The 2018-2019 flu season killed 34,200 out of 35.5 million infected over 21 weeks. That is 0.9634% of all infected died from the flu in the United States. COVID-19 has killed 65,435 (as of May 1, 2020) from 1.13 million infected over 12 weeks. That is 5.791% of all infected so far that have died from COVID-19 in the United States.
R0 (R-Naught) measures the number of cases of any contagious disease on average that an infected person will cause during the infectious period to a susceptible population. R0 < 1 means each new infection causes less than 1 new infection which causes the disease to decline and eventually die out. R0 = 1 means each existing infection causes one new infection which causes the disease to stay alive and stable but will not become an outbreak or epidemic. R0 > 1 means each existing infection causes more than one new infection which will cause it to spread and may cause an outbreak or epidemic.
The R0 value of the flu is ~1.3. The higher the R0 the more contagious it is. Ebola is 2, HIV is 4, Polio is 5-7, and Measles is highly contagious at 12-18.
Scientist from Los Alamos National Laboratory have a research article in Emerging Infectious Diseases titled High Contagiousness and Rapid Spread of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2 is the virus that causes COVID-19) that the median of estimated R0 = 5.7. An earlier estimate of R0 was ~ 2.2 to 2.7.
R0 of 5.7 means that a single infected person can potentially transmit SARS-CoV-2 virus to 5 to 6 people which means that at least 82% of the population needs to be immune to COVID-19 to stop the transmission through vaccination and herd immunity.