A comparison of alternative vaccination strategies for protecting those vulnerable to illness, hospitalization, or death upon infection with SARS-CoV-2.

U.S. vaccination policymakers initially prioritized healthcare, other frontline essential workers, and those at greatest risk of severe COVID-19. Protection may be direct, by decreasing susceptibility to infection if exposed, or indirect, by decreasing the risk of exposure through the vaccination of others. We modified our published SARS-CoV-2 transmission model by including immunity-moderated disease, updating some parameters, and making others time-varying. Then we derived the metapopulation reproduction numbers and related analytical quantities and compared the respective impacts of protecting vulnerable people directly and indirectly from 14 December 2020, when vaccination began, to 11 May 2024, when the U.S. Public Health Emergency officially ended. We found that proportionately more young people, particularly adolescents, had antibodies to nucleocapsid protein than adults before schools reopened for the 2021-22 school year, their vaccination lagged far behind that of older people, especially during the Omicron epoch, and they caused the most secondary infections per capita. Nonetheless, while vaccinating younger people would have reduced transmission more, we found that - if the effectiveness of vaccination and duration of vaccine-induced immunity were age-independent, as limited available evidence suggests - the actual strategy averted more illnesses, hospitalizations, and deaths. Because of the age-dependence of severe COVID-19, which policy was designed to mitigate, evidently vaccinating older people was more effective than vaccinating the younger ones who might otherwise have infected them. This result would not hold if, due to immune senescence, the effectiveness of vaccination or duration of vaccine-induced immunity varied inversely with age.