For patients with new-onset systemic lupus erythematosus (SLE) who also developed antiphospholipid syndrome (APS), most thromboembolic events occurred during the first year after diagnosis, a ...
This and these are used in different ways when you are referring to people, things, situations, events, or periods of time. They can both be determiners or pronouns.
You use these when you refer to something which you expect the person you are talking to to know about, or when you are checking that you are both thinking of the same person or thing.
This and these are demonstratives, which means they indicate a specific noun in a sentence. The two words are similar because they refer to nouns that are near in space and time.