@Lucrecia : I don't mean to trivialize the 1918 flu pandemic, it was certainly a big deal as you say, and you'll get no argument from me about that. But if you look at per capita deaths especially, it wasn't of the same magnitude as the plague. There were outbreaks of the plague that killed over half the population in some places in the same amount of time. (See
The Italian Plague of 1629–1631)
Although, in terms of per capita deaths, small pox in the Americas might beat the plague, but I haven't done the math on that one in particular.
Still, as far as the story goes, the precautions Letizia came up with for dealing with general illness would already be the best defense against the flu. The flu vaccine wasn't invented until 1938 so it wasn't used in the 1918 outbreak, and wouldn't help the people already sick if she did create one; whereas the plague is readily handled with antibiotics.