In total, 3646 records were identified through systematic literature searches and an additional 453 records via grey literature, manual search of the reference lists of the included research articles and reviews, and articles received through professional contacts. This added up to a total of 4099 records which after duplicate removal yielded 3185 unique studies for screening. Out of these, 48 studies (described in 51 articles) were included in this review (Fig. 1).

Almost half of the included studies (n = 22) [19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,43] were published before 2000, 16 between 2000 and 2009 [43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,59] and 10 from 2010 to 2020 [59,60,61,62,63,64,65,66,67,69] (Table 1). The majority of studies were conducted in the US (n = 37) [19,20,21,22,23,24,26, 27,28,29,30,31,32,34, 37, 38,39,40,42, 43,44,46, 49, 52, 53, 54,55,56,57,58,59,60,61,62,63,64,65,66,68]. Most interventions focused on physicians, among whom also residents and interns [19,20,21,23, 24,25,26,28, 29,30,31,32,34, 36,37,38,39,40,41,42,43,45, 46,47,48,50, 52, 53, 58, 60, 61, 62,63,64,66]. Some studies targeted both physicians and nurses [24, 35, 36, 57, 59, 70]. Nurses would offer vaccination to patients through standing order procedures (SOP) enabling healthcare personnel other than physicians to evaluate immunization status and administer vaccines [24, 46, 57, 59]. In some cases, both physicians and nurses received reminders [57, 70]. In one study nurses called patients to remind them they were due for vaccination [35, 36]. The study of MacIntyre, Kainer and Brown [51] engaged both primary care physicians and hospital staff in their intervention. Furthermore, one study aimed their intervention on long-term care facilities [56], another on pharmacist-technicians [62] and one on community HCW [69]. There were also some studies where it was not entirely clear what HCW-types were part of the intervention [29, 51, 54, 67, 68], referring for example to hospital staff or primary care clinics. The most frequently studied educational interventions were tailored reminders for vaccination [21, 25, 29, 31, 32, 33,34,35,36,37,39, 41,