The Delta delta

Covid-19 Covid-19

The Delta delta

Posted on: 12th June 2021
By Multiple Authors
Ian Mulheirn
Executive Director for UK Policy and Chief Economist
David Britto

A month ago we were anticipating the government’s next move to Step 3 of the lockdown exit roadmap. Now we await their decision on whether or not to move to Step 4 of the roadmap on 21 June, as planned. But a month is a long time in Covid. The picture has darkened substantially with the Delta variant becoming the dominant force driving a galloping third wave – well before any further easing steps.

First the good news. Vaccine rollout has been progressing at pace, and slightly faster than SAGE assumed in their early May analysis, so vaccine protection levels are now higher across the country than was expected (see Figure 1).

Figure 1: Estimated past infection and vaccination rates across local authorities

Figure 1 notes: Estimates for past infection rates are based on Imperial College’s paper Evaluating the roadmap out of lockdown – step 3, submitted to SAGE on 5 May. We calculate scale factors between Imperial’s cumulative infection estimates for each NHS England region and positive tests as recorded in the UK coronavirus dashboard, and apply these scale factors to positive tests in each local authority to calculate LTLA-level cumulative infection rates as of 9 June

But the bad news comes in two parts. The Delta variant is much more resistant to one dose of any available vaccine and a little bit more resistant even to a second dose. That means the population protection afforded by vaccines has been lowered compared to what it would have been against the Alpha variant (see Figure 2). What does that do to levels of population protection to Delta versus Alpha in different parts of the country?

The right-hand panel of the map below shows our estimates of the level of population protection in each local authority in England from either past infection or vaccination for the Delta variant. The left-hand panel shows the same for the Alpha variant. Comparing the much darker colours for Alpha to the lighter shades for Delta illustrates just how much of a step backwards the new variant is for the level of population protection. On average protection across England now stands at 61% vs Alpha but just 56% versus the now-dominant delta.

Figure 2: Estimated protection levels against Alpha and Delta Covid variants

Figure 2 notes: Protection levels for each LTLA are estimated as a combination of local vaccination rates and past infection rates as of 9 June. For vaccination protection, we assume the latest symptomatic vaccine efficacy results against Delta for AstraZeneca and Pfizer as reported by Public Health England and others on 22 May (AZ: 32.9% first dose and 59.8% second dose; PF: 33.2% first dose and 87.9% second dose). To adjust for vaccine efficacies against infection, we assume that first and second doses further limit onward Delta transmission by 20% and 45% respectively for both AstraZeneca and Pfizer, estimates carried from Imperial’s 5 May paper. We assume a 55:45 split between AstraZeneca and Pfizer rollout based on MHRA disclosures of vaccine delivery by manufacturer to date. We calculate past infection rates based on the previous note, and assume past infection provides equivalent protection to a second dose of AstraZeneca, in line with previous research

The second piece of bad news is that because the Delta variant is perhaps 60% more transmissible, the fabled herd immunity threshold (HIT) – the level of population protection required to end the exponential growth of cases – after Step 4 has increased to around 82%. Putting this together with the protection level estimates shows just how far from herd immunity we currently are even under Step 3, let alone Step 4 (see Figure 3). At current protection levels against Alpha, almost every local authority in England would be above the HIT given current restrictions in place, and converging on the HIT level for Step 4. But against Delta, not only do protection levels melt back, but every area is well short of the substantially higher HIT level even before Step 4 – hence the current surge in cases.

Figure 3: Protection levels and herd immunity thresholds for Alpha and Delta variants

Figure 3 notes: Protection levels for each LTLA are estimated based on the notes under the figures above. Herd Immunity Thresholds are calculated based on Imperial’s 5 May R excluding immunity estimates after Steps 3 and 4 for Alpha, with extra 60% transmissibility applied to adjust for Delta

As we have argued in the past, if protection levels are too low by early June, the obvious call would be to delay Step 4 until the school summer holidays. By then vaccine protection will be higher and transmission rates lower as pupils disperse. This would also buy time for the rollout of vaccinations to adolescents before the autumn term begins. Whether that strategy is enough now seems unlikely. If the pace of vaccine rollout can be accelerated, such a delay combined with other local measures in Delta hotspots might be enough to mitigate a third wave. But even once the process is complete, a large surge in infections looks unavoidable.


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