Satsuki was never one to get sick, well, according to Nui, anyway, so her catching the flu was something of a surprise but it wasn't impossible. I suppose, between caring for her sisters and trying to keep things afloat, she might have been neglecting her health or, to the very least, failing to get any vaccinations. Of course, in her being ill and aware of it, she told Nui to tell me to get them to a pharmacy or clinic to get them flu shots so they won't contract her illness.
Nui, as I would imagine, could withstand the flu, however, Ryuuko would leave much to be decided, considering her being a feral child and having not much of, if any, exposure to disease. Regardless, the current strain of the influenza virus did leave Satsuki practically bedridden, so, in which case, it was probably more crucial for Ryuuko to get a vaccine, as catching the flu could land her in the hospital. Now, as we could see, that would not be good, so getting her a vaccine is a must, however, she isn't known for cooperation.
Nui could and would cooperate but Ryuuko is a bit more stubborn, especially when she knows there will be pain involved. Of course, during her stay at the hospital and institution, she was poked and prodded at with needles and, as we could tell, she hated every bit of that and, once, decided to pull one out. She didn't do that again because, well, the pain from doing it once shocked her, however, it didn't stop her from gnawing at the tubes or sites where they put them at. She's concluded that she didn't like needles and neither did she like intravenous tubes or anything in which she'd view as causing pain.
I dreaded this bit, as, apparently, it seems, I would have to hold her down so she'd cooperate. I know she wouldn't like that but it'd have to be a necessary evil, after all, she's come a long way and it'd be devastating if she were to come down with the flu and end up hospitalized or worse. She has no concept of necessary evils. Vaccines are a necessary evil, regardless, if she hated the concept. I didn't want to put her through any semblance of pain, however, to keep her from getting sick, she'd have to experience something of a pinch.
The moment she caught sight of the clinic, I could tell she was going to put up a fight and she did, complete with her usual trimmings of kicking and scratching and screaming, "No!" at the top of her lungs. We got her into the waiting room and we tried to calm her down, however, she wasn't having any of that, screaming "No!" and then "Pr'ick!" I was intrigued and I asked her if she meant something else but she just repeated the word with her usual upset inflection, leaving me to wonder if the word was her way of saying, "Ow!" or expressing pain, either that or she had learned a new name for someone, regardless, she's learned to associate the word with something negative.
Likewise, she's associated hospitals and similar places to be negative, of course, that was a given, as, being poked and prodded at by strange people in a loud and bright place was one of the first things she's come to experience, judging it thusly. As said before, with her being such a rather fragile child with an equally fragile psyche, I would hate to make to her relive a traumatizing experience but, in this case, with her sister being ill with a virus that could put her in the hospital or worse, getting her a shot was a necessary evil.
In simple enough terms, I tried to reassure her that, while it would hurt, getting sick would hurt more, along with that Satsuki said that she had to get a shot and the reason why was that she loved her, that she didn't want her to get sick. It seemed, for a bit, that she understood and, thus, she calmed down, ceasing her screams. It seemed she would cooperate, regardless if she was upset or had tears in her eyes. In that moment, she merely asked for a hug and she wouldn't let go.
As I held her and while she cried out (and tried to swat the nurse giving it to her), she took her flu shot. She was upset, understandably so, but it had to be done. At least, I managed to get her to cooperate and, for a good year, she was safe from the same illness that plagued her sister.