Revaccination strategies to maximize duration of herd immunity (DHI).

<p>(<b>A</b>) Recurring mass vaccination events (arrows) with 100% coverage of susceptible people every year (dashed line) or two years (dotted line) is shown to periodically achieve then lose herd immunity, designated by the horizontal line at <i>R</i>_<i>e</i> = 1. Faded horizontal bars show times with herd immunity under each strategy and the total DHI is annotated to the right of each. (<b>B</b>) Routine vaccination of 2.4% (green), 3.6% (teal), or 4.8% (purple) of the population per month achieve herd immunity for 0, 4.4, and 4.3 years, respectively. (<b>C</b>) A “Mass and Maintain” strategy with one-time vaccination at 75% coverage followed by routine vaccination of 2.4% (green), 3.6% (teal), or 4.8% (purple) of the population per month can render herd immunity for 1.6, 5.2, and 4.3 years, respectively. The following are held constant for all simulations: population size = 10,000; maximum vaccine courses = 30,000; <i>R</i>₀ = 1.5; migration rate = ; and birth and death rates = .</p

Vaccine targeting optimized in settings with intermediate rates of migration.

<p>Vaccine impact, as measured by the difference in the cumulative probability of an outbreak comparing a mass kOCV campaign (coverage 100%) versus no vaccination, is shown to reach maxima (triangles) at intermediate levels of mobility (x axis). The time since vaccination (colored lines) modifies these maxima. Grey dashed lines denote the estimated migration rates for Calcutta, Bentiu PoC Camp, and Dhaka. In this example, <i>R</i>₀ = 1.5 and the average probability that a migrant is infected is 1/<i>N</i>, where <i>N</i> is the population size.</p

Dynamics of population susceptibility and herd immunity.

<p>Dynamics following mass vaccination (100% coverage) with kOCV (left column) or a hypothetical vaccine with VE = 1 indefinitely (right column). (<b>A-B</b>) Population susceptibility increases over time in the presence of migration rates of (solid line), (dashed line), and zero (dotted). (<b>C-D</b>) The effective reproductive number changes over time with X(t) differently for settings with basic reproductive numbers of 2 (red), 1.5 (green), and 1 (blue). (<b>E-F</b>) The probability that a single case sparks an outbreak of more than 10 cases. Birth and death rates are set to zero in each simulation.</p

Relative contribution of four potential drivers of waning herd immunity in Bentiu PoC Camp.

<p>Relative contribution of four potential drivers of waning herd immunity in Bentiu PoC Camp.</p

Bentiu PoC Camp case study.

<p>(<b>A</b>) Reported population size of the Bentiu PoC Camp (blue line), approximate number of people vaccinated assuming two-dose coverage (green bars), and monthly case counts from October to January (inset grey bars). IOM began reporting entries and exits in December 2015, which are represented by the faint green and red ribbons around the blue line. (<b>B</b>) The proportion susceptible over time (green line) decreases due to mass vaccination events and increases over time since vaccination. (<b>C</b>) The probability that a single case sparks an outbreak of more than 10 cases increases with <i>X</i>(<i>t</i>) and R₀, as represented by line color: R₀ = 1 (blue); 1.5 (green); 1.8 (black); and 2 (red). For reference, R_e = 0 yields an outbreak probability of 0; R_e = 1.01 yields a probability of 0.25; R_e = 1.35 yields a probability of 0.50; R_e = 1.84 yields a probability of 0.75; and R_e>4.66 yields an outbreak probability over 99%.</p