Table 1.
Study characteristics of included studies (N=41)
| Count of studies | % of studies | ||
| Year of publication | ≤2010 | 8 | 20% |
| 2011–2015 | 12 | 29% | |
| 2016–2020 | 15 | 36% | |
| 2021 | 6 | 15% | |
| Geographical region | USA | 19 | 46% |
| Europe | 6 | 15% | |
| Asia | 4 | 10% | |
| Canada | 1 | 2% | |
| Russia | 1 | 2% | |
| Australia | 1 | 2% | |
| Multicountry | 9 | 22% | |
| Study centres | Multicentre study | 34 | 83% |
| Single-centre study | 5 | 12% | |
| Not reported | 2 | 5% | |
| Setting | Community | 20 | 49% |
| Nursing homes or long-term care | 5 | 12% | |
| Clinic | 4 | 10% | |
| Outpatient centres | 1 | 2% | |
| Research centres | 1 | 2% | |
| Not Reported | 10 | 24% | |
| Study design | RCT | 38 | 93 % |
| Cluster RCT | 3 | 7 % | |
| Frequency of interventions examined | IIV3-HD/IIV3-SD | 9 | 22% |
| IIV3-Adj/IIV3-SD | 4 | 10% | |
| IIV3-SD/Placebo | 4 | 10% | |
| IIV3-SD/IIV3-SD-CC | 3 | 7% | |
| IIV3-HD/IIV4-SD | 2 | 5% | |
| IIV3-SD/IIV4-SD | 2 | 5% | |
| RIV3/IIV3-SD | 2 | 5% | |
| IIV3-Adj/IIV3-HD | 1 | 2% | |
| IIV3-Adj/IIV3-Adj/IIV3-Other-Adj/IIV3-Other-Adj/IIV3-Adj/IIV3-Adj/IIV3-Adj | 1 | 2% | |
| IIV3-Adj/IIV3-Adj/IIV3-Adj/IIV3-Adj | 1 | 2% | |
| IIV3-HD/IIV3-Adj/RIV4 | 1 | 2% | |
| IIV3-SD+IIV3 HD/IIV3-SD+IIV3-Adj/IIV3-HD/IIV3-Adj | 1 | 2% | |
| IIV3-SD/IIV3-other/IIV3-Adj/IIV3-Other-Adj | 1 | 2% | |
| IIV3-SD/IIV3-SD | 1 | 2% | |
| IIV3-SD-CC/IIV4-SD-CC | 1 | 2% | |
| IIV4-Adj/Tdap | 1 | 2% | |
| IIV4-Adj/IIV3-Adj | 1 | 2% | |
| IIV4-HD/IIV4-HD/IIV4-SD | 1 | 2% | |
| IIV4-HD/IIV3-HD | 1 | 2% | |
| IIV4-HD/IIV4-SD | 1 | 2% | |
| IIV4-SD/IIV3-Adj/IIV3-HD/RIV4 | 1 | 2% | |
| RIV4/IIV4-SD | 1 | 2% | |
| Funding | Industry | 23 | 56% |
| Public | 11 | 27% | |
| Public and industry | 4 | 10% | |
| Not reported | 3 | 7% | |
| Number of influenza seasons studies | 1 season | 31 | 76% |
| 2 seasons | 6 | 15% | |
| 3 seasons | 1 | 2% | |
| 4 seasons | 1 | 2% | |
| Not reported | 2 | 5% | |
Adj, adjuvanted; HD, high dose; IIV3, trivalent inactivated influenza vaccine; IIV4, IIV quadrivalent ; RCT, randomised controlled trial; SD, standard dose.