The data is showing vaccination to be highly effective, but the numbers need to be understood as they can, at a quick glance, look paradoxical.
Public Health England (PHE) regularly publishes data on cases, hospitalisations and deaths in vaccinated and unvaccinated people for all COVID-19 cases and for the Delta variant. Some of the numbers can be confusing at first glance. For instance, out of 257 deaths reported, 118 had received two doses of vaccine but out of 1165 hospitalisations only 173 had received two doses. Why the difference?
Tables from PHE report:
The reason this looks puzzling is that the table does not show the denominator – i.e. how many people were fully vaccinated (two doses) to produce these figures?
If you look more carefully you can see that the deaths are divided into people over 50 years (where there are very high vaccination rates in England) and under 50 years (which are quite a bit lower and currently underway). In unvaccinated people over 50 years there were 71 deaths but 116 deaths in two dose recipients.
Does this mean that fully vaccinated people are more likely to die? Answer: NO
As there are many more older people now vaccinated than unvaccinated you have to compare the rates of death by the total number vaccinated versus the total number unvaccinated overall.
If you look at English figures for numbers of people > 50 years and vaccination, there are about 22.8M people who have received two doses, but only about 1.5M who are completely unvaccinated. By adding the denominator to the numerator, you end up with:
This data shows a nearly a 10 fold higher chance of dying in the unvaccinated.
Furthermore, this does not take into account the numbers in the table are for any death occurring within 28 days after a positive COVID test. Especially in the oldest age group (say over 80 rather 50 to 80), many of these deaths may not be due to COVID even though they occurred within 28 days of a test – that is dying but not of COVID.
So, the true difference between vaccinated and unvaccinated COVID deaths is probably even greater.
What about overnight hospital admissions?
For the overnight admissions, the relative numbers of <50 years vs >50 years are reversed from what we would expect without vaccination: 651 for unvaccinated <50 (33 in two dose recipients) vs 82 unvaccinated >50 (140 in two dose recipients).
This reflects the different stage the vaccine rollout in England is at by age – the data show only 3 people aged over 50 who are within 21 days of dose 1 were admitted to hospital overnight (when the vaccine is not expected to be doing much) because the rollout is largely complete for this age group, while 71 people aged under 50 have been recorded with overnight hospital admissions during a time when this age group is actively receiving vaccines.
What can we learn from these data?
It shows that when vaccination rates are high, the relative proportion of those vaccinated with disease changes dramatically. This does not mean the vaccine is not effective, it is just related to the fact that a large proportion of the population is vaccinated. But also important to be aware is that no vaccine is 100% effective so will not prevent every infection.
An old example of this perceptual issue with numbers is the measles vaccine - if you have a high school with 1,000 students of whom 95% are vaccinated and there is a measles outbreak, although the vaccine is 95% effective, you get about the same number of cases in the 950 vaccinated kids (48 = 95% of 950) as you do in the 50 unvaccinated (48 also because not everyone who is unvaccinated gets infected). However if you only had 50% of the school having received measles vaccine, the total number of cases would go up over 500.
Media reports about booster (third) doses being needed for the Pfizer vaccine are based on COVID infections, especially due to concerns with the Delta variant, being seen in places like Israel and England in fully vaccinated people and the fact that total antibody levels go down over, say 6 months, even after two doses.
However, this is not the most important consideration for booster doses in a highly vaccinated population – Israel and England both have over 60% of their populations vaccinated with two doses. The important outcome is reduction in severe disease. In the age group most at risk of severe disease (death and hospitalisation), vaccination rates are over 90%. This means that for them the link between infection and severe disease has now almost disappeared because the age groups most at risk of severe disease are fully vaccinated.
What would prompt the need for booster doses would be if fully vaccinated people not only got infections (which they will – no vaccine is 100% and the percentage protection against infection does decrease with lower antibody levels especially for the Delta variant,) but also got severe disease – needing hospitalisation or ICU or in the worst case dying. Promisingly, there is no sign of that at present, although for people with weakened immune systems there may be an earlier need to consider booster doses.
It is informative to look at what has happened in the Sydney outbreak with the Delta COVID-19 variant, as it could easily happen for example in Auckland – which is a lot like Sydney but with about 20% of the population.
Again, the data from Public Health England is helpful. In England, about 51% of the total population has received two COVID-19 vaccine doses but in Sydney only 9% of the population has. In England, there were 123,000 cases of Delta between 1 February and 21 June 2021, of whom 1.5% were hospitalised. In Sydney, there have been 626 cases to July 12 - 10.1% are hospitalised and 3% are in ICU. This is the difference effective vaccines can make, but of course you have to have received them for that effectiveness to be realised.