On the 8th of August 2022, data collection began for the 12th wave the COVID-19 Impact Monitoring series, with a total of 3,510 responses collected between the 8th and 22nd of August. The aim of this paper is to try to understand some of the reasons why booster uptake in Australia may have stalled, with a focus on the socioeconomic, demographic, and geographic determinants of third and fourth-dose booster uptake. A particular focus of this paper is the relationship between COVID-19 infection and booster uptake, the first time this appears to have been studied quantitatively in Australia. Those with low education, low income, who were born overseas in a non-English speaking country, who do not trust key institutions, who identify as right wing, and who do not consume news from public broadcasters, radio, and newspapers are all less likely to have received a third or fourth COVID-19 vaccine. This paper also shows that those who have had a COVID-19 infection are less likely to have received a COVID-19 booster, potentially explaining why booster uptake has stalled as infections in Australia reached their highest level across the first two-and-a-half years of the pandemic.