The COVID-19 pandemic struck Spain severely from the beginning. Prevention via information that fosters knowledge, reasonable concern, control, and personal care is the most effective means to slow down the pandemic. In this intervention field study, first, we assessed actual knowledge, concern, control, and care about the COVID-19 in 111 Spanish university teachers and students. Subsequently, we randomly assigned them to two groups. One group (n = 53) received uncertain information about prevention measures, whereas the other group (n = 58) received certain information. Analysis of covariance, using baseline measures as covariates, revealed that the group receiving the certain information reported an immediately increased perceived control and personal care about the pandemic. These findings suggest that measures that are known to be effective in COVID-19 prevention, if communicated with certainty (i.e., solid evidence), could influence people's attitudes, possibly through the schematic organisation of new information.