[gtest]

Thank you for asking in a way that is not derogatory unlike many of the comments here. There are two reasons generally. I am a Christian, and morally natural to the vaccine.

The first (and rarely claimed as the reason) is the mark of the beast interpretation in Revelation. Interpreting much of Revelation is hard, whether the mark is metaphorical or actual, and what the mark is, and what the beast is, are all questions with not extremely clear answers.

The second, and by far the most claimed reason, is the use of fatal tissues in testing, developing, or producing the vaccine. Here, Christians consider supporting and taking this vaccine is similar to supporting abortion. In a similar way, one might think that buying products produced by child labor is supporting child labor, or that eating meat is supporting animal cruelty.

The same Christians who do not support covid vaccine for the second reason, might be fine with taking other vaccines, even if they were developed by aborted tissues. The main reason here is that it has been such a long time between the development and today that taking the vaccine is not a support of such behavior. Similarly, one today might use ride a volkswagon beetle without being considered a supporter of nazi, even though it was developed by the nazi.

Some Christians see that the vaccine is good despite of using fatal tissues, because they see that there is no alternative to preventing getting covid, and so it is permissible in this exceptional circumstance.

I hope this answers your question.