This is written as a closure (for now) for the previous influenza articles (here and here), which reported on the influenza A(H3N2) epidemic in the U.S.A and the similar high proportion of influenza A(H3N2) among the Singapore influenza isolates that were typed. Various indicators suggest that the epidemic in the U.S.A. is coming to an end. The proportion of visits for influenza-like illness (ILI) reported by the ILINet has continued to decrease, and more importantly, the proportion of influenza cases caused by influenza A(H3N2) has dropped dramatically – most cases are now caused by influenza B instead. Interestingly, the circulating influenza B strains correspond closely to the strains included in the 2014-2015 Northern hemisphere vaccine (70.6% belong to the B/Yamagata/16/88 lineage while the remained belong to B/Victoria/02/87 lineage).

Percentage of influenza-like illness cases reported by the U.S. Outpatient Illness Surveillance program.

Replacement of influenza A(H3N2) by influenza B viruses, with an overall drop in cases of influenza in the past few weeks (data and graphic from the CDC Weekly Flu Summary).

In Singapore, the situation remains unchanged (and from previous years) in terms of number of influenza-like cases. There has been a corresponding drop in influenza A(H3N2) cases, with a trend towards its replacement by influenza B, but not to the extent seen in the U.S.A.

Acute respiratory infections reported by the polyclinics in Singapore (graphic from the MOH weekly infectious diseases bulletin).

Influenza subtypes in Singapore showing a fall in influenza A(H3N2) in Feb and Mar 2015 (graphic from the MOH weekly infectious diseases bulletin).

The 2015 Southern Hemisphere influenza vaccine should be available later this month, and includes all the influenza virus types listed above.