Former Presidents, First Ladies Encourage Vaccinations

The video discusses a public service announcement promoting the coronavirus vaccine featuring former presidents Obama, Bush, Clinton, and Carter, along with their wives. Donald Trump is notably absent due to the ad being filmed at President Biden's inauguration, which Trump did not attend. Despite not attending the inauguration, former President Carter is included in the ads.

What is the purpose of the public service announcements mentioned in the text?

Which former president was not present at President Biden's inauguration?