Table 1.

Considerations in the Use of Vaccines for Direct Protection of Vulnerable People vs. Herd Immunity

	Strategy
	Direct Protection	Herd Immunity
Goal	To protect the vaccinated directly against infection, illness, hospitalization, or death	To protect the population (including the nonvaccinated) against infection (and its consequences) by reducing transmission
Criteria for who should receive priority for vaccination	Individuals who will benefit most from the vaccine's effects: groups at high per capita risk of severe outcomes (infection risk × severity)	Individuals at high risk of becoming infected and transmitting infection to others (initially, schoolchildren; also, certain healthcare workers)
Data used to identify priority groups	Predictors of high risk of severe outcome (eg, risk factors for death or hospitalization, compared with the general population) Evidence that the vaccine is effective in the high-risk groups (difficult to obtain in the pandemic setting, but possible to extrapolate from seasonal vaccines)	Incidence rate and/or force of infection by age group22 Estimates of potentially infectious contacts per day in different groups136,137
Factors favoring the strategy	Convincing data on who is at highest risk (Note: Predictors of high risk need not be causal, only reliable markers of high risk.) Good immunogenicity of the vaccine in the high-risk groups Limited quantities of vaccine Late availability of vaccine	High-risk groups are unknown or vaccine has limited effectiveness in them (eg, the elderly during seasonal influenza17) Large supplies of vaccine available in time to significantly reduce transmission7,22,23 Evidence available on the key transmitters22 (Note: This may change over time, as most affected groups become increasingly immune and contribute less to transmission as the epidemic progresses.)