A framework for monitoring population immunity to SARS-CoV-2

Abstract

In the effort to control SARS-CoV-2 transmission, public health agencies in the United States and globally are aiming to increase population immunity. Immunity through vaccination and acquired following recovery from natural infection are the two means to build up population immunity, with vaccination being the safe pathway. However, measuring the contribution to population immunity from vaccination or natural infection is non-trivial. Historical COVID-19 case counts and vaccine coverage are necessary information but are not sufficient to approximate population immunity. Here, we consider the nuances of measuring each and propose an analytical framework for integrating the necessary data on cumulative vaccinations and natural infections at the state and national level. To guide vaccine roll-out and other aspects of control over the coming months, we recommend analytics that combine vaccine coverage with local (e.g. county-level) history of case reports and adjustment for waning antibodies to establish local estimates of population immunity. To do so, the strategic use of minimally-biased serology surveys integrated with vaccine administration data can improve estimates of the aggregate level of immunity to guide data-driven decisions to re-open safely and prioritize vaccination efforts.

Keywords: Covid-19; Herd Immunity; SARS-CoV-2; Serology; Vaccines.

Fig 1

Paths to population immunity through recovery from natural infection and vaccination.Population immunity as a combined function of fraction recovered (x) and fraction vaccinated (y). The heat map (with contours) denotes the estimated fraction of the population that is likely immune to severe infection (f = x + (1-x)*y). Contour lines denote equivalent levels of population level immunity, f. Vaccination initiatives that start with an estimated 30% of population recovered (estimated via serosurveys when accounting for seroreversion) reach 80% population level immunity given an intensive campaign (green; 2/3 vaccinated, 10% more infected); intermediate campaign (yellow; 1/2 vaccinated, 30% more infected), and an incomplete campaign (red; 1/4 vaccinated, 43% more infected). Estimated fatalities associated with start of vaccination campaign and after reaching target population level immunity, albeit with different coverage (fatalities are scaled with the increase in infected population for each scenario). A dashboard estimating population immunity amongst US states is available at https://popimmunity.biosci.gatech.edu. (For interpretation of the references to colour in their figure legend, the reader is referred to the web version of this article.)