Vaccinating people over 60 is the most effective way to mitigate mortality from COVID-19, a new age-based modeling study suggests. Although vaccination of younger adults is projected to avert the greatest incidence of disease, vaccinating older adults will most effectively reduce deaths, the analysis shows. Less than one year after SARS-CoV-2 was identified, deployment of multiple vaccines against the virus has been initiated in several countries. Although vaccine production is being rapidly scaled up, demand will exceed supply for the next several months. An urgent challenge is the optimization of vaccine allocation to maximize public health benefit. To quantify the impact of COVID-19 vaccine prioritization strategies on cumulative incidence of the disease, mortality, and years of life lost, Kate Bubar and colleagues used a mathematical model to compare five age-stratified prioritization strategies. The approach varied assumptions about the total available vaccine supply, country-specific age structure, and age-varying efficacy of a hypothetical vaccine. It used data from countries around the world. In one of the modeling strategies using a highly effective, transmission-blocking vaccine prioritized to adults ages 20-49 years, cumulative disease incidence was minimized. However, in most scenarios where the vaccine was prioritized to adults over 60, mortality and years of life lost were minimized. This suggests optimal benefit - in the terms Bubar et al. evaluated - comes from prioritization of older individuals. If a vaccine is less efficacious in older adults, however, priority could be given to younger age groups, the authors say. To increase the available doses, further priority should be given to seronegative individuals, say the authors, whose work also assessed target-based vaccinations focused on serological status; vaccinating seronegative individuals improved efficiency of the vaccine in reducing overall transmission. The framework the authors applied can be used to compare impacts of prioritization strategies in other contexts, they say. They note several limitations of their study, including that it considers variation in disease risk only by age. They note that "other considerations are crucial, from equity in allocation between countries to disparities in access to healthcare, including vaccination, that vary by neighborhood."
In a related Perspective, Meagan Fitzpatrick and Alison Galvani discuss how the study by Bubar and colleagues suggests the optimal approach for COVID-19 vaccination is different than the optimal strategy for influenza vaccination, which indicates giving vaccines to school-age children as the priority. "Although it may seem intuitive that the optimal strategies against influenza and COVID-19 would be identical," Fitzpatrick and Galvani write, "vaccine optimization is not one size fits all, even for apparently similar pathogens." Shifts necessitating vaccinating older people first for SARS-CoV-2 are related to various factors, they say, including that the average number of secondary infections arising from a single SARS-CoV-2 case, when everyone is assumed to be susceptible, is typically double that of influenza virus. What's more, influenza vaccines have variable age-specific efficacy, with reduced protection for older people; so far, data from ongoing phase III clinical trials of a SARS-CoV-2 vaccines has shown similar efficacy across age groups.