The 2021–2022 influenza season was unusual with two epidemic waves separated by a period of very low influenza activity. This period coincided with the first, intense wave of infection with the Omicron variant of SARS-CoV-2 in Sweden. Subtyping of samples within the laboratory reporting and sentinel surveillance showed that influenza A(H3N2) dominated the season. Altogether, the first wave is considered to have been at a medium level of activity and the second wave at a low level. In total, just over 13,000 laboratory-confirmed influenza cases were reported in the 2021–2022 season.
The spread and surveillance of influenza have both been affected by the COVID-19 pandemic since March 2020, particularly in terms of health care-seeking behaviour and frequency of testing. Interventions and behaviour changes to reduce transmission of COVID-19 have also likely had an effect on the spread of influenza and other respiratory pathogens. Testing for influenza has increased and reached populations with mild disease that previously had a low coverage of testing, primarily due to the widespread availability and use of multiplex PCR analyses for SARS-CoV-2, influenza, and RSV. The increased testing means that a higher proportion of people with an influenza infection have been included in laboratory-based reporting than previously, particularly among older children and adults under 65 years of age. In comparison with previous seasons, weekly incidence was lower than seasons before the pandemic among people aged 65 years and older, but was considerably higher among those aged 5–14 years and 15–39 years.
The spread of influenza was very low from the spring of 2020 and throughout the 2020–2021 season, both in Sweden and globally. During the summer of 2021, sporadic influenza cases were reported and the autumn of 2021 resembled the start of a regular season, with an increase in cases beginning in November. Several systems showed increasing influenza activity at levels that were normal for that time of year. The first influenza wave started in the beginning of December, and the number of cases reached its first peak in week 50. At the same time, the Omicron variant of SARS-CoV-2 was introduced to Sweden, and as the number of COVID-19 cases increased in the beginning of January, the number of influenza cases decreased drastically. The usual increase after the end-of-year holidays did not occur, and an unusually low number of influenza cases was reported during February and March. A second influenza wave started in the beginning of April, and a lower peak was reached in week 16. Influenza continued to circulate into the summer weeks and reached inter-seasonal levels in week 25 of 2022.
There were marked geographical differences in the spread of influenza during the season. In the initial wave, the middle parts of Sweden (Svealand) had higher incidence in comparison to the southern (Götaland) and the northern (Norrland) parts of the country. During the second wave, the incidence was higher in the southern (Götaland) part and the northern (Norrland) part of the country, but was overall lower than in the first wave. These trends were also seen in web search data.
During the season, 110 patients with influenza were reported as having received intensive care, which was lower than the 2016–2017 to 2019–2020 seasons, during which an average of 311 patients were reported. Of the patients in intensive care, 76 patients were admitted during the first wave of the epidemic and 34 patients during the second wave. Excess mortality was noted during week 1, 2022, which could be related to the first wave of the influenza season and to COVID-19. No excess mortality was seen in the second wave. Among patients who received a laboratory-confirmed influenza diagnosis, 2 percent died within 30 days, which was lower compared to the previous five seasons (range 3–5.5 percent). Within the season, most deaths among laboratory-confirmed cases occurred during the first wave of the season (72 percent), with the highest number of deaths occurring in week 52, 2021.
Genotypic and phenotypic characterisation was performed on viruses from a selection of influenza-positive samples collected in sentinel sampling and from influenza-positive samples sent from laboratories around Sweden. In total, 158 A(H3N2), 6 A(H1N1)pdm09, and 11 B/Victoria viruses were genetically characterised with respect to the hemagglutinin (HA) gene. The vast majority of the investigated viruses belonged to genetic groups that in antigenic analyses were poorly recognised by ferret antisera raised against the vaccine viruses included in the influenza vaccine for the northern hemisphere 2021–2022. Genotypically, no evidence of reduced susceptibility to the neuraminidase (NA) inhibitors oseltamivir or zanamivir was seen in these 175 viruses. In addition, all 11 viruses [10 A(H3N2) viruses and 1 A(H1N1)pdm09 virus] that were phenotypically analysed with respect to sensitivity to oseltamivir and zanamivir were shown to be sensitive to both substances. Analysis of the PA-gene identified no genetic markers associated with reduced sensitivity to baloxavir in any of the 167 analysed viruses [151 A(H3N2), 6 influenza A(H1N1)pdm09, and 10 B/Victoria]. The amantadine resistance mutation S31N in the matrix protein was present in all 170 analysed influenza A viruses.
The Public Health Agency of Sweden (PHAS) participates in the European Influenza Monitoring Vaccine Effectiveness (I-MOVE) network with data from the sentinel system. In the end-of-season report, the overall vaccination effect was estimated to be 75 percent for influenza A(H1N1)pdm09 (all ages) and 31 percent for influenza A(H3N2) (all ages), but the vaccination effect for influenza A(H3N2) was not significant in the age group 65 years and older (1).
During the 2021–2022 season, the average vaccination coverage among people 65 years and older was 70 percent, which was five percentage points higher than the previous season. The increase was likely due to a combination of increased interest in influenza vaccination during the pandemic and the concomitant administration of influenza vaccines with COVID-19 booster vaccines for those aged 65 years or older.