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: The HBsAg positive rate in neonates and in infants before two months were 3 % and 2 % respectively .
### Output Text:
The <span class="chemical">HBsAg</span> positive rate in neonates and in infants before two months were 3 % and 2 % respectively . 