  We quantified the probability of developing symptoms (respiratory or fever
\geq 37.5 {\deg}C) and critical disease (requiring intensive care or resulting
in death) of SARS-CoV-2 positive subjects. 5,484 contacts of SARS-CoV-2 index
cases detected in Lombardy, Italy were analyzed, and positive subjects were
ascertained via nasal swabs and serological assays. 73.9% of all infected
individuals aged less than 60 years did not develop symptoms (95% confidence
interval: 71.8-75.9%). The risk of symptoms increased with age. 6.6% of
infected subjects older than 60 years had critical disease, with males at
significantly higher risk.
