But then why didn't the numbers explode in Feb? We saw a 100x increase in deaths between mid and end of March - in circa 2 weeks. That's what happens with exponential growth when numbers are doubling every three or four days, as they were.
So if we had 100 die in Jan, we'd have seen 10,000 dead by mid Feb and 100,000 by end of Feb (without a lockdown in Feb). i.e. there cannot have been 10,000 infected in Jan. Widespread infection in Jan is not possible.
How do we know the doubling time was 3 days?
The more you ramp up testing the more it would appear the virus is doubling.
Example
1st day of testing 1000 tests 10 infected
7th day of testing 2000 tests 40 infected
The figures would have you believe the virus has doubled every 3 days..... in reality if 1000 tests had been carried on day 7 u may only have found 20 people infected, giving a doubling rate of 6 days