Public Health England (PHE) released its latest technical briefing (number 15) yesterday on “variants of concern” which claimed the Delta (Indian) variant is 64% more infectious than the Alpha (British) variant. But look closer and you find this headline finding is not all it seems.
In the underlying study the researchers admit they did not control for the crucial factor of household size – bigger households will tend to have more secondary infections because there are more people in the household being exposed. The authors acknowledge that many of the Delta variant households may have been larger than Alpha variant households (say, because of different proportions of different ethnic groups), and also that many of the matched controls may have lived alone – they have no way of knowing.
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The latest figures for secondary attack rates are 11.3% for the Delta variant versus 8% for the Alpha variant. In relative terms that’s 41% more infectious, but in absolute terms it’s only 3.3% more contacts becoming infected, which is tiny. The fact that 88.7% of contacts of people infected with the Delta variant (and 92% with the Alpha variant) do not become infected is once again an indication of the high levels of immunity that exist in the community to this disease, whatever the variant.
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in just two weeks the Delta variant secondary attack rate has reduced by 16.3%, from 13.5% to 11.3%.
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the Alpha variant itself has reduced in infectiousness from a 12.9% secondary attack rate in January to 8% in May, four months later. While the vaccines will have helped with this to some extent, the sharp drop in the Delta variant attack rate in the last two weeks indicates that it is not just vaccine related. It appears to be related to the phase of the epidemic, with burnt-out variants having lower attack rates and newer variants that are currently causing spikes and surges having higher attack rates.
This means that comparing the infectiousness of a new variant to a burnt-out variant will always come to the conclusion that the new variant is “50% more infectious” or whatever, though this is not an innate property of the variant.
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the Indian variant already has a lower secondary attack rate than the British variant did in January (11.3% versus 12.9%). So much for 64% more transmissible.
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Nonetheless, it does seem to be the case that new variants can evade our immunity (whether from infection or vaccine) slightly more than the old variants and so cause a slightly greater number of infections (most of them mild) and that this may cause spikes of varying sizes (partly depending on the season) as they circulate before burning out.