Vaccines, overconfidence and 98% cuts
An open letter makes a strong claim about vaccine effectiveness.
How much do vaccines cut deaths from COVID-19? An open letter from scientists, published in the Mail on Sunday, asserts:
We can be very confident that they will reduce Covid deaths by around 98 per cent and serious illness by 80–85 per cent.
This figure does not come from a vaccine effectiveness study. This post looks at those two claims.
How well do vaccines protect against death?
Vaccines can limit deaths in three ways:
- Infection prevention: it stops people becoming infected with the virus.
- Reducing severity: among people who still get infected, that infection is less severe — with fewer deaths.
- Dampening transmission: of those infected, it can lower the likelihood of passing on the virus. That reduces further cases and later deaths.
The UK daily COVID-19 deaths measure reached a peak of 1,360 on 19th January. In the United Kingdom (without Wales), that counts deaths within 28 days of a positive test. Public Health Wales uses a different definition of confirmed deaths. That agency counts deaths with clinical suspicion COVID-19 contributed to the death. The centred seven-day average was 1,285.
By 13th April, that deaths measure fell to 20. The centred seven-day average was about 21. The reduction in rolling average deaths was around 98%. The letter appears to attribute the entire fall in recorded deaths to vaccines.
That ignores the impact of public restrictions. In England, there have been three national stay-at-home orders. Those lock-downs started on: 23rd March 2020, 5th November 2020, and 6th January 2021. Other restrictions, including local lock-downs and tiers, have also permeated.
Lock-downs and vaccines are a tag-team: cutting cases and making infections less severe.
In their analysis, Public Health England estimate from their effectiveness studies:
Vaccine effectiveness against mortality was based on PHE estimates of effectiveness of vaccination against symptomatic infection (58%) and of death given infection (54%) which combined gives 81% protection against death[.]
The mortality impact analysis does not account for dampened transmission.
What about the “serious illness” claim?
If we take “serious illness” to mean hospitalisations, the second range is about right:
Combined with the reduced risk of becoming a case (Section 3) this is consistent with vaccine effectiveness against hospitalisation of around 80%.
A prospective cohort study in Scotland estimated vaccine effectiveness against hospitalisations. Different periods after vaccination came to different numbers. After vaccination, there was an adjusted fall in hospitalisations:
- 21 to 27 days after the first dose: 76% (70% to 81%)
- 28 to 34 days: 89% (83% to 92%)
- 35 to 41 days: 78% (68% to 85%)
It is very likely both vaccines and restrictions played a role in cutting deaths. In England, vaccine effectiveness against COVID-19 deaths is around 80%. That figure is for one dose: protection is likely to be higher after the full course.
Isolating the effect of one intervention is challenging. We should stress uncertainty, and avoid overconfidence.
