Summary

What is already known about this topic?

CDC collects, compiles, and analyzes data on influenza activity year-round in the United States. Substantial influenza activity generally begins in the fall and continues through the winter and spring months. However, the timing and severity of influenza activity varies by geographic location and season.

What is added by this report?

The 2015–16 influenza season was less severe overall compared with the preceding three seasons. The cumulative hospitalization rate for all ages of 31.3 per 100,000 population was lower than those for the previous three seasons (64.1 in 2014–15, 35.1 in 2013–14, and 44.0 in 2012–13), and the number of influenza-associated pediatric deaths (74) also was lower compared with previous seasons (148 in 2014–15, 111 in 2013–14, and 171 in 2012–13). Influenza activity began later and continued for a longer period, peaking in mid-March. During the most recent 18 influenza seasons, only two other seasons have peaked in March (2011–12 and 2005–06). Influenza A (H1N1)pdm09 viruses predominated during the 2015–16 influenza season, with influenza B viruses, and to a lesser extent, influenza A (H3N2) viruses cocirculating. Antigenic and genetic characterization showed that most circulating viruses were well-matched to the 2015–16 Northern Hemisphere vaccine.

What are the implications for public health practice?

Influenza surveillance, including for novel influenza viruses, should continue throughout the summer months, and health care providers should consider influenza as a cause of respiratory illness even outside the typical season. Although influenza viruses typically circulate at low levels during the summer months, antiviral treatment is recommended for all patients with confirmed or suspected influenza who have severe, complicated, or progressive influenza-like illness; those who require hospitalization; and those at higher risk for influenza-related complications, including adults aged ≥65 years. These medications work best when administered early in the course of illness.