Please see attached instructions and article. Please keep all discussions US/American related. Please use the article to answer the following questions in 225-250 words:

1. Explain why vaccine efficacy is not always a good predictor of vaccine effectiveness.
2. According to the article, which type of epidemiologic study design is the most efficient to show vaccine efficacy and why?
3. Which population poses the greatest challenge for recruitment in COVID-19 vaccine trials, and why?