In the United States mask wearing is positively correlated with vaccine acceptance. This correlation introduces an important bias into real-world estimates of vaccine efficacy. I derive the formulae for vaccine efficacy that correct for this phenomenon and show that such biases explain some of the differences between higher estimates of vaccine efficacy observed in the US studies, on one hand, and lower estimates from Israel and Pfizer trials, on the other hand. Control for such biases is important for currently-debated public health decisions regarding COVID19 vaccine booster doses.