Protection of mice against challenge with virulent could be induced by intraperitoneal or subcutaneous vaccination with heat-phenol-or acetone-inactivated suspensions of strain Ty2 or of a Vi-producing strain of , or with Vi antigen prepared from either of these.

In all cases, protection rose to a maximum at about 4 days after vaccination. By day 18 this had fallen to the level at day 2.

The rise and fall of Vi agglutinins in mice vaccinated either intraperitoneally or subcutaneously with heat-phenol- or with acetone-killed suspensions of Ty2 or paralleled closely the rise and fall in protection, whereas the rise of O agglutinins corresponded with the decline in protection.

It was concluded that the potency tests for typhoid vaccines according to procedures SCM and IPM (WHO type-N procedures) in mice are chiefly a measure of the response to Vi antigen and consequently not a measure of factors responsible for eliciting immunity in man.


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