Question: The least effective prevention measure in the appearance of an outbreak of salmonellosis is: 
 Options:  <1> Active immunization to the population at risk. <2> Rigorous and frequent hand washing, as well as utensils and surfaces that will be used for cooking. <3> Keep food in the refrigerator. <4> Cook food with sufficient temperature (at least 70ºC). <5> Quick refrigeration of cooked foods.
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