The 2019–2020 influenza season was mild, and three viruses circulated: influenza A(H3N2), influenza A(H1N1)pdm09, and B/Victoria. The epidemic started in early December (week 49) and continued at low intensity for 16 weeks, with a peak in week 10. Following the implementation of measures to reduce the spread of COVID-19 in weeks 11-12, the number of cases dropped quickly and the epidemic came to an end in week 13 despite an increase in testing for influenza during this period. In the past 20 years, no season has gone from peak activity to end-of-epidemic as fast.
During the 2019–2020 season, 7,941 laboratory-confirmed cases were reported, of which the majority were influenza A (69 percent). Of the subtyped influenza A-samples, 64 percent were A(H3N2) and 36 percent were A(H1N1)pdm09. Samples of influenza B assigned to a lineage were exclusively B/Victoria.
The majority of cases overall were among individuals aged 0–39 years. However, the majority of influenza A cases were in individuals aged 40 years and older. The youngest children aged 0 to 4 years had the highest cumulative (146 per 100,000 population) and weekly incidence of influenza, followed by individuals aged 65 and older (105 per 100,000 population). The median age for individuals with laboratory-confirmed influenza A and B were 51 and 24 years, respectively.
Web searches reflected a low level of influenza activity during the season. Phone calls to the medical advice line 1177 indicated a medium level of activity during weeks 6 to 10. Both systems showed a sharp drop in activity towards the end of the season, similarly to the laboratory data.
During the season, 175 patients with influenza were reported as having received intensive care across the country, which is fewer than during the previous four seasons. The majority of patients (80 percent) had influenza A, with a median age of 61 years. The median age for patients with influenza B was 15, which is unusually low, and 20 out of 34 patients with influenza B were under 18 years of age. Samples were subtyped for 23 percent of patients with influenza A, and the results showed that 75 percent had influenza A(H1N1)pdm09. Of all reported cases in intensive care, 62 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 of patients in intensive care was most similar to the 2016–2017 season, which was dominated by influenza A(H1N1)pdm09.
During the 2019–2020 influenza epidemic, influenza-related excess mortality was measured during weeks 7 to 12 (FluMoMo model) in the age group 65 and older, followed by excess mortality due to COVID-19 from week 13 (see weekly reports for covid-19 [in Swedish]). Among patients who received a laboratory-confirmed influenza diagnosis, 3 percent died within 30 days, which is similar to previous mild seasons; the percentage who died has ranged from 3 to 5.5 percent during the previous four seasons. In total, 89 percent of deaths were among people aged 65 years and older.
In sentinel surveillance, influenza was detected in 18 percent of all samples. Of these, 57 percent were influenza A and 43 percent were influenza B. As in the laboratory-based reporting, both influenza A(H1N1)pdm09 and influenza A(H3N2) circulated from the start of the epidemic (week 48) until week 5. From week 6 onwards, influenza A(H3N2) dominated. Influenza B/Victoria circulated during the whole season. Cumulatively, the distribution was 33 percent A(H1N1)pdm09, 23 percent A(H3N2), and 43 percent B/Victoria. Vaccine break-through infections were detected in 7 percent of patients with influenza A(H1N1)pdm09 (median age 58 years) and 7 percent of patients with influenza A(H3N2) (median age 82 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 2019–2020 season, the vaccination effect was 48 to 75 percent for influenza A(H1N1)pdm09, <0 to 57 percent for influenza A(H3N2), and 62 to 83 percent for influenza B (1).
During the 2019–2020 season, the average vaccination coverage among people 65 years and older was 53 percent, compared to 52 percent in the previous season. Due to delays in vaccine production, the 2019–2020 vaccination campaign started two weeks later than usual (19 Nov, week 47). The shortened period of vaccination does not seem to have affected the coverage rate among people 65 years and older.
Genetic characterisation of a subset of viruses collected through sentinel sampling and from laboratories around Sweden showed that of the characterised influenza A(H3N2) viruses, the majority (72 percent) belonged to one of three groups within the 3C.2a1b subgroup, while the remaining 28 percent belonged to subgroup 3C.3a. Of the characterised influenza A(H1N1)pdm09 viruses, the majority (92 percent) belonged to the 6B.1A5A subgroup, with 30 percent of these viruses having the amino acid substitution N156K, and 45 percent having the D187A+Q189E substitutions. The remaining A(H1N1)pdm09 viruses belonged to subgroup 6B.1A5B or 6B.1A7. Of the characterised influenza B/Victoria viruses, the vast majority (99 percent) belonged to subgroup 1Adel162-164B.
Three influenza A(H1N1)pdm09 viruses with the H275Y mutation known to confer clinical resistance to oseltamivir (Tamiflu/Ebilfumin) were detected. In one of these three viruses, a minority population with the N295S substitution, associated with reduced or highly reduced inhibition to oseltamivir, was present, along with the H275Y substitution, which was present in the vast majority of the virus population. The remaining 289 influenza A and B viruses for which the NA gene was sequenced did not carry any amino acid substitution known to confer reduced or highly reduced inhibition to the neuraminidase inhibitors oseltamivir (Tamiflu/Ebilfumin) or zanamivir (Relenza). An additional 94 A(H1N1)pdm09 viruses were analysed exclusively for the H275Y substitution, and none of these viruses carried this substitution.