Mid-season real-time estimates of seasonal influenza vaccine effectiveness in persons 65 years and older in register-based surveillance, Stockholm County, Sweden, and Finland, January 2017

Abstract

Systems for register-based monitoring of vaccine effectiveness (VE) against laboratory-confirmed influenza (LCI) in real time were set up in Stockholm County, Sweden, and Finland, before start of the 2016/17 influenza season, using population-based cohort studies. Both in Stockholm and Finland, an early epidemic of influenza A(H3N2) peaked in week 52, 2016. Already during weeks 48 to 50, analyses of influenza VE in persons 65 years and above showed moderately good estimates of around 50%, then rapidly declined by week 2, 2017 to 28% and 32% in Stockholm and Finland, respectively. The sensitivity analyses, where time since vaccination was taken into account, could not demonstrate a clear decline, neither by calendar week nor by time since vaccination. Most (68%) of the samples collected from vaccinated patients belonged to the 3C.2a1 subclade with the additional amino acid substitution T135K in haemagglutinin (64%) or to subclade 3C.2a with the additional haemagglutinin substitutions T131K and R142K (36%). The proportion of samples containing these alterations increased during the studied period. These substitutions may be responsible for viral antigenic change and part of the observed VE drop. Another possible cause is poor vaccine immunogenicity in older persons. Improved influenza vaccines are needed, especially for the elderly.

Heterogeneity of Circulating Influenza Viruses and Their Impact on Influenza Virus Vaccine Effectiveness During the Influenza Seasons 2016/17 to 2018/19 in Austria

“…This may also partly explain the absence of influenza vaccine protection in Austria, where negative IVEs throughout the season were observed. This differs to data from other countries, where overall IVE's for this season of around 40% have been described (12)(13)(14)(15). Beyond viral genomic variation, birth cohort effects, prior vaccination in addition to the epidemic period may account for regional differences in IVE estimates (16,17).…”

“…The proportion of this newly evolved subclade of the circulating viruses showed regional differences, depending on the start of the influenza season in the different European regions. In the northern parts of Europe, in Sweden and Finland, the influenza season 2016/17 started early in weeks 47/2016 to 49/2016 and peaked already in week 52/2016 (14). In these countries the genetic influenza surveillance revealed the presence of the genetic A(H3N2) subclade 3C.2a1 in 24% of the samples already at the beginning of the season, with an increase to 75% of the circulating viruses in the final weeks 52/2016 to 02/2017 (14).…”

“…In the northern parts of Europe, in Sweden and Finland, the influenza season 2016/17 started early in weeks 47/2016 to 49/2016 and peaked already in week 52/2016 (14). In these countries the genetic influenza surveillance revealed the presence of the genetic A(H3N2) subclade 3C.2a1 in 24% of the samples already at the beginning of the season, with an increase to 75% of the circulating viruses in the final weeks 52/2016 to 02/2017 (14). This circulation pattern is in contrast to that observed in Austria, where the influenza season started several weeks later with its peak in week 02/2017.…”

End-of-season influenza vaccine effectiveness in adults and children, United Kingdom, 2016/17

“…The available genetic characterisation data indicate that the circulating strains clustered to the same genetic lineages as the 2016/17 A(H3N2) inactivated vaccine virus strain, which was an A/Hong Kong/4801/2014 (H3N2)-like virus. Some authors have suggested that reduced VE may relate to the emergence of the genetic sub-clade 3C.2a1 [17,18]. We found a non-significantly lower VE for this sub-clade compared with the 3C.2a clade, although the antigenic characterisation data of circulating viruses remained limited, and there was no suggestion of mismatch based on the available information.…”

“…A small number of studies have currently reported effectiveness in those aged 65 and above for the 2016/17 season, with the I-MOVE network also reporting a non-significant VE in the ≥ 65-year-olds of 23.4% (95% CI: − 15.4 to 49.1) [14], as did study teams from Sweden and Finland [17]. The observation of reduced VE in this vulnerable population may relate to the circulating virus strains, the vaccine itself and/or host factors.…”