The 2015-2016 influenza season (Oct. 4, 2015 to May 21, 2016) in Los Angeles County was moderate overall. Peak activity occurred during mid-February, substantially later compared to previous seasons where peak activity usually occurs from December to January.

During the week of Feb. 14-20 (surveillance week 7), percent positive tests for influenza reached a high of 34.1 percent for the season. In addition, syndromic surveillance detected the highest proportion of visits to emergency rooms for influenza-like illness (ILI) that same week. The greatest number of influenza-associated deaths (IAD) also occurred during week 7.

Overall IADs increased from last season (N=70), however did not surpass the number of deaths during the last A (H1N1) season of 2013-14 (N=105). While influenza A (H1N1) viruses predominated, overall influenza A and B viruses were almost equally represented in laboratory surveillance testing throughout the season which is uncommon.

California data show that influenza activity across the state was similar to what was seen in LAC, in terms of the timing of peak activity and representation of influenza A/B viruses. Conversely, nationwide influenza activity peaked in mid-March (surveillance week 10), almost a month later than in L.A. County. Influenza A (H1N1) predominated throughout the season followed by a typical later season increase of influenza B viruses.

The majority of viruses characterized by the Centers for Disease Control and Prevention were similar to the ones included in this season’s vaccine. This resulted in an estimated vaccine efficacy of 47-60 percent, which is considered average, and a substantial improvement from last season (23 percent).

Influenza-Associated Deaths

To date, a total of 70 IADs have been confirmed in L.A. County this season. The majority of deaths (61 percent) occurred in those under 65 years old (median 59 years old), which is consistent with other A (H1N1) predominant seasons that more severely affect the uner-65 years old population.

More deaths overall were reported in L.A. County this season compared to last season. Of the three pediatric influenza-associated deaths reported this season, two had no past medical history identified, highlighting the potential for severe influenza outcomes in otherwise healthy children.