For the umpteenth time: The fact that there are no checkpoints on the land borders between EU countries does not to mean that there are no Covid-19 public health restrictions that people ought to inform themselves about and comply with. Throughout 20+ months or so, the Spanish government had been particularly clear about their requirements for entering their country by air travel, by sea travel and by land travel where they even clearly specified whether it concerned travel by land from Portugal or travel by land from all of France or from specific regions of France. While they had at no point wasted human resources or financial resources to put border control on the Route Napoleon.
Also, at this point in time, don't airlines inform their passengers during the booking process about the requirements for boarding of the international flights they sell to their customers and don't they provide the links to relevant Covid-19 related public health requirements for each passenger booked to enter their destination country? At this point in time, passengers should not need to ask on social media.
But, ok, for reassurance, although
@trecile and
@Harland2019 have said it already: You fly from the USA to France. You walk from France to Spain. You had a first injection for Covid-19 immunisation on 2 April 2022 in the USA and it was the Johnson&Johnson vaccine (I don't understand what H&M means). So you need your vaccination certificate and also the filled in Passenger Locator Form for France. See
https://www.diplomatie.gouv.fr/fr/c...ations-pratiques/article/coronavirus-covid-19, updated on 2 April 2022. When you click on the link and have switched to English and start the form you will see what is shown in the screenshot below.
But all this ought to have been provided to you by your airline either during the booking process or when you got the confirmation email.
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