Are vaccinations less effective in the under 50s?

New data from Public Health England

PHE released their latest “SARS-CoV-2 Variants of Concern” technical briefing last week. The document contains a very useful breakdown of cases of the current main (delta) variant. A particular statistic that caused consternation in some parts was the fact that the mortality rate in the vaccinated group (68 deaths) was higher than in the unvaccinated (38 deaths) in the over 50s. Although this sounds concerning, it is likely to be an artefact: those at risk of dying are the ones who have been vaccinated. The younger age groups, who were more likely to be unvaccinated, were unlikely to die before or after vaccination, and deaths per positive test are lower in the vaccinated than the unvaccinated.

However, the question needs to be asked — are vaccinations less effective in the under 50s?

It is important to note that the outcome in the under 50s is generally benign. The technical briefing states that the household transmission rate is 9% for the alpha variant and 12% for the delta variant, i.e. 88-91% of household contacts do not get infected by an ill family member. The hospitalisation rate for diagnosed cases is less than 1% for the under 50s.

The impression of the severity and extent of COVID-19 is entirely dependent on how testing is carried out. Defining a case as someone who is severely ill with symptoms of a severe acute respiratory syndrome or clotting abnormalities in the presence of the SARS-CoV-2 virus would utterly transform the perception of the extent of disease. The number of tests carried out in the vaccinated versus the unvaccinated cohorts has the potential to create significant bias in the data.

In the PHE data, “vaccinated” means more than 21 days post first dose at the point PHE collected this data on the delta variant. 90% of the over 50s and 32% of the under 50s fall into this category. We can now estimate a hypothetical figure for how many cases there would be altogether in the absence of vaccination by using the numbers in the unvaccinated cohort as a control. The number of cases in the unvaccinated over 50s would represent 10% of this hypothetical total number of cases. This figure can be compared to the actual number of cases in the vaccinated group to give an indication of the effectiveness of vaccination. Using this methodology, the vaccination effect can be calculated for cases, admissions and deaths in both the unvaccinated and vaccinated as shown in table 1.

Under 50 year oldsOver 50 year olds
Positive test results (referred to as cases by Government)46% reduction due to vaccination16% reduction due to vaccination
Admissions (excluding those diagnosed on admission)65% reduction due to vaccination74% reduction due to vaccination
DeathsToo few to measure80% reduction due to vaccination
Table 1: Effectiveness of vaccination against the delta variant based only on those more than 21 days post first vaccination. 

The methodology used above will exaggerate the benefit of vaccination because cases, hospitalisations and deaths in those fewer than 21 days post vaccination have been totally excluded. Also, the percentage vaccinated was taken from 1 June but the data on cases, hospitalisations and deaths started in February when vaccination rates were lower. 

The impact of vaccination on case numbers is not as much as the trial data predicted. However, a case in the trials was diagnosed based on symptoms, whereas current cases are diagnosed based on any positive test. PHE estimated that — for the delta variant  — vaccines resulted in a 79% reduction for symptomatic disease and 96% for hospitalisation. 

Mortality rates are impossible to judge in the absence of information about age. For example, the total number of delta cases in the vaccinated cohort between 7 and 21 June was 17,848 and of these there were 49 deaths. If 18,000 people over 50 were given a blue sticker and any that died within 28 days were labelled as a blue-sticker-death, we would expect 37 deaths. If blue stickers were given out on admission to hospital and in care homes, selecting for people more likely to die, the background figure would be higher than 37 — it is important to remember that many COVID-19 deaths are in fact coincidental deaths “with COVID” not “of COVID”. 

However, vaccination has been so extensive that 43% of vaccinated people are under 50. The figures above would look very different if a high proportion of these deaths in the vaccinated were in the under 50s. 1 in 14 of recent overall COVID-19 deaths and of delta variant COVID-19 deaths were in under 50 year olds, so the assumptions made here are probably fair.

In summary, the data contained in the technical briefing is no cause for undue alarm. While there are many confounding factors and error bars are wide (especially for the younger cohorts as hospitalisation rates are so low), the data indicates that those people who are at least 21 days after their first vaccination are less likely to get hospitalised with COVID-19. Further analysis, including age-stratified data and more information on outcomes within 21 days of their first vaccination, would help shed more light on vaccine effectiveness.

Please follow and like us:
Visit Us