A "major difference is that COVID-19 causes more severe disease than seasonal influenza.
While many people globally have built up immunity to seasonal flu strains, COVID-19 is a new virus to which no one has immunity. That means more people are susceptible to infection, and some will suffer severe disease.
Globally, about 3.4% of reported COVID-19 cases have died. By comparison, seasonal flu generally kills far fewer than 1% of those infected."
- Tedros Adhanom Ghebreyesus, World Health Organization's Director-General
The WHO Director's full opening remarks on novel coronavirus can be found here.