All laboratory-confirmed cases of influenza have statutory reporting requirements (as of Dec 1, 2015), but subtyping is not required. Denominator data (the total number of samples analysed) are reported voluntarily via e-mail. Samples analysed for influenza at the laboratories in Sweden have historically primarily been done in hospital settings, but the proportion of cases reported from outpatient settings has increased in recent years.
The 2020–2021 influenza season was unusually mild, and only 29 sporadic cases of influenza were reported (see Figure 2). The number of cases stayed below the threshold value for epidemic start throughout the period from week 40, 2020, until week 20, 2021. Due to the ongoing COVID-19 pandemic and restrictions implemented both in Sweden and elsewhere, the spread of influenza virus has drastically decreased.
Of the 29 cases reported, 19 cases were influenza B and 10 were influenza A. Of the influenza B cases, six were typed for lineage and all were influenza B/Victoria. Of the seven subtyped influenza A cases, all were influenza A(H3N2).
The median age of cases should be interpreted with caution because the number of reported influenza cases was low overall and by type. The overall median age for laboratory-confirmed influenza was 35 years. For influenza A and B the median age was 32 years and 54 years, respectively (see Table 2). More men (59 percent) than women (41 percent) were reported with influenza, but the difference was not statistically significant.
Although the number of reported cases was unusually low, the number of analysed samples (approximately 175,000) was higher in comparison to the average number of samples analysed during the last four seasons (79,000). Figure 3 shows the number of samples analysed per week and season. Table 3 summarises the laboratory reporting results over the last six seasons, including the number of analysed samples and the proportion of positive samples as well as the total number of samples positive by type.
Table 2. Median age (in years) of laboratory-confirmed influenza cases by type and season with dominant type listed.
Table showing median age (in years) of laboratory-confirmed influenza cases by type and season, five seasons.
Indicator | 2016–2017 | 2017–2018 | 2018–2019 | 2019–2020 | 2020–2021 |
Dominant type |
A(H3N2) |
B/Yamagata |
A(H1N1)pdm09 |
Mixed season |
- |
Median age influenza A |
73 |
70 |
60 |
51 |
32(a) |
Median age influenza B |
58 |
68 |
37 |
24 |
54(a) |
(a) The median age should be interpreted with caution because the number of reported influenza cases was low.
Figure 2. Total number of laboratory-confirmed cases of influenza (all types) per week and the dominating influenza type(s) per season, 2016–2017 to 2020–2021.
Figure 3. Number of samples analysed per week, 2016–2017 to 2020–2021.
Table 3. Laboratory results of samples analysed and reported through the laboratory reporting system over the last five seasons.
Table showing the number of samples analysed, proportion positive and total positive for influenza by type and subtype or lineage, five seasons.
Indicator | 2016–2017 | 2017–2018 | 2018–2019 | 2019–2020 | 2020–2021 |
Analysed samples |
68,241 |
88,837 |
83,325 |
75,819 |
175,048 |
Proportion positive samples |
19% |
23% |
17% |
10% |
0,02% |
Total positive for influenza A |
12,361 |
7,406 |
13,664 |
5,441 |
10 |
Total positive for influenza B |
708 |
13,280 |
93 |
2,500 |
19 |