Instead of exploding relative to a baseline of 0 cases (like in March), the new strain will be exploding relative to a baseline of around 200,000 cases per day. As a result, day-to-day random noise will completely mask any increase in infections from the new strain until it becomes dominant — around day 40 in the above chart. This means that if people use the overall numbers to guide their levels of precaution, our reaction time could lag by two or three weeks compared to March — as if we had locked down in early April instead of mid-March.

Now, this isn't entirely a correct comparison because the rate of exponential growth will be much below 1.36; a reasonable guess might be 1.12. (You can get this by assuming R = 1.6 and assuming a generation time of 4 days.) But the overall point is the same: because any increase in the prevalence of the new strain will be masked by noise in current COVID levels, the new strain won't be evident in overall numbers until it starts contributing hundreds of thousands of daily infections.

That is from Eric Neyman, good points thought note we will have independent measures of the spread of the new strain, with shorter lags than is currently the case.