We have shown a clear difference in development of the COVID-19 epidemic between countries with different containment policies.
Most importantly, we note a dramatically accelerated accumulation of deaths in the United States. Of note, the forecast for the United States is likely to apply to the cumulative number of deaths in the states of New York, New Jersey, and Michigan. This is because the epidemic centered in this area also largely drove the cumulative number of reported deaths so far. If during the coming weeks the entire United States are affected similarly to this area, the total numbers are likely to be 15-fold higher (i.e. over 200,000). Importantly, this number reflects only deaths reported as COVID-19 related, while the true number of deaths to which COVID-19 contributed is likely to be much higher. However, the other parts of the United States might still be able to stay closer to the pattern of temporal development observed in Europe, if strict containment measures are applied immediately. Failing this, the only way in which a total number of deaths in excess of 200,000, by May 15th 2020, seems avoidable would be through a depletion of susceptibles. In this case, the death rate would slow down dramatically before day 50, not because containment measures stopped the spread of the virus, but because everybody who was susceptible has already died. The total number of susceptibles is currently almost impossible to estimate and hoping for a depletion of susceptibles, to end the rapid accumulation of deaths, therefore doesn’t seem advisable.
Further, although the cumulative number of deaths estimated for day 80 in South Korea is low, the epidemic is likely to last much longer there, due to its slow development. Whether the final number of deaths per inhabitant will also be lower therefore remains to be seen.
To appreciate our results, it is important to note that data from different countries are not directly comparable, for at least five distinct reasons. First, the virus did not arrive in all countries simultaneously, causing a desynchronized development of the epidemic in different countries.
Second, absolute numbers are incomparable due to different population sizes. Third, rates per 100,000 of the population are incomparable, because not all countries are affected homogeneously. Especially in the larger countries, like China and the United States, epidemics can be (temporarily) focused on a localized level. For example, in China, the province of Hubei was severely affected, while the rest of the country was not. Therefore, correction for the total size of the Chinese population would not provide a representative figure. Fourth, susceptibility to death by COVID-19 can differ between populations, depending on the demographic composition of a country’s population. For example, in Italy, older people are known to be relatively overrepresented in the population, and to be more likely to be in a single household with relatives from a younger generation, causing increased numbers of elderly to be infected and therefore relatively more COVID-19 mortality. Fifth, a death during the COVID-19 epidemic will only be reported as a COVID-19 related death if the patient was diagnosed with SARS-CoV-2 infection. Therefore, differences in testing policy and guidelines for clinical diagnosis (i.e. in the absence of laboratory testing), will also cause differences in estimated numbers of COVID-19 related deaths.
The first problem was addressed by choosing an appropriate index date for each country and setting this date to day 1, for the start of the epidemic in that country. As an index date, we choose the date of the first reported COVID-19 related death in each country. Admitted, chance processes play a role here, causing some misclassification. The remaining four problems all pertain to the size and the susceptibility of the population, or the probability of a COVID-19 related death being reported as such. Adequately control for all factors influencing these problems is a practical impossibility.
Therefore, we choose to normalize the cumulative number of deaths, by a reference number of deaths. The number of actually reported COVID-19 related deaths is clearly a direct function of the size and susceptibility of the population and the probability of a COVID-19 related death being reported as such. Therefore, taking the reported number of COVID-19 related deaths on a synchronized reference date as a standard will correct results for all these factors simultaneously. In conclusion, although the future development of the epidemic remains difficult to predict accurately, due to changing containment policies, changing seasonal influences, and the possibility of a depletion of susceptibles, or the development of herd immunity,[10, 11] current data suggest the United States to expect an explosive increase in cumulative mortality due to COVID-19, with containment policies still lagging behind. Drastic measures are needed immediately to curb the unprecedented epidemic which is currently unfolding across this country.