The scale of the Omicron escalation

A huge escalation in cases, but what has been the mortality impact?

Since mid-December 2021, the world has experienced a major escalation of the pandemic, which reflects the combined spread of Omicron and Delta at a time where the Northern Hemisphere experiences cooler weather and the risk of indoor exposure rises. The general pattern thus far has been for the trajectory of cases to be explosive while the impact on mortality remains muted compared to earlier waves. But how well do these patterns hold up when we examine them more closely?

In what follows, we document the scale of escalation of the Omicron/Delta episode through three different lenses. First, we examine at the country level the levels of peak case and mortality rates before November 1, 2021 and after through May 1, 2022. Second, we examine the relative escalation in case and mortality rates by World Bank income classification. Third, we do the same by World Bank region. 

The relative perspective usefully complements the absolute one as it helps control for inequalities in data quality and other structural features as well as previous pandemic performance. 

Absolute comparisons

Let’s start off with the absolute comparisons. The chart below plots the peak case rate before and after November 1, 2021. The date is chosen to stay well clear of the December period where the major escalation really started. Note that case rates are measured as the 7-day trailing average of daily confirmed cases per 100,000 people. We calculate the peak value of this indicator for both periods. The size of the balloons reflects population size and they are colored according to the World Bank income group the country belongs to.

The chart shows a number of things. First, the peaks during earlier waves have generally been higher for high-income and upper-middle-income countries (HICs and UMICs). Second, the peaks during the current episode have been especially high for almost all HICs, as is demonstrated by their values being well above the 45-degree line. So, in absolute terms, we see the highest degree of escalation among countries that had high case rates to begin with!

We can show the same for the mortality rate (see above). Measured mortality rates in HICs and UMICs tends to be larger than for lower-middle-income and low-income countries (LMICs and LICs) – even though that is not true for excess death rates. But the impact of the most recent wave on mortality rates has been quite different compared to cases. Where mortality rates have been high, they have generally remained high even though there’s been a tendency for the recent peaks to be lower. Unlike the picture for cases, there is no escalation in the mortality rate. 

Relative comparisons

Let’s now take the relative view and look at how population-weighted averages behave. We first start with the country groupings of the World Bank income classification. Note that the peaks are calculated on these weighted averages. We show the relative increase in peak case and mortality rates before and after November 1, 2021 (through May 1, 2022).

The chart clearly shows that on the cases side the escalation has been especially major in HICs and to a lesser extent in UMICs and LICs. For LMICs so far the peak has remained well below the previous peak (as indicated by a ratio below 1). 

On the mortality side, we see that throughout the mortality peak has remained at a fraction of the earlier value. That fraction is not negligible, but compared to the scale of escalation in cases it does appear to be more muted. Note here also that the mortality impact reflects the combined effect of Omicron and Delta. 

Interestingly, the degree of relative escalation is rather similar across country groups, which is a curious result to interpret as it will be the combined effect of differences in vaccination profiles, immunity from prior infection, seasonality and other environmental and host-specific factors. 

In the chart above, we replicate the same analysis at the level of World Bank regions (see footnote for acronyms). The most serious escalations have happened thus far in EAP, ECA, NAM and LAC. That’s for cases. The mortality escalation has thus far remained under 1, meaning lower impact than previous peaks (thus far). But we can already see that the relative mortality impact is very considerable for some regions. 

In sum

The above charts capture the scale of the Omicron/ Delta escalation as it happened between November 2021 and May 2022.  The general pattern is one of a dramatic escalation of cases, which centered on high-income and low-income countries. Luckily, while there are important exceptions at the country level, the mortality impact has remained muted across income groups and regions at a fraction of previous waves.

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