http://economics.harvard.edu/concentration-requirements says that those who do not earn B- or higher grades in those courses “must take Ec 975a or Ec 975b or an economics elective that has 1010a/1011a or 1010b/1011b as a prerequisite.”
Ec 1010a/1010b appear to be typical intermediate economics courses with single variable calculus (Ma 1a). Ec 1011a/1011b appear to be more math intensive ones with multivariable calculus (Ma 21a). Whether such courses are a “walk in the park” depends on the student.
http://www.registrar.fas.harvard.edu/courses-exams/courses-instruction/economics