Adds support for vaccine tracking.
New methods for Covid
vaccine_coverage(last_days='all')
-> returnsList[VaccineTimeline]
vaccine_countries(last_days='all')
-> returnsList[VaccineCountry]
vaccine_country(last_days='all')
-> returnsVaccineCountry
New classes in covidstatistics
VaccineTimeline
date
:datetime
value
:int
VaccineCountry
country
:str
timeline
:List[VaccineTimeline]