Under Texas law, you qualify as having a disability if you are (1) sick, (2) pregnant, or (3) if voting in person will create a likelihood of injury to your health.
The Texas Supreme Court has ruled that lack of immunity to COVID-19 can be considered as a factor in your decision as to whether voting in person will create a likelihood of injury to your health, but it cannot be the only factor. You do not qualify to vote by mail as “disabled” if you have a fear of contracting COVID-19 but are otherwise healthy. To qualify, you must have an accompanying physical condition.