A PHEIC declaration does not apply to a specific area - it is just that, a declaration by the WHO that the Directory-General has determined that a public health emergency of international concern exists pursuant to Article 12 of the International Health Regulation (2005). Paragraph 5 of Article 12 states that
If the Director-General, following consultations with the State Party
within whose territory the public health emergency of international
concern has occurred, considers that a public health emergency of
international concern has ended, the Director-General shall take a
decision in accordance with the procedure set out in Article 49.
Paragraph 6 of Article 49 states that
The Director-General shall communicate to States Parties the
determination and the termination of a public health emergency of
international concern, any health measure taken by the State Party
concerned, any temporary recommendation, and the modification,
extension and termination of such recommendations, together with the
views of the Emergency Committee.
Here, States Parties refers to all state parties to the IHR - so any declaration of a PHEIC is therefore applicable globally, and obligates state parties to notify the WHO of any local spread of the disease. Notably, the declaration of a disease to be a PHEIC does not actually introduce new obligations to state parties directly, but as states are required under Article 6 to
notify WHO, by the most efficient means of communication available, by
way of the National IHR Focal Point, and within 24 hours of assessment
of public health information, of all events which may constitute a
public health emergency of international concern within its territory
in accordance with the decision instrument, as well as any health
measure implemented in response to those events
the designation of the disease to be a PHEIC means that the event is notifiable, and as a result, the declaration of a PHEIC affects all state parties by definition.
However, if we refer back to Article 49, note that the Director-General is obligated to communicate temporary recommendations with regard to public health. These temporary recommendations are non-binding and time-limited, and are described further in Article 15, paragraph 2 of which states that
Temporary recommendations may include health measures to be
implemented by the State Party experiencing the public health
emergency of international concern, or by other States Parties,
regarding persons, baggage, cargo, containers, conveyances, goods
and/or postal parcels to prevent or reduce the international spread of
disease and avoid unnecessary interference with international traffic.
Therefore, these temporary recommendations need not be globally applicable, and can apply to just one, or a few state parties.
In conclusion then, while the declaration of a PHEIC is applicable globally, the temporary recommendations issued as a result of the declaration are not required to be, and in general, aren't.