I read this quote from the CDC reported by jcaron:

If I have one or more connecting flights to the US, does the 3-day period apply to the first flight or the last one? Do I need to get another test if I have a connecting flight?

If your itinerary has you arriving to the US via one or more connecting flights, your test can be taken within the 3 days before the departure of the first flight.

If the 3-day testing period expires before one of your connecting flights, you only need to get retested before boarding connecting flights if:

  • You planned an itinerary incorporating one or more overnight stays en route to the US. (NOTE: You do not need to be retested if the itinerary requires an overnight connection because of limitations in flight availability.), OR
  • The connecting flight is delayed past the 3-day limit of testing due to a situation outside of your control (e.g., delays because of severe weather or aircraft mechanical problem), and that delay is more than 48 hours past the 3-day limit for testing.

How does the CDC define more precisely the following condition:

if the itinerary requires an overnight connection because of limitations in flight availability.

E.g. in the case of my friend flying to the US, his flight has a layover of around 8 hours from very early AM (quite close to midnight) to the beginning of the morning: the layover is inevitable if flying with his preferred airline and departing at a specific day. However, if he decides to flight the next day with the same airline, then the layover is just around 3 hours. In that context, does the CDC think that his itinerary requires the 8h layover? (I don't intend to discuss whether this 8h layover is overnight: see How does the CDC define an overnight stay? for that question. Let's just assume it is.)

  • 1
    Require 1) specify as compulsory 2) cause to be necessary . Use definition 2 and you'll see that @Hilmar needs a checkmark on their answer.
    – Dave
    Aug 14, 2021 at 14:49
  • @Dave I've already upvoted it, but just waiting a bit in case of other answers since it's just 13h old. Aug 14, 2021 at 15:32

1 Answer 1


You are overthinking this.

The most reasonable interpretation of the CDC rules is that you don't need to retest unless you do an intentional stopover or have a massive delay.

There is no requirement to book the flight with the "shortest possible layover".

If you want to be absolutely safe just test less than 72 hours before the last leg to the US departs. The "3 day" rule is pretty generous and most countries can turn a PCR test is in 24 hours or less.

  • 1
    Even better, rapid antigen tests are also acceptable.
    – phoog
    Aug 15, 2021 at 2:40

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