The data from the two most recent technical briefings (No. 18, published July 9th, and No. 19, published July 23rd) from Public Health England shows that vaccine effectiveness against infection in the over-50s has plummeted to just 17% in the month between June 21st and July 19th, which corresponds to the main Delta surge.
The data is derived by subtracting the figures in table 5 in briefing 18 (up to June 21st) from those in briefing 19 (up to July 19th) to give just the data for the intervening period.
Briefing 18 gives 9,885 ‘cases’ of the Delta variant in those over 50 who have received at least one vaccine dose and 1,267 ‘cases’ in the unvaccinated over 50 up to June 21st. Briefing 19 gives 18,873 ‘cases’ in the vaccinated and 2,337 in the unvaccinated for the period up to July 19th. Subtracting the earlier from the later gives 8,988 ‘cases’ in the vaccinated and 1,070 in the unvaccinated for the month from June 21st to July 19th.
To calculate the vaccine effectiveness we need to know the infection rate in the vaccinated and unvaccinated groups. According to the PHE week 29 surveillance report, as of July 18th, 20,331,977 people over 50 in England were vaccinated and 2,016,818 were unvaccinated. This gives an infection rate in the vaccinated of 0.044% and an infection rate in the unvaccinated of 0.053%.
Vaccine effectiveness can be calculated by dividing the infection rate in the vaccinated by the infection rate in the unvaccinated and taking away from 100%. Thus 0.044/0.053 gives a vaccine effectiveness of 16.7%, which rounds to 17%.
PHE has recently published a study in the NEJM claiming vaccine effectiveness against the Delta variant of 67% for AstraZeneca and 88% for Pfizer. However, that was using data from April and May when prevalence was very low. This data is from the recent surge in the Delta variant, which is when vaccine effectiveness is really tested. Like recent data from the surge in Israel, it shows that vaccine effectiveness against infection is not holding up.
As noted yesterday, vaccine effectiveness against serious disease and death appears to be holding up much better. This means that those who see vaccines as the necessary pre-condition to removing restrictions don’t need to change course. However, it does indicate that vaccines do little to prevent infection or transmission and so there is no justification for vaccinating young people and children to protect others, or for any coercive measures like vaccine passports. Each person can be free to weigh up the risks and benefits of being vaccinated for themselves.
This article has been updated.