Results
| Type of predictor | Predictor | Number of studies that included the predictor | mhRR (95% CI) |
| Demographic | Male gender | 810 15–17 19 21–23 | 1.18 (1.13 to 1.23) |
| 40 years and older | 415 16 20 23 | 1.23 (1.17 to 1.28) | |
| Being a physician | 89 15 16 18 20 21 23 24 | 1.16 (1.13 to 1.20) | |
| Being a nurse | 89 15 16 18 20 23–25 | 0.90 (0.88 to 0.91) | |
| Knowledge | The vaccine is effective | 315 16 21 | 2.22 (1.93 to 2.54) |
| Being aware of the CDC recommendations | 1* 22 | 1.29 (0.98 to 1.70) | |
| Getting influenza is safer | 1* 15 | 0.83 (0.56 to 1.23) | |
| Influenza appears as a heavy cold | 1* 16 | 1.53 (1.18 to 1.98) | |
| Perceived benefits | Willing to prevent influenza transmission | 315 16 21 | 2.31 (1.97 to 2.70) |
| Willing to protect at-risk patients | 210 21 | 1.12 (1.06 to 1.18) | |
| Willing to protect oneself | 1* 15 | 1.89 (1.40 to 2.54) | |
| Perceived risks and severity | Being at risk for influenza complications | 410 15 16 23 | 1.13 (1.08 to 1.19) |
| Believing that the vaccine transmits influenza | 315 16 22 | 0.67 (0.55 to 0.83) | |
| Believing that the risk of getting influenza is small | 315 16 22 | 0.73 (0.52 to 1.03) | |
| Believing that influenza is not a serious illness | 1* 15 | 1.26 (0.87 to 1.83) | |
| Believing that influenza is highly contagious | 1* 16 | 2.25 (1.66 to 3.05) | |
| Perceived barriers | Fear related to vaccination | 1* 15 | 0.74 (0.49 to 1.15) |
| Fear related to injection | 215 16 | 1.05 (0.84 to 1.29) | |
| Vaccination is unpleasant | 215 16 | 0.68 (0.49 to 0.92) | |
| Convenient vaccination clinics | 1* 15 | 2.25 (0.95 to 5.34) | |
| Vaccination free of charge | 215 22 | 1.52 (1.27 to 1.81) | |
| Previous influenza and influenza vaccination | Previous influenza vaccination | 510 16 20–22 | 1.51 (1.47 to 1.55) |
| Having a history of influenza | 210 22 | 1.07 (1.02 to 1.13) | |
| Other predictors | Believing influenza prevention is important | 215 16 | 3.63 (2.87 to 4.59) |
| Preferring natural/complementary medicine | 215 16 | 0.26 (0.12 to 0.55) | |
| Family is usually vaccinated | 1* 16 | 2.32 (1.64 to 3.28) | |
| Having contact with children | 1* 10 | 1.08 (1.04 to 1.13) | |
| Having direct patient contacts | 410 16 17 21 | 1.04 (0.99 to 1.08) | |
| Smoking status | 1* 23 | 1.01 (0.66 to 1.53) |
↵* When the information about a predictor was available from only one study, risk ratios (RR) and not Mantel-Haenszel risk ratios (mhRR) were calculated.
CDC, Centers for Disease Control and Prevention; mhRR, Mantel-Haenszel risk ratio.