Deep Impact? Analysing the Effect of the COVID 19 Vaccine on UK Case Numbers & Deaths

Female scientist with syringe

It’s been almost 9 weeks since Margaret Keenan became the first person in the UK to receive a COVID-19 vaccine as part of a vaccination programme. The UK government had set themselves an ambitious target of offering a vaccine to its top four priority groups in the UK by 15th February. This was the equivalent of 15 million people and on 14th February we learned that this milestone had been reached.

Getting over this first hurdle is one thing, but understanding the impact of the UK vaccination programme is going to be essential in easing lockdown restrictions. PHASTAR’s Vice President for Statistical Research and Consultancy, Jennifer Rogers, is being asked more and more whether vaccines are having an impact on COVID-19 case numbers and deaths. But what does the data show?

Before we look at the data in more detail, let’s remind ourselves of those priority groups identified by the UK government. The top four priority groups for UK COVID-19 vaccination are:

  1. Residents in a care home for older adults and staff working in care homes for older adults
  2. All those 80 years of age and over and frontline health and social care workers
  3. All those 75 years of age and over
  4. All those 70 years of age and over and clinically extremely vulnerable individuals (not including pregnant women and those under 16 years of age)

If we therefore examine falls in cases and deaths by age group and observe differences in the rates of change, this should give us a good indication that vaccines are starting to have an effect. If cases and death rates are falling quicker in those groups that have been vaccinated compared to those that haven’t, this would suggest that the vaccines are having an impact. Data from the government daily update dashboard is available for download here and Public Health England collects daily COVID-19 cases (by specimen date) and COVID-19 deaths by age group. The analysis that we carry out will therefore be on English data only, but there is no reason to suspect that the conclusions wouldn’t be generalisable to other nations within the UK. Also note that the most up to date data available was to 8th February 2021.

The first key thing to consider is that for the top four priority groups, we would expect to see the biggest impact of vaccines on COVID-19 deaths. If we look at all the COVID-19 case data and death data that has been collected to date, those aged 70 and over have accounted for only 11% of COVID-19 cases, but 83% of deaths. So let’s start by looking at COVID-19 deaths in 2021 first:

Looking at the plot, it certainly does look like the gradient of the curve for the older generations is steeper than for the younger generations. Considering the last two weeks of data and comparing death figures on 26th January with the figures for 8th February, a crude analysis shows that the 90+ age group has seen COVID-19 deaths fall by 46%, compared with the 50-59 which saw a reduction of 37%. To examine the fall in deaths further, we can fit quasi-Poisson regression models to the data and estimate the weekly fall in deaths over the two-week period. This gives us:

We see that those aged 90+ have had weekly reductions in COVID-19 deaths of 35%, compared with 22% for those aged 50-59. Plotting the fitted curves shows clearly that deaths seem to be coming down faster in those groups that have been vaccinated versus those that haven’t.

One vaccinated age group that doesn’t seem to have seen as dramatic a fall in COVID-19 deaths is the 70-79 age group. Looking at the latest vaccination data from NHS England (announced 11 February for data up to 7th February), we can calculate that 91.26% of the estimated 80+ population have received their first dose of vaccine, 95.62% of those aged 75-79, but just 74.02% of those aged 70-74. Furthermore, the vaccination programme has been working its way through the priority groups in order, meaning that someone in the 70-74 age group would likely receive their vaccine after someone in the 80+ age group. There is also a substantial time lag between an individual receiving a vaccine and seeing the impact on subsequent COVID-19 deaths as there is a lag between a person being vaccinated and them receiving protection, and lags between a person being infected then presenting with symptoms, then requiring hospitalisation, and then dying. So it may be a few more weeks before we see the full effect of vaccination on this age group, but there are indications of a positive response.

Some further questions that I have been asked with respect to the impact of the vaccine rollout are:

 “Could the reductions we are seeing not just be because we are in lockdown?”

We are all currently under lockdown, so there is no reason to expect that staying at home would affect some age groups more than others. Lockdown is having an effect on cases and deaths coming down overall, but the differences that we’re seeing in reductions between age groups is likely to be, at least in part, due to vaccines.

 “You have taken a look at the effect of vaccines on deaths, but what about cases?”

If we carry out the same analysis on COVID-19 cases, we see again that falls in the vaccinated groups are larger than the unvaccinated groups, but the differences aren’t as impressive. Those aged 90+ have seen a weekly reduction in cases of 35%, compared with a weekly reduction of 32% for those aged 50-59. There are a couple of reasons why this might be the case. Firstly, we know from clinical trial results that vaccine efficacy has been typically larger for severe disease and death when compared with vaccine efficacy associated with mild COVID-19. This means that we may expect to see bigger impacts of vaccines on deaths than we would cases. Also, if we look at the age distribution of cases, 52% of all COVID-19 cases occur in individuals who are aged 20-49, so we’re not going to see any big reductions in cases until we start to vaccinate these age groups.

 It’s going to be some time yet before the full impact of the vaccination programme is thoroughly understood. And, as with so many other questions associated with this pandemic, answering them in an ever-changing environment is not a trivial task. Trying to tease out the impact of vaccines whilst being in the midst of a lockdown which is also driving down counts makes the task just that little bit trickier. But I think we’re starting to see the positive effect of vaccines and it will be interesting to see how this develops in the coming weeks and months as more and more individuals receive their vaccinations.