Vaccines against the SARS-CoV-2 virus appear, finally, to offer nations around the globe a pathway out of the COVID pandemic. Yet vaccines themselves come with potential risks of side-effects. In the aftermath of some three dozen or so cases of deaths from blood clots in recently vaccinated individuals, a handful of European nations recently paused their delivery of the AstraZeneca vaccine pending further study.
Was this a sensible precaution? Or, perhaps, an over-reaction given what we know about the risks from SARS-CoV-2 infections and vaccine side-effects?
In a new paper (still under peer review), LML External Fellow Davide Faranda and colleagues address these questions. Their aim is to give a rough yet plausible estimate of the number of excess deaths likely to result from pausing vaccinations, taking into account both potential reduction in deaths from blood clots (linked to the medical condition of Deep Vein Thrombosis), and the likely additional deaths linked to unvaccinated individuals becoming infected. They focus on Italy and France, two nations which temporarily paused vaccinations.
To make their estimates, Faranda and colleagues employ a standard epidemiological model – the Susceptible-Exposed-Infected-Recovered (SEIR) model – as well as statistical analyses of risks from Deep Vein Thrombosis. Importantly, given the limited understanding of any risks associated with blood clots, they take an extremely conservative approach and ensure their model almost certainly overestimates by a significant margin the number of lives saved by avoiding blood clotting side-effects. Despite this simplification, however, their modelling still finds that suspending the AstraZeneca vaccination for a week (without substitution with another vaccine) would lead to approximately 800 additional deaths in Italy and 160 in France. The difference between the countries arises because the number of infections is currently growing faster in Italy.
The authors stress that these estimates are preliminary, and should be backed up with more detailed modelling studies. Even so, the simplest plausible estimates suggest that the benefits of deploying the vaccine greatly outweigh the associated risks, and increase rapidly with the temporal duration of the vaccination pause. It’s good news that most European nations which had paused AstraZeneca vaccinations have now resumed them. Even so, the estimates presented here show that a pause of a few days will have had real consequences – tens or hundreds of further deaths. Moreover, in nations in which the infection reproduction number is greater than one, infections of the unvaccinated would have led to infections of still more people. A pause of just a few days is a real set back – it takes more vaccinations than those missed out to get back to the same level of epidemic control.
The paper is available as a pre-print at https://doi.org/10.13140/RG.2.2.15954.89285