Here, you can just use a share of official cases to true cases. How to decide which one? For the Bay Area, they were testing everybody who had traveled or was in contact with a traveler, which means that they knew most of the travel-related cases, but none of the community spread cases. By having a sense of community spread vs. travel spread, you can know how many true cases there are.
I looked at that ratio for South Korea, which has great data. By the time they had 86 cases, the % of them from community spread was 86% (86 and 86% are a coincidence).
With that number, you can calculate the number of true cases. If the Bay Area has 86 cases today, it is likely that the true number is ~600.
France and Paris
France claims 1,400 cases today and 30 deaths. Using the two methods above, you can have a range of cases: between 24,000 and 140,000.
The true number of coronavirus cases in France today is likely to be between 24,000 and 140,000.
Let me repeat that: the number of true cases in France is likely to be between one and two orders or magnitude higher than it is officially reported.
Don’t believe me? Let’s look at the Wuhan graph again.
Testing as many people as possible, isolating anyone who has come into contact with an infected individual and curtailing travel everywhere is the only way we are going to see a decline in the number of new cases. The pressure health services come under as the number of new severe cases explodes is non trivial and is the primary reason the death rate in Italy is so high. Unfortunately, the USA’s reliance on private healthcare, motivated by profits rather than potential need, is hampering the ability to get tests done.
Early action in locking down community transmission is the only solution and even if it is not early it is still the only solution. My family has been on a social distancing regime for the 10 days week. I cancelled all travel, but we are going on full self-quarantine after school today. Even then our school has not yet closed but that seems inevitable at this stage.