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 2018–2019 influenza season was entirely dominated (99 percent) by influenza A. The epidemic started in week 50 (early December) and continued for 21 weeks until it ended in week 18. As in previous seasons, the number of laboratory-confirmed influenza cases increased throughout the month of December and then reached a plateau during the holidays at the end of the year. A sharp increase in the number of cases was seen in week 4 leading to the peak in week 6, when 1,296 cases were reported (see Figure 2). Only a limited number of cases of influenza B were reported during the season.
The majority of the cases (>99%) during the peak (week 6) were influenza A, and based on the samples that were subtyped influenza A(H1N1)pdm09 was the dominating subtype during the season overall. After the peak in week 6, however, the proportion of influenza A(H3N2) started to increase, and in week 10 the dominating subtype of influenza A shifted from A(H1N1)pdm09 to A(H3N2). Influenza A most recently dominated the season in Sweden during 2016–2017 and 2015–2016, when influenza A(H3N2) and influenza A(H1N1)pdm09 circulated, respectively.
In comparison with recent seasons, the number of reported laboratory-confirmed influenza cases during the current season (13,757) was similar to the number of cases in 2016–2017, but lower than the intense season of 2017–2018 (Table 3). During the peak week (week 6), the number of cases (1,296) was also similar to the peak week of 2016–2017 (see Figure 2). During the previous season, 2017–2018, there was a significant increase of samples taken for influenza diagnosis; a high number of samples taken were also reported during the 2018–2019 season. Overall, 16.5 percent of the samples taken were positive for influenza, which is lower compared to previous seasons, in part reflecting the lower intensity of the season (Table 3, Figure 3).
An analysis of the level of care for reported influenza cases over the past four seasons has shown that the proportion of cases from outpatient settings has increased, whereas the proportion of cases from inpatient settings has decreased slightly (Figure 4). Faster and increased availability of virological analysis of samples has most likely contributed to the increase of both sampling and the reported number cases from outpatient care settings. During the last few years, diagnostics have also been implemented at hospitals that did not previously have microbiological laboratories, and this change has thus facilitated fast “around-the-clock” analysis for influenza in more hospitals than ever before. The increased sampling might also contribute to the low percentage of positive samples of influenza this current season.
Figure 2. Total number of laboratory-confirmed cases of influenza (all types) per week and the dominating influenza type(s) per season, 2015–2019.

Figure 3. Percentage of samples testing positive for influenza, per week, 2015–2019.

Figure 4. Number of laboratory-confirmed influenza cases stratified by level of care at sampling, 2015–2016 to 2018–2019.

Level of care could not be determined for a small number of cases, which have been excluded.
Viral distribution
During the 2018–2019 season, 13,757 laboratory confirmed cases were reported, almost exclusively influenza A (>99 percent). Very few cases of influenza B were reported (93 cases). Of the subtyped influenza A-positive samples, 63% were A(H1N1)pdm09 and 37% were A(H3N2). Samples of influenza B assigned to a lineage were 50% B/Yamagata and 40% B/Victoria (see also the Subtyping and lineage determination section). Table 3 summarises the laboratory reporting results over the last five seasons, including the number of analysed samples and the proportion of positive samples as well as the total samples positive by type, subtype, and lineage.
Table 3. Laboratory results of samples analysed and reported through the laboratory reporting system over the last five seasons.
| 2015–2016 | 2016–2017 | 2017–2018 | 2018–2019 |
Analysed samples |
48,135 |
68,241 |
88,837 |
83,325 |
Proportion positive samples |
19% |
19% |
23% |
17% |
Total positive for influenza A |
6,730 |
12,361 |
7,406 |
13,664 |
A(H1N1)pdm09 * |
2,031 |
14 |
294 |
1,228 |
A(H3N2) |
112 |
2,061 |
556 |
721 |
A, not subtyped** |
4,587 |
10,286 |
6,556 |
11,715 |
Total positive for influenza B |
2,409 |
708 |
13,280 |
93 |
B/Victoria lineage*** |
65 |
11 |
0 |
4 |
B/Yamagata lineage*** |
6 |
30 |
144 |
4 |
B, not typed to any lineage |
2,338 |
667 |
13,136 |
85 |
* Not typed as N1, but classified as A(H1N1)pdm09 based on H1 typing.
** For the period 2013–2015, influenza A cases not subtyped but A(H1N1)pdm09 negative were considered to be influenza A(H3N2) cases. Data on subtype for the 2015–2016 and 2016–2017 seasons are from the Public Health Agency and the three regional laboratories that regularly perform subtyping.
*** All typing for lineage was performed at the Public Health Agency laboratory.
Age and sex distribution
During the 2018–2019 season, the majority (69 percent) of the laboratory-confirmed influenza cases were among individuals aged 40 years and older (Table 4). Young children have been affected by influenza to a greater extent this current season in comparison to previous ones (Figure 5). Because influenza A(H1N1)pdm09 has not circulated since 2015–2016, the youngest cohorts of children had not previously been infected by the virus. Children infected by influenza, however, usually recover at home and do not need to seek medical care for influenza. The age distribution of the laboratory-confirmed cases thus only reflects the burden in terms of children who have sought care for their influenza infection.
The age distributions in 2015–2016 and 2018–2019 were similar, with individuals aged 65 and older and children being the most affected (see Figure 6).
Those aged 65 years and over had the highest cumulative incidence with 300 cases per 100,000 individuals, followed by children aged 0–4 years with a cumulative incidence of 203 cases per 100,000 individuals (see Table 5). Since the pandemic in 2009, the age distribution has shifted and an increasing proportion of the elderly have been affected by influenza A(H1N1)pdm09. In week 10, as the dominance shifted from A(H1N1)pdm09 to A(H3N2), the incidence among the elderly was elevated for a long period and decreased much slower compared to younger age groups. The cumulative incidence for individuals aged 65 and older was lower compared to the two previous seasons (2016–2017 and 2017–2018) when influenza A(H3N2) and influenza B/Yamagata circulated, respectively (Figure 7).
The median age for individuals with laboratory-confirmed influenza A was 60 years, which was higher than 2015–2016, for which the median age was 48 years (see Table 6). Significantly more women (52%) than men (48%) had laboratory-confirmed influenza (p < 0.001).
Few cases (
Figure 5. Weekly incidence of influenza (dominating type) for children aged 0–4 years in Sweden, 2014–2019 seasons.

Figure 6. Weekly incidence of influenza A per age group in Sweden, 2018–2019 season.

Figure 7. Weekly incidence of influenza (dominating type) for individuals aged 65 and older in Sweden, 2014–2019 seasons.

Table 4. Incidence (per 100,000 population) and percentage of cases, by age group, of laboratory-confirmed influenza A cases for seasons when influenza A(H1N1)pdm09 has circulated (2015–2016, and 2018–2019).
| 2015–2016 | 2018–2019 |
Age group |
Incidence |
Percentage |
Incidence |
Percentage |
0–4 |
130.3 |
11% |
203.3 |
9% |
5–14 |
26.2 |
4% |
55.4 |
5% |
15–39 |
53.3 |
25% |
73.4 |
17% |
40–64 |
71.2 |
33% |
103.7 |
24% |
65+ |
92.9 |
27% |
300.1 |
45% |
Total |
68.2 |
100% |
134.8 |
100% |
Note: Data do not include sentinel cases or cases where the age is unknown. |
Table 5. Number and incidence (per 100,000 population) of laboratory-confirmed influenza cases per age group and influenza type, Sweden, 2018–2019.
Age group | Population ‡ | Influenza A | Influenza B |
|
|
Cases |
Incidence |
Cases |
Incidence |
0–4 |
604,498 |
1,229 |
203.3 |
9 |
1.5 |
5–14 |
1,215,231 |
673 |
55.4 |
10 |
0.8 |
15–39 |
3,228,688 |
2371 |
73.4 |
28 |
0.9 |
40–64 |
3,146,057 |
3,261 |
103.7 |
14 |
0.4 |
65–69 |
549,311 |
884 |
160.9 |
7 |
1.3 |
70–74 |
563,648 |
1,267 |
224.8 |
6 |
1.1 |
75–79 |
400,619 |
1,168 |
291.5 |
7 |
1.7 |
80–84 |
261,527 |
1,135 |
434.0 |
3 |
1.1 |
85–89 |
162,303 |
972 |
598.9 |
5 |
3.1 |
90–94 |
76,247 |
539 |
706.9 |
3 |
3.9 |
≥95 |
22,056 |
145 |
657.4 |
0 |
0.0 |
Total |
10,230,185 |
13,644 |
405.3 |
92 |
15.2 |
*The table does not include sentinel cases or cases where age is unknown.
‡ Population on December 31, 2018. Source: Statistics Sweden, Statistikdatabasen.
Table 6. Median age (in years) of laboratory-confirmed influenza cases by type and season.
| 2015–2016 | 2016–2017 | 2017–2018 | 2018–2019 |
Influenza A |
48 |
74 |
70 |
60 |
Influenza B |
33 |
58 |
68 |
37 |
Geographic distribution
During the initial weeks of the epidemic, the northern parts of Sweden (Norrland) had higher incidence in comparison to the middle (Svealand) and southern (Götaland) parts of the country (Figures 8 and 9). During the Christmas and New Year holidays (weeks 52), the incidence reached a first peak for the northern parts of the country (Norrland), and then a second peak was seen in week 7. The incidence in the middle (Svealand) and southern (Götaland) parts of the country peaked in weeks 5 and 6, respectively. The percentage of samples positive for influenza showed a similar geographic trend with peaks in weeks 1–2 and 7 for the northern part of the country (Norrland), weeks 6–7 for the middle (Svealand) and southern (Götaland) parts of the country.
Altogether, the northern parts of the country (Norrland) had the highest cumulative incidence of laboratory-confirmed cases per 100,000 individuals this season, with 162 cases, followed by the middle parts (Svealand) with 149 cases and the southern parts (Götaland) with 116 cases. The greatest numbers of cases were reported from the largest urban areas (Stockholm, Västra Götaland, and Skåne). However, in relation to population size, some smaller counties had a higher cumulative incidence. The number of laboratory-confirmed cases might be affected by healthcare-seeking behaviour as well as differences in sampling in the various regions; thus, no direct conclusions can be drawn regarding actual influenza activity using the measured incidence.
Figure 8. Weekly incidence of laboratory-confirmed influenza per 100,000 population and county from week 40, 2018, to week 20, 2019. Note: The colour scale indicates the incidence. The scale has been changed from the previous season.

Figure 9. Weekly incidence of laboratory-confirmed influenza per 100,000 population and county from week 40, 2018, to week 20, 2019.
