In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Recent development of vaccine for prevention is reported to be successful in reducing the size of chronically infected carriers , although the standard medical therapies have not been established up to now .
### Output Text:
Recent development of vaccine for prevention is reported to be successful in reducing the size of chronically infected carriers , although the standard medical therapies have not been established up to now . 