When will vaccines let us reopen society?

With every passing day, more Covid immunity is being gained as hundreds of thousands receive the vaccine. Of course, vaccines take time to mature in the body and offer protection, but with roughly a quarter of the population having now received their first inoculation, our approach to dealing with the virus will inevitably need to shift. The big question is how vaccination has changed the equation for how quickly society can be reopened. Modelling from the PCCF project at Bristol University, on cautious assumptions, suggests that the pace of the vaccine rollout would allow significant reopening with herd immunity achieved in July.

First, let’s say how much vaccine immunity has been induced by the vaccine. The latest findings of the PCCF model developed at Bristol University – which is matched against recent ONS antibody survey data – puts the figure today at 44 per cent: 23 per cent coming from recovery from infection, 13 per cent from prior, T-cell immunity, plus a rapidly growing share, already 8 per cent, resulting from the vaccination campaign (see the below chart). So the herd immunity threshold of 70 to 80 per cent is certainly becoming within reach.

When deciding how much freedom to allow, the government should take this into consideration. Over the last 10 days the PCCF has shown, by tracking its Social Distancing Index, that lockdown fatigue may be setting in, with mixing having risen by 25 per cent (see the below chart). Should we be concerned by this? Not at present because the R-rate remains very low (we estimate 0.6, chart three). So infections have been falling (fast) even with more socialisation.

The PCCF model does not simply track how the virus – and society – is behaving. It also offers us projections of what could happen as we reopen society and exit the third lockdown.

Models are only as good as the assumptions that are fed into them. For the PCCF, these are the assumptions being used:

• 92 per cent of those offered a vaccine will have taken or will take one (as per to the latest ONS survey).
• The first dose of any of England's vaccines will prevent 67 per cent of new infections and 95 per cent of deaths once they have matured in the body (as per the AstraZeneca vaccine data).
• The infection fatality rate for Covid-19 is 1.3 per cent. A cautious estimate that is higher than previous estimates. But it reflects the greater threat posed by the new Kent (B.117) variant, which, according to ONS data, is currently responsible for 60 per cent of infections in England.
• The current vaccination trajectory means that those over-50 and above will have received their first injection by the end of April, the (cautious) timetable outlined by the government.
• A two-step release from lockdown is assumed, with the R-rate kept at 0.95 in the first stage starting at the end of January. Keeping the R-rate at this set point means opening up society and the economy ever more as the nation’s overall susceptibility to infection falls.

We will know more on Monday about the strategy for loosening lockdown and the timetable proposed when Boris Johnson delivers his Downing Street speech. At the forefront of government policy is likely to be the goal of keeping the R rate below 1.0 (ie, the virus keeps falling). However, the PCCF model shows that, by the end of May, the nation’s immunity will be such (from the vaccine effect) that the death rate will stay down even if there are no restrictions.

The infection rate, however, is a different matter. Although Covid-19 will have lost a lot of its threat by the end of May, it will still be around, and the nation will not yet have achieved herd immunity. So we can expect infections to rise again, and infections in the community may reach levels not too far below what they were at the start of January (see the below chart). But there is a big difference: fewer people will die of Covid because of the vaccine – which is effective at stopping serious infections. Therefore, even if deaths increase for a time, they will never reach the levels we saw in January and in April last year (see chart five). Although hospitalisations have not been modelled explicitly, the documented success of the vaccines in preventing serious illness should keep these well down too.

But vaccination brings even better news: not only will deaths have dramatically dropped but the herd immunity threshold of 75 per cent should be hit in July. As always, there is some overshoot in the eventual immunity achieved, and it settles out at 85 per cent (see the below chart). This would make it very hard for a new variant to take hold in the way that we have seen with the Kent variant in the months running up to Christmas and at the beginning of this year.

Of course, all models are simplified idealisations of the world – they need to be if they are to help us understand what is going on. And what this does is show just how much immunity vaccination will give us. This should help devising a good exit strategy.

We know that the cost of lockdown is vast, from the loss of education to mental health to the loss of lives from other illnesses and increased poverty. Undoubtedly, there must be some caution shown in order to ensure that the virus does not spiral out of control again. But, on the whole the speed of the vaccine rollout offers an opportunity for normal life to resume far quicker than many might have feared.