The announcement of Pfizer/BioNTech’s COVID-19 vaccine success on 9 November 2020 led to a global stock market surge. But how did the general public respond to such good news? We report results of a nation-wide natural experiment in the US and the UK on how the vaccine news influenced citizens' government evaluations, anxiety, beliefs and elicited behaviors. While most outcomes were unaffected by the news, trust in government and elected politicians (and their competency) saw a significant decline in both countries. As the news did not concern the government, and the government did not have time to act on the news, our results suggest a dispositional response to positive news more likely to be explained by a form of the psychological mechanism of motivated reasoning. They also offer a novel insight regarding the association between trust in government and compliance with its policies: anxiety might explain both.