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: Immune escape variants of the hepatitis B virus ( HBV ) represent an emerging clinical challenge , because they can be associated with vaccine escape , HBV reactivation , and failure of diagnostic tests .
### Output Text:
Immune escape variants of the hepatitis B virus ( HBV ) represent an emerging clinical challenge , because they can be associated with vaccine escape , HBV reactivation , and failure of diagnostic tests . 