Today, New York State reported a 30% increase overnight in confirmed cases of Covid-19. That is a stunning rise in the number of case. Or is it? The data shown above from Washington State suggest another possible explanation.
This is a plot of the number of cases and deaths related to Covid19 in Washington state over the past ten days based on data from the Johns Hopkins Corona Virus Center. This seems to show an explosion in the number of cases starting around March, 10. But that date coincides with a major increase in the state’s capacity for testing as the Gates Foundation and UW teamed up to address the need to improve surveillance. Note that, throughout this period, deaths were steadily rising, but not exhibiting any concurrent jump in numbers. At the same time, the case fatality rate (CFR), which is the ratio of total deaths to total cases, appears to drop dramatically. Although the number of cases was probably rising through this period, most of what we are seeing is an artifact resulting from increased testing. In New York State, we are probably seeing a similar phenomenon due in part to increased testing in New Rochelle related to a local outbreak.
It is interesting to note the actual numbers for the CFR in Washington under the initial, inadequate screening scenario were about 14%. This is seven to fourteen times the current estimates of CFR and if true, would have dire implications. We might expect a somewhat higher rate here because most of the original cases were in nursing home patients, which is the group known to have the highest CFR, but not that high. We don’t currently know what the CFR is here, but it is not 14% because initial reports on the number of cases were gross underestimates.
The takeaway here is that the only numbers in which we can have confidence are the number of deaths, since almost all cases now dying with Covid19 are being correctly diagnosed. Estimates of the number of cases depend heavily on the level of screening, which has been woefully inadequate. That means any data describing either the prevalence of Covid19 or the case fatality rate are suspect and should be interpreted with caution. More importantly, the complete inadequacy of our testing programs are making it impossible to evaluate the threat from the disease or control its spread.
Addendum: The Seattle City Attorney reported that even he had a hard time getting promptly tested when a member of his staff tested positive for Covid-19.