | 4 | 0 | 1 | ||
| 41 | 0 | 0 | 0 | 0 | 5 | 68 | 7 | 0 | 4 | ||
| 42 | 0 | 0 | 0 | 0 | 6 | 47 | 6 | 0 | 0 | ||
| 43 | 0 | 0 | 0 | 0 | 7 | 50 | 1 | 0 | 0 | ||
| 44 | 0 | 0 | 0 | 0 | 8 | 23 | 0 | 0 | 0 | ||
| 45 | 0 | 0 | 0 | 0 | 9 | 12 | 0 | 0 | 0 | ||
| 46 | 0 | 0 | 0 | 0 | 10 | 16 | 0 | 0 | 0 | ||
| 47 | 0 | 0 | 0 | 0 | 11 | 0 | 0 | 0 | 4 | ||
| 48 | 3 | 0 | 0 | 0 | 12 | 2 | 0 | 0 | 0 | ||
| 49 | 8 | 0 | 0 | 3 | 13 | 2 | 0 | 0 | 2 | ||
| 50 | 31 | 0 | 0 | 1 | 14 | 0 | 0 | 0 | 2 | ||
| 51 | 28 | 11 | 0 | 1 | |||||||
| 52 | 119 | 10 | 0 | 2 |
| YEAR | WEEK | SCHOOLS | NURSERY SCHOOLS | YEAR | WEEK | SCHOOLS | NURSERY SCHOOLS |
| 1999 | 38 | 2.2% | 11.7% | 2000 | 1 | C | 6.2% |
| 39 | 2.7% | 8.5% | 2 | 2.2% | 8.4% | ||
| 40 | 2.4% | 8.4% | 3 | 2.3% | 9.1% | ||
| 41 | 2.9% | 9.1% | 4 | 2.4% | 11.9% | ||
| 42 | 1.9% | 8.9% | 5 | 4.3% | 11.8% | ||
| 43 | 2.9% | 7.7% | 6 | 3.7% | 14.9% | ||
| 44 | 2.7% | 8.1% | 7 | 2.2% | 12.1% | ||
| 45 | 2.4% | 10.5% | 8 | NA | 9.0% | ||
| 46 | 2.7% | 10.8% | 9 | 3.0% | 8.4% | ||
| 47 | 2.5% | 8.9% | 10 | 2.7% | 12.2% | ||
| 48 | 2.1% | 9.7% | 11 | 2.6% | 10.4% | ||
| 49 | 2.2% | 10.1% | 12 | 2.3% | 10.7% | ||
| 50 | 2.8% | 8.3% | 13 | 3.1% | 8.8% | ||
| 51 | C | C | |||||
| 52 | C | C |
| YEAR | WEEK | ADULTS | CHILDREN | YEAR | WEEK | ADULTS | CHILDREN |
| 1999 | 38 | 68 | 76 | 2000 | 1 | 100 | 39 |
| 39 | 74 | 42 | 2 | 84 | 41 | ||
| 40 | 58 | 87 | 3 | 79 | 105 | ||
| 41 | 83 | 67 | 4 | 73 | 92 | ||
| 42 | 65 | 52 | 5 | 68 | NA | ||
| 43 | 66 | 61 | 6 | 79 | NA | ||
| 44 | 66 | 54 | 7 | 66 | NA | ||
| 45 | 49 | 58 | 8 | 71 | NA | ||
| 46 | 64 | 34 | 9 | 51 | NA | ||
| 47 | 55 | 90 | 10 | 58 | NA | ||
| 48 | 75 | 93 | 11 | 53 | NA | ||
| 49 | 86 | 50 | 12 | 64 | NA | ||
| 50 | 62 | 81 | 13 | 48 | NA | ||
| 51 | 78 | 34 | |||||
| 52 | 113 | 103 |
| YEAR | WEEK | CASES | YEAR | WEEK | CASES |
| 1999 | 38 | 351 | 2000 | 1 | 524 |
| 39 | 397 | 2 | 533 | ||
| 40 | 422 | 3 | 652 | ||
| 41 | 520 | 4 | 535 | ||
| 42 | 476 | 5 | 535 | ||
| 43 | 501 | 6 | 509 | ||
| 44 | 364 | 7 | 298 | ||
| 45 | 446 | 8 | 307 | ||
| 46 | 373 | 9 | 341 | ||
| 47 | 369 | 10 | 368 | ||
| 48 | 389 | 11 | 272 | ||
| 49 | 344 | 12 | 305 | ||
| 50 | 414 | 13 | 318 | ||
| 51 | 339 | ||||
| 52 | 459 |
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 | Five amino acid substitutions (red, spacefilled) in the HA1 domain of virus haemagglutinin differentiated A/Finland/715/00 (isolated in January 2000) from the consensus sequences of the two virus sublineages that were detected during the 1998/99 epidemic season in Finland (26 strains altogether were analysed). Residues of the receptor binding site (RBS) are shown in green. |
 | 13 amino acid substitutions (red, spacefilled) in HA1 differentiated the epidemic virus A/Finland/715/00 and the vaccine virus of autumn 1999 (A/Sydney/5/97). Four of the differentiating residues (137, 226, 192, 194) were within or in the vicinity of RBS. |
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