The 2018–2019 season in Sweden was dominated by influenza A and reached a moderate level of intensity. According to several surveillance systems, the season was less intense than the five previous seasons. The epidemic started in the second week of December (week 50) and reached its peak in mid-February (week 6). The laboratories in Sweden reported 13,757 influenza cases during the season, which was fewer cases than in the previous season. Ninety-nine percent of these were influenza A. Influenza A(H1N1)pdm09 dominated until week 10, when dominance shifted to influenza A(H3N2). The distribution within subtyped laboratory samples was 63 percent A(H1N1)pdm09 and 37 percent A(H3N2).
At the national level, the number of reported cases increased after the epidemic started in December and then reached a plateau around the Christmas and New Year holidays. A similar trend is seen most seasons. During the initial weeks of the influenza epidemic, the number of cases primarily increased in northern Sweden, where a pronounced peak was seen in week 52 and another peak in week 7. The incidence in the middle and southern parts of Sweden was highest during weeks 5 and 6, respectively.
Both phone calls to the medical advice line 1177 and web searches reflected a moderate level of influenza activity during the season. According to phone call data, many parents called about fever among children this season, particularly children 0–4 years of age. The proportion of calls about fever in this age group was significantly higher than the previous five seasons. The incidence of laboratory-confirmed cases was also high in this age group compared to previous seasons. Because influenza A(H1N1)pdm09 has not circulated to a greater extent in Sweden since the 2015–2016 season, many younger children who had not yet been infected with this type of influenza were affected this season.
Fewer people in the age group 65 years and older fell ill during the season than during the previous two seasons. Prolonged influenza activity was seen in this age group towards the end of the season, as influenza A(H3N2) began to dominate the season. No influenza-related excess mortality was seen among people 65 years and older, although a small peak in all-cause excess mortality was seen in week 1, 2019, in northern Sweden. Because the season was dominated by A(H1N1) pdm09 and reached only a moderate intensity level, mortality was lower than during a typical A(H3N2) season, as well as the 2017–2018 season, which was dominated by influenza B/Yamagata. Among patients who received a laboratory-confirmed flu diagnosis, 505 (3.8 percent) had died within 30 days. In total, 88 percent of deaths were among people aged 65 years and older. The proportion of patients who died increased with increasing age.
During the 2018–2019 season, the average vaccination coverage among people 65 years and older was 52 percent, which was higher than the previous seven seasons. Coverage increased in all age groups despite a vaccine shortage that emerged at the end of November 2018. During the vaccine shortage, the Public Health Authority published updated recommendations regarding prioritization of and among risk groups (1). In January 2019, additional doses were delivered to Sweden, but by then efforts to vaccinate and interest in vaccination had decreased.
During the season, 359 patients with influenza were reported as having received intensive care across the country, which is fewer than during the previous season. Nearly all patients (99.5%) had influenza A. Samples were subtyped for only 11 percent of patients with influenza A and results showed that 85 percent had influenza A(H1N1)pdm09. The greatest number of patients were admitted to intensive care during week 7 (36 patients), which is the week after the laboratory-confirmed cases of influenza peaked. A large number of patients (34 patients) were also admitted to intensive care in week 52 at the start of the seasonal epidemic.
Of all reported cases, 74 percent were in a risk group for severe influenza illness, either due to age (65 years and older) or due to one or more medical risk factors. The age distribution among intensive care patients under 65 was similar to the 2015–2016 season, which was dominated by influenza A(H1N1)pdm09, but a greater proportion of patients overall were people 65 years and older. Of those who belonged to a risk group and needed intensive care during this season, only 29 percent with known vaccination status were vaccinated.
Characterisation of the viral strains collected through sentinel sampling and from laboratories around Sweden showed antigenic similarity to the A(H1N1)pdm09 vaccine strain. The majority of the A(H3N2) strains showed antigenic similarity to cell-cultured but poor similarity to egg-cultured vaccine strains. For the upcoming season, the A(H3N2) and A(H1N1)pdm09 components of the vaccine have been changed. Only a few influenza B strains have been characterized. Of these, the B/Yamagata strains were antigenically similar to the vaccine strain in the tetravalent vaccine (but not included in the trivalent vaccine) and most of the B/Victoria strains belonged to genetic group 1Adel162–164, which has poor antigenic similarity to the vaccine strain. None of the 260 strains analysed had any mutations associated with resistance to neuraminidase inhibitors.
In sentinel surveillance, influenza was detected in 31 percent of all samples. Influenza A was detected in 99 percent of these and influenza B in 1 percent. As in the laboratory-based reporting, the start of the season was dominated by A(H1N1)pdm09, but in week 10 the proportion of A(H3N2) increased. Cumulatively, the distribution was 76 percent A(H1N1)pdm09 and 24 percent A(H3N2). Vaccine break-through infections were detected in 5 percent of patients with A(H1N1)pdm09 (median age 64 years) and 11 percent of patients with A(H3N2) (median age 60 years). The Public Health Agency participates in the European Influenza Monitoring Vaccine Effectiveness (I-MOVE) network with data from Swedish sentinel sampling. In the interim report for the 2018–2019 season, the vaccination effect was good for A(H1N1)pdm09 (45 to 71 percent) and low for A(H3N2) (<0 to 24 percent). There were too few influenza B strains to determine the vaccination effect.