Fear generally works best for motivating one-time acts, especially those that feel risky. Early this year, fear was probably an effective way to motivate people to get their first vaccine shot. But it tends to be less effective for driving repeated behaviors such as getting a second dose and a booster.
Of course, the problem isn’t just being oversaturated with fear messages. Safety behaviors have become so politicized that many people are skeptical not only of vaccines and face masks but also even of the threat that Covid-19 presents. For a fear message to get through and change behaviors, people need to be confident both that there’s a clear and present danger and that taking action will protect them.