Generally, this is a word that means without cells. In the context of early childhood it usually has reference to acellular pertussis vaccine. The original pertussis vaccine (DTP) included parts of the whole bacterial cell (Bordetella pertussis) that causes pertussis. (Recent) research has shown that only certain parts of the bacteria are required to produce immunity; not the whole bacterial cell. The new vaccine is called an acellular vaccine since it contains only those proteins that are important for producing immunity and not the whole bacterial cell. The acellular vaccine has fewer side effects than the old, whole-cell vaccine and is the recommended type of vaccine for preventing pertussis.
The old vaccine was called DTP; the acellular form is called DTaP.