I have never had that level of responsibility and can imagine that the responsibility alone can cause a spike in anxiety which would make you prone to making errors, as in the flu vaccine case, and then knowing that you're making errors would make you fearful, anxious yet more, and guilty. With that kind of a negative cocktail you you would make even more errors, going into a downward spiral.

Reducing the work week was an awesome move. Let us learn from other high responsibility occupations.

1. Aircraft pilots may not work more than a certain number of hours straight.
2. Pharmacists in California must take a 30 min lunch break (maybe even an hour, not sure)
3. Nurses at nursing stations at hospitals have an uninterrupted time for preparing medications.

So you need to do those things - sleep enough, eat balanced meals regularly, and drink liquids throughout the day. Even a small degree of dehydration diminished mental acuity in studies, and where you have reduced mental acuity, you become prone to making errors.

Also walking for 45 a day produced an improvement in memory function in one study (Middle-aged participants walking for 45 mins a day showed functional improvement after three months).