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TopConfirmed Cases Is What We Do Know
What we do know is the number of confirmed cases. The World Health Organization (WHO) explains that a confirmed case is “a person with laboratory confirmation of COVID-19 infection, irrespective of clinical signs and symptoms” (Maier & Brockmann 2020). The WHO also speaks of ‘suspected cases’ and ‘probable cases’, but the WHO Situation Reports do not provide figures on ‘probable cases’, and only report ‘suspected cases’ for Chinese provinces (‘suspected cases’ by country is not available) (Vaidya et al., 2016). The daily Situation Reports list the number of confirmed cases; we rely on these reported numbers for the regular updates of our own datasets presented below. As explained, the number of confirmed cases is lower than the number of total cases because not everyone is tested.
Growth of cases: How long did it take for the number of confirmed cases to double? In the section below we present the latest data on the number of confirmed cases by country, and how this has changed over time. But in an outbreak of an infectious disease it is important to not only focus on the number of cases, but also the growth rate at which the number of cases is increasing. The growth rate is an essential metric to understand and to monitor. This is because a fast growth rate can lead to very large numbers rapidly, even if the current numbers of cases and deaths are small when compared with other diseases. A helpful metric to measure the rate of change is to look at each country and ask: How long did it take for the number of confirmed cases to double? Let’s take an example: if the number of confirmed cases as of today is 1000, and there were only 500 cases three days ago then we would say that it took three days for the number of confirmed cases to double (Danon et al., 2020). The doubling time of cases has changed and it will change in the future. It would be wrong to extrapolate current growth into the future. But it is important to keep focusing on the doubling time. As long as cases are doubling at a constant rate the growth is exponential. We humans tend to think in linear growth processes even when the growth is exponential, as psychological research has shown for decades. Below we give some intuition about exponential growth and provide the referenced psychological research on this.