Montezuma’s Revenge

A recent paper, published at the end of last month, did the obvious, a scatter plot of covid vaccination rates against recent cases for 68 countries, and got the sort of plot that looks like a loo pan after it has been used by a person with a particularly bad case of gas powered Montezuma’s revenge. Dots everywhere, but very little focus. Such a scatter plot suggests there is no connection between the variables, or in the case of covid vaccination, it has no discernable effect on case numbers, despite the fact the authors fitted a trend line that suggests a slight increase in case numbers in countries with higher vaccination rates. As ever, we have to remember that cases is a misnomer: they are merely positive PCR tests, that may or may not also be cases, and of course, just as correlation is not proof of causation, so too is the opposite true: lack of correlation is not necessarily proof of no connection. This applies particularly in a cross sectional study such as this, where the epidemiologist’s sword has cut a slice through a single moment in time.

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