This report describes the 2016–2017 influenza season, which was dominated by influenza A(H3N2). The season came to a first, intense peak in the northern part of the country during the Christmas holidays, followed by a second peak in the south in February. The season was intensive and particularly severe for those aged 65 years and over, with high mortality.
The Public Health Agency of Sweden has prepared this report for the World Health Organization (WHO) as part of the agency's function as a National Influenza Centre (NIC).
The 2016–2017 season was dominated by influenza A(H3N2) that came in two waves. Overall, 95% of cases were influenza A, and of those subtyped more than 99% were A(H3N2). The season was intensive with more laboratory-confirmed cases compared with previous seasons. At first, the season came to an intense peak in the northern part of the country, followed by a second peak in the south. The median age of influenza A cases was 73 years, reflecting the dominance of A(H3N2). Like the 2014–2015 season, which was also dominated by A(H3N2), the majority (65%) of the influenza cases this season were among people 65 years and older.
The web-search based system Webbsök signalled the start of this early season in week 45, which coincided with the start of the vaccination campaign in Sweden. The epidemic threshold was crossed in other surveillance systems in week 47 or 48. The highest incidences were seen in the north (Norrland) and middle parts (Svealand) of the country at the beginning of the season and at the peak (week 52), whereas the highest incidences during the second peak (week 8) were seen in the southern parts of the country (Götaland). The number of laboratory-confirmed influenza A cases was at a low level from week 10 onwards. Influenza B dominated from week 17 to the end of the epidemic in week 20, but at a very low level.
The average vaccination coverage among people 65 years and older was 49% and has been between 49% and 50% during the last three seasons. Data on vaccination coverage is collected in different ways, and small differences from year to year can occur due to methodological differences. The coverage rate is highest among people 75 years and older (55%). This is encouraging because increasing age raises the risk of severe influenza infections. There is great variation among county councils/regions, moreover vaccination coverage increased in 9 out of 21 county councils. It is estimated that 5–10% of people under the age of 65 belong to a risk group, but the vaccination coverage in this age group is only 2%. Only 32% of those with known vaccination status who belonged to a risk group and needed intensive care during this season were vaccinated.
The sales pattern for antiviral medications, used for the treatment of severe influenza disease and as prophylaxis, in Sweden follows laboratory data and shows the same two peaks (week 52 in 2016 and week 8 in 2017). The number of packages sold increased significantly this season, mainly through increased requisitions in healthcare. This is probably due in part to communication efforts regarding low vaccine effectiveness and the intensity of the season in the older age group, leading to an increased use of antivirals. No resistance to neuraminidase inhibitors was detected among the 266 virus strains characterized.
Compared with the previous season, fewer patients (259 vs. 362) required intensive care this season, and the vast majority (69%) were aged 65 years or older. The age distribution among intensive care patients is different compared with the previous
season, which was dominated by influenza A(H1N1)pdm09, as older individuals are more affected by influenza A(H3N2).
Influenza infection is often a contributing factor to deaths among elderly individuals during seasons dominated by influenza A(H3N2). An analysis of the 734 deaths that occurred within 30 days of a laboratory-confirmed influenza diagnosis, showed that 95% of deaths were among those aged 65 years and over, followed by those aged 40-64 years (4.8%). Of those aged 65 years and over who had been diagnosed with influenza A, 8.5% had died within 30 days. The proportion of deceased cases increased with increasing age and varied between 0.05% for individuals under age 40 years and 18% for those between 90 and 94 years. Excess mortality modelling also showed a high mortality rate among those aged 65 years and over.
A selection of 240 strains collected through sentinel sampling and from laboratories in Sweden were genetically characterized. Characterization showed that all influenza A(H1N1)pdm09, A(H3N2), and B strains belonged to genetic groups where most strains have been found to be antigenically similar to the vaccine strains in the trivalent (for A and B/Victoria strain) and quadrivalent vaccines (for all strains including B/Yamagata lineage). Vaccination breakthrough infections were detected in 24% of patients sampled within the sentinel sampling system, with a median age of 74 years. Vaccination effectiveness depends on various factors, such as age, immune system function, and time between vaccination and disease. In addition, vaccine effectiveness varies due to the degree of matching between circulating and vaccine strains. The proportion of A(H3N2) vaccination breakthroughs reported within the sentinel system rose at the same time as the Stockholm County Council reported decreasing vaccination effectiveness (28%). The Public Health Agency's analysis of circulating influenza strains found that the proportion of vaccine breakthroughs rose as the proportion of circulating A(H3N2) strains with the T135K mutation increased.