Indian Variant Infectiousness Falls Again, PHE Data Shows – Just 10.7% of Contacts Become Infected. So Why Isn’t COVID-19 More Infectious?

The latest Public Health England (PHE) Technical Briefing on the variants of concern is out, and it shows that the infectiousness of the Delta (Indian) variant has dropped again, so that it is now at the same level as the Alpha (British) variant was at the end of March.

Data from PHE

The Delta variant secondary attack rate (SAR – the proportion of contacts an infected person infects) is down to 10.7% this week, from 11.4% a week ago (see graph above). That means that around 90% of the close contacts of people infected with the Delta variant are not infected by them. Once again, this shows how claims that the Delta variant is “60% more transmissible” or similar fail to make clear that transmissibility changes and often declines over time, and that even if the Delta variant is currently more infectious than the Alpha variant it is still less infectious than the Alpha variant was when it first appeared on the scene.

You might be wondering, though, how a disease that only infects 10-15% of close contacts, even at the height of an epidemic wave, can be called highly infectious. And indeed, how such a disease can cause an explosive epidemic wave at all, and why those waves crash down as quickly as they arise despite not exhausting the pool of susceptible people.

If you do wonder that, you would not be alone. A famed epidemiologist called Robert Edgar Hope-Simpson spent his life observing the idiosyncrasies of seasonal influenza and summarised some of his thinking in a 1981 article entitled “The role of season in the epidemiology of influenza“. More recently, in 2008, a group of scientists took up Hope-Simpson’s investigations (which were inconclusive) and summarised seven conundrums of influenza that he identified, all of which apply equally to COVID-19: