Language teacher immunity, as a new notion in language teacher psychology, is a strong indicator of how teachers behave when dealing with difficulties and challenges; and it has a profound effect on teachers' careers. Research on language teacher immunity is in its nascent stage. This study employed an explanatory sequential mixed-methods design to explore the changes in EFL teachers' immunity for a semester in the COVID-19 era. In so doing, the language teacher immunity questionnaire (Language Immunity Questionnaire; Hiver [2016. Tracing the Signature Dynamics of L2 Teacher Immunity: A Retrodictive Qualitative Modeling Study." Unpublished Ph.D. thesis, University of Nottingham, England]) was administered to 30 male and female Iranian EFL teachers using SHAD application (an Iranian application developed to address the emergency remote teaching need to be created by the COVID-19 crisis) in the beginning, middle, and at the end of the Fall semester 2020. After that, follow-up interview data were collected to help explain the results of the quantitative phase. The results of the repeated-measures ANOVA indicated that there were differing changes in different aspects of participants' immunity. The follow-up qualitative data analysis showed that the changes in teachers' immunity were mainly due to familiarity with online teaching, scepticism of online learning, and limited student participation. The implications for future research in the field are discussed.