Which strain of influenza is coded as J09.X2?

The correct code J09.X2 corresponds to the specific strain of influenza known as H1N1, more commonly referred to as swine flu. In this coding system, J09 pertains to pandemic influenza due to identified influenza viruses. The 'X2' in the code further specifies the H1N1 strain.

This code is designated for cases where influenza due to the H1N1 virus is confirmed. Accurate coding allows for better tracking of the impact of particular strains of influenza on public health and informs treatment and preventive measures.

The other options do not align with this specific coding. Avian influenza pertains to a different group of viruses, seasonal influenza is often assigned different codes within the J10 or J11 categories for seasonal flu strains, and unspecified does not provide the specificity required, as J09.X2 denotes a defined and specific type of influenza.