Global Influenza Activity Update

The World Health Organization (WHO) has issued its global 2015—2016 influenza season summary. The report tracks influenza activity from October 2015 until the end of October 2016, and gives stakeholders access to patterns of viral transmission characteristics, illness, and mortality data by geographic region and/or country.

In North America, the 2015—2016 influenza season peaked later than usual. The highest percentage of respiratory specimens testing positive for influenza viruses didn’t occur until mid-March. In the southern hemisphere, Argentina and South Africa reported early influenza activity.

Laboratories determined that influenza A(H1N1)pdm09 viruses predominated in the United States, accounting for 81% of influenza A viruses. Laboratories reported a small number of patients with influenza B virus throughout the season. Canadian laboratories reported an uptick in patients with influenza B toward the end of the season.

In the United States, peak influenza positivity in March mirrored that seen in the 2014—2015 season, but patient visits to healthcare providers reached 3.6% at the peak—a number that was lower than in the previous season.

Patients were less likely to die from pneumonia and influenza mortality was also much lower than with the previous 3 seasons.

Influenza-hospitalizations in elderly adults (aged ≥65 years) were also significantly less likely in the 2015-2016 season than in the previous year (84.8/100 000 vs. 322.8/100 000 respectively). Influenza A(H1N1)pdm09 infections were the predominant culprit causing most patient hospitalizations.

The report also indicates that vaccines provided moderate protection against infection for all subtypes. Data were not available from all countries, but in those that had compiled statistics:

• Vaccine effectiveness against influenza A(H1N1)pdm09 was slightly lower than the previous season.
• Vaccine effectiveness against influenza A(H3N2) was slightly higher than the last season when antigenic differences between the circulating influenza A(H3N2) viruses and the vaccine virus existed.
• Vaccine effectiveness against influenza B virus infection was similar to 2014—2015 estimates in the United States.

Viruses identified in most influenza outbreaks were similar to strains included in influenza vaccines.

Reference

Review of global influenza activity, October 2015—October 2016. Wkly Epidemiol Rec. 2016; 16;91(51-52):604-22.