Vaccination and non-pharmaceutical interventions for COVID-19: a mathematical modelling study
University of Warwick study d
ated 18 Mar (I have precised the summary and drawn some key bits from the paper.)
"Under plausible assumptions for efficacy and uptake, the UK is unlikely to reach the herd immunity threshold through vaccination. We predict that only gradual release of non-pharmaceutical interventions (NPIs) coupled with high uptake of a high-efficacy vaccine can prevent subsequent waves of infection."
Predicted daily deaths from COVID-19 in the UK after the start of an immunisation programme and relaxation or removal of NPIs
Shading indicates the level of NPIs implemented. (A, B) The effect of relaxing current NPI measures down to those implemented in early Sep 20. The dashed line indicates the point of partial NPI relaxation—Feb 21, in panel A and Apr 21, in panel B.
Protection against infection [and hence onward transmission] was varied from 0% to 85% in model input.
Study used epidemiological data from the UK together with estimates of vaccine effectiveness to predict the possible long-term dynamics of COVID-19 under the planned vaccine rollout.
Method
Mathematical model structured by age and UK region, fitted to a range of epidemiological data in the UK,
Assumes:
- Vaccination programme as planned: all adults by end July.
- Vaccine uptake of 95% in 80+, and assumed 85% in 50–79 and 75% in 18–49s.
- Vaccine effectiveness against symptomatic disease was assumed to be 88%
- No change in dominant SARS-COV-2 variant transmissibility or mortality.
Considered the combined interaction of the UK vaccination programme with relaxations of NPIs, to predict the contemporary reproduction number (
R) and pattern of daily deaths and hospital admissions due to COVID-19 from Jan 2021 to end 2023.
Findings
Though efficacy against disease is of specific individual benefit (protecting against severe symptoms), it is the
vaccine protection against infection [and hence onward transmission] that leads to a reduction in the intrinsic growth rate and R.
But vaccination alone is insufficient to contain the outbreak.
With no NPIs and optimistic assumption of 85% prevention of infections,
R is estimated to be 1·58 (plus 0.26 or minus 0.22) even with all adults vaccinated. [Vaccination of 12-17s (if trials are successful) makes little difference, maybe 0.05 to
R.] Removal of
all NPIs in late 2021, once the vaccination programme is complete, is predicted to lead to 21,400 COVID-19 assigned deaths over 2 years (2022 and 2023) (very low confidence - wide range: 1,400–55,100) - much worse (quadruple) if vaccine only prevents 60% of infections.
Although vaccination substantially reduces total deaths, it only provides partial protection for the individual. For the default scenario and only 60% protection against infection, 16% of the deaths will be individuals who had been vaccinated (with 2 doses).
Several key vaccine parameters within the model are based on (study's words)
parsimonious assumptions: 3 limitations:
- determining whether the vaccine prevents infection is key for the development of population immunity and the potential for the vaccine to further reduce viral shedding from vaccinated individuals, reducing onward transmission,
- determining if the vaccine offers greater protection against the most severe disease: this will reduce predictions for hospital admissions and deaths.
- efficacy estimates are emerging, with the estimated ranges of efficacy for each vaccine subject to revision as new data emerge.
Maintaining low levels of infection is likely to be key to the success of test, trace, and isolate strategies and in reducing the risk of vaccine escape.