This Q&A was co-authored by Charlotte Milbank [@epidummyology] and Dr Hazel Wallace [founder of @thefoodmedic]
Efficacy vs. effectiveness
Both terms are thrown around a lot but there are important DIFFERENCES between them.
EFFICACY…
refers to the REDUCTION in disease that we see in vaccinated individuals compared to unvaccinated individuals (ie those given a placebo). Importantly, efficacy is calculated based on the results of our highly controlled clinical trials, which might differ from what we see in real-life.
EFFECTIVENESS…
is best thought of as “efficacy in real-life” & is important to the overall success of our vaccine rollout.
How do effectiveness studies work?
Effectiveness against hospitalisation
A recent study from Scotland (1) compared the hospitalisation rates in people who have and have NOT had the first dose of (any) vaccine.
For Pfizer-BioNTech vaccine…
the estimated effectiveness against hospitalisation was 85% at 28-34 days after dose 1, and 66% over 35 days after vaccination
For the Oxford vaccine…
vaccine effectiveness against hospitalisation at 28-34 days after dose 1 was 94%
Effectiveness against infection and transmission
The “SIREN study” (2) – looked at the effectiveness of vaccines against INFECTION (asymptomatic or symptomatic) in healthcare workers.
This estimated vaccine effectiveness was 72% at three weeks after a single dose of the vaccine. This increased to 86% at seven days after a second dose.
The findings from the SIREN study are supported by recent data from Cambridge (3), UK, that suggested 75% effectiveness against asymptomatic infection amongst healthcare workers who have been vaccinated for more than 12 days with a single dose of the Pfizer-BioNTech vaccine.
Data from Israel (4) shows a similar (75%) reduction in infection in vaccinated healthcare workers at 15-28 days after one vaccine dose.
Summary of findings
Conclusions
These are still early findings but this is all very good news, demonstrating high effectiveness against severe disease and infection. Since we are talking about the prevention of any infection, this means we can be confident our vaccines are highly effective against transmission too.
These results also give us confidence that our vaccines are highly effective against dominant variants of COVID-19 here in the UK (the “Kent variant”)
As always, these studies experience issues of bias and confounding – but nonetheless, these results give us confidence in the effectiveness of the vaccines. This is not our only solution to eliminating the virus. It is still important we abide by our gradual roadmap out of lockdown.
References
[1] Vasileiou et al 2021. Effectiveness of First Dose of COVID-19 Vaccines Against Hospital Admissions in Scotland: National Prospective Cohort Study of 5.4 Million People
[2] Hall et al 2021. Effectiveness of BNT162b2 mRNA Vaccine Against Infection and COVID-19 Vaccine Coverage in Healthcare Workers in England, Multicentre Prospective Cohort Study (the SIREN Study)
[3] Weekes et al 2021. Single-dose BNT162b2 vaccine protects against asymptomatic SARS-CoV-2 infection
[4] Amit et al 2021. Early rate reductions of SARS-CoV-2 infection and COVID-19 in BNT162b2 vaccine recipients. The Lancet.
