In 1918, a crude influenza vaccine made from chemically inactivated, mixed cultures of respiratory bacteria was widely used prior to the understanding that influenza was caused by a virus. Such vaccines contained no viral material and probably consisted largely of bacterial endotoxin. The Australian military used such a vaccine on Samoa in December 1918 and thought it was valuable. Post hoc analyses suggest that the mixed respiratory bacteria vaccine may have actually been of some benefit, but the mechanism of such protection is unknown. Although such a crude vaccine would not be considered in a modern setting, the rapid use of problematic vaccines still remains a risk when new influenza types suddenly appear, as in 1976 and 2009.
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