TLDR: It can guide it but it cannot determine it.
In a recent MedPage Today OpEd “What Does ‘Follow the Science’ Mean, Anyway?“, Vinay Prasad argues that science alone is not sufficient to guide policy and that and that to inform decision making, it needs to be supplemented with an appropriate value system. In his words:
… science will never be sufficient to guide choices and trade-offs. Science cannot make value judgments.
If we replace “guide” with “determine”, I agree and I would like to clarify how a value judgment can be incorporated in the context of probabilistic inference. Probabilities alone are not sufficient to guide decision-making as they generally do not account for the costs and benefits of a set of possible actions. In other words, knowing the probability that it is going to rain is not enough to decide if you should carry an umbrella — you need to weigh that by the cost of the umbrella and by how much you hate getting wet. From this, you can see that it could be perfectly rational for two different people to act differently under the same weather forecast.
Decision theory, a science that is concerned with making rational decisions, has a long literature on how to encode these costs and benefits — economists call these utility functions, and statisticians, being a more pessimistic bunch, call them loss functions (U = -L). There is nothing unscientific about utility functions as we can study how closely they match people’s risk and reward preferences. So given that we can specify a utility function for say a vaccination policy, we can integrate it over our uncertainty (from the probabilistic model that includes Pr(efficacy)) and maximize this function with respect to the set of contemplated actions. This process can then guide policy by choosing the action with the highest utility. See, for example, Lin et al. (1999) which works out a policy recommendation for home radon measurement and remediation.
Of course, there is a caveat. Even assuming you can write down a set of realistic utility functions, a very difficult task in itself, who’s utility should we choose to maximize? This is where science is completely silent. It does not take a lot of imagination to realize that the utilities of any set of individuals, a utility of a corporation, and a utility of a population as a whole, are likely different. They may be similar but they are not the same. It is in that sense that science can not determine policy — the final choice of one utility function from a set of possible utilities must incorporate the most relevant value system in a society where it is to be applied. People must choose that, science can’t help you there.
References
Lin, C.-Y., Gelman, A., Price, P. N., & Krantz, D. H. (1999). Analysis of Local Decisions Using Hierarchical Modeling, Applied to Home Radon Measurement and Remediation (No. 3; pp. 305–337).