how to apply this approach
our model was designed to provide insight into how vaccines work, and not to comment on specific policy recommendations. there are also important caveats to our study. for instance, we did not study a strategy of prioritizing residents of long-term care facilities, most of whom are older than 60 years of age. we also assumed that covid-19 immunity is long-lasting, although evidence on this is still accumulating. however, the effect we identify relies upon well-validated principles of infectious disease epidemiology, and we found the effect persisted under the various scenarios explored by our modelling.
to apply these ideas in practice, public health authorities would first need to assess population immunity through a survey that tests population blood samples for covid-19 antibodies a few months before vaccination starts. they could then plug these numbers into a mathematical model to determine whether they should prioritize the elderly, or instead prioritize groups that spread the infection the most.
we think this approach could be feasible. standard mathematical modelling frameworks could be used, and their predictions are more reliable when predicting months ahead instead of an entire year. also, antibody surveys are a more accurate way to determine a population’s infection history than case notifications, which would make the model predictions less subject to uncertainties.
ethical dimensions, and looking to the future
some might argue that the clear ethical choice is to give the vaccine first to those who are most vulnerable to fatality from the disease. we argue that decisions should be based on best available evidence and consider both direct and indirect effects. if the evidence suggests we can prevent more deaths in the elderly by vaccinating other age groups first, then that is the ethical decision. in other words, the right thing to do requires careful and sometimes nuanced thinking, but always evidence.
indirect protection may seem like an abstract concept in the grip of a fast-moving pandemic. however, looking ahead , and with hope, we think it will be practically impossible to globally eradicate covid-19 without it, just as in the case of smallpox. vaccination policies will have to take advantage of indirect protection sooner or later, in one context or another.