Decisive Conditions for Strategic Vaccination against SARS-CoV-2

Böttcher, Lucas; Nagler, Jan

While vaccines that protect against SARS-CoV-2 are being approved, the number of available doses is limited as it may take months until the production of vaccines can meet the actual demand. The majority of available SARS-CoV-2 vaccines elicits strong immune responses when administered as prime-boost regimens. Since the immunological response to the first (“prime”) injection may provide already a substantial reduction in infectiousness and protection against severe disease, it may be more effective—under certain conditions—to vaccinate as many people as possible with only one shot, instead of administering a person a second (“boost”) shot. Such a strategic vaccination campaign may help to more effectively slow down the spread of SARS-CoV-2, reduce hospitalizations, and reduce fatalities. Yet, the conditions which make single-dose vaccination favorable over prime-boost administrations are not well understood. Here, we formulate a model that helps explore these decisive conditions as a function of the various time scales and epidemiological mechanisms at work. We study how these conditions arise from disease prevalence, vaccination rates, basic reproduction number, prime and prime-boost efficacies, prime-boost intervals, and waning rates. By combining epidemiological modeling, random sampling techniques, and decision tree learning, we find that prime-first vaccination is robustly favored over prime-boost vaccination campaigns, even for high vaccination rates, high disease prevalence, and a relatively low single-dose efficacy.