If you have a connecting flight in the US to another country then it is advisable to still complete a US Attestation Form

If you have a connecting flight in the US to another country then it is advisable to still complete a US Attestation Form. You may proceed with you itinerary if you wish to take a connecting flight from the US to another country, but you must still fill out and sign the attestation in case your intentions are altered or postponed. The CDC has more details - https://www.cdc.gov/coronavirus/2019-ncov/travelers/proof-of-vaccination.html#faq-attestations