This paper reports the findings of a 606-participant study where we analyzed the perception and engagement effects of COVID-19 vaccine rumours on Twitter pertaining to (a) vaccine efficacy; and (b) mass immunization efforts in the United States. Misperceptions regarding vaccine efficacy were successfully induced through simple content alterations and the addition of popular anti COVID-19 hashtags to otherwise valid Twitter content. Twitter's misinformation contextual tags caused a "backfire effect" for the skeptic, vaccine-hesitant participants reinforcing their opposition stance. While the majority of the participants staunchly refrained from engaging with the COVID-19 rumours, the vaccine-hesitant ones were open to comment, re-tweet, like and share the vaccine efficacy rumors. We discuss the implications of our results in the context of broadening the effort for dispelling rumors about COVID-19 on social media.