Might be dumb questions, but what's the background rate of these rashes in the general population? What fraction of that population received a first dose of the Pfizer-BioNTech vaccine?

These two cases occurred 7 days and 8 days after the affected individual's first vaccine dose. How many other people experienced the same type of rash due to some unknown trigger without recent vaccination? How were these two rash cases actually linked to the vaccine? Is the 1+ week delay implicitly deemed close enough to assume a causal link?

tldr:

  We present two cases of COVID-19 vaccine-induced pityriasis rosea. Skin lesions appeared in a 49-year-old female 8 days after the first dose of the BNT162b2 mRNA vaccine and in a 53-year-old male 7 days after the second dose of the same vaccine.
  ...
  Her rash gradually improved and at follow-up, 4 weeks later, it had resolved completely.
  ...
  The itching was self-limiting and therefore the patient did not receive any antihistamines. His rash resolved completely over a period of 4 weeks.

> The activation of immunological mechanisms triggers PR in both cases: infection and vaccination.