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 proportion of infants who were vaccinated in January and February 2000 ( 48 . 5 % and 67 . 5 % , respectively ) was less than any other months , whereas the proportion who were vaccinated in December 2000 ( 88 . 2 % ) was the highest .
### Output Text:
The proportion of infants who were vaccinated in January and February 2000 ( 48 . 5 % and 67 . 5 % , respectively ) was less than any other months , whereas the proportion who were vaccinated in December 2000 ( 88 . 2 % ) was the highest . 