In August, we witnessed a gradual but constant increase in the infection from Covid-19. The number of new positives (in the definition of Civil Protection or daily incidence in epidemiological terminology) rose from about 250 cases at the end of July to about 1350 cases of the first week of September. The increase in daily incidence, in addition to almost tripling the positive totals numbers (in the definition of Civil Protection or prevalence in epidemiological language) going from about 12,500 cases to about 32,000 cases, it is projecting on the number of people hospitalized in intensive care.
In the first graph (Fig. 1), we show an indicator useful for controlling the trend changes in our interest quantity. The indicator is based on the crossing of two delayed moving averages: a short one (at seven days) and a long one (at 14 days). This method is widely used in the analysis of financial markets to confirm changes in the direction of the trends of the securities in which one is interested. Take, for example, two moving averages calculated on the value of a company's shares or a financial fund's value. The first of these two moving averages concerns a short duration while the second a more extended period. When the short moving average passes above the long moving average, there is a buy signal; the stock is increasing in value. Conversely, when the short moving average goes below the long average, there is a sell signal—the value drops.
In this graph, we analyze the number of hospitalized cases that occupy intensive care every day from the beginning of July to the first week of September. The crossing of the short and faster moving average (in red) from bottom to top with the longer and slower moving average (in blue) confirms a change in trend in intensive care admissions at the beginning of August, currently showing a strong tendency to increase.
To analyze this trend change in intensive care units' occupation, it is also interesting to look at the relationship between the number of cases admitted to intensive care and the number of patients hospitalized with milder symptoms. Suppose we consider the whole group of hospitalized people (in intensive care and with milder symptoms). In that case, this ratio represents an estimate of a crucial epidemiological indicator. That is the probability share of the "hospitalization in intensive care" event (in the epidemiological terminology odds, a term derived from the language of betting). The probability share of an event of interest is the ratio between the probabilities in favor of that event and the odds against it. To better understand how odds are used, let's think about betting. Imagine we have 100 men and 80 women; in these two groups, 90 men drank wine last week, and only 20 women did the same. The odds of drinking wine among men are 90 against 10, 9 (90: 10 = 9), while among women, it is 30 against 50, that is, it is 0.6 (30: 50 = 0.6). So if I had to bet who will drink wine during the next week between a man and a woman, I would undoubtedly prefer to bet that it will be a man. Indeed the share of men drinking wine is much higher than that of women. I can also calculate the ratio between the two quotas and see that men have an aptitude for drinking wine 15 times stronger than women (9: 0.6 = 15).
Building on this way of thinking, we try to analyze hospitalizations in intensive care. We are now observing the second graph (Fig. 2), where we report the respective probability share trend from the beginning of July to the first week of September. We can see that the level is still relatively stable, ranging between 5 and 9 (out of 100). The latter shows that there is still no significant change between critical and mild in the composition of total hospitalizations.
One sign of concern comes from the odds ratio. From the beginning of August, you can see a gradual increase in the level of critical hospitalizations compared to mild ones; it rises from about five against 100 to a value of about nine against 100. Between the low at the end of July and the first weeks in September, the ratio between the two quotas is approximately 1.75. In the last month, the latter means that the composition between intensive care admissions and regular ward admissions has changed, with intensive care admissions increased by approximately 75% compared to hospitalizations with mild symptoms.
*This post is born from a chat with Enrico Bucci https://cattiviscienziati.com/author/eb72enrico/
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