Intervention Serology and Interaction Substitution: Modeling the Role of 'Shield Immunity' in Reducing COVID-19 Epidemic Spread

Joshua S Weitz; Stephen J Beckett; Ashley R Coenen; David Demory; Marian Dominguez-Mirazo; Jonathan Dushoff; Chung-Yin Leung; Guanlin Li; Andreea Magalie; Sang Woo Park; Rogelio Rodriguez-Gonzalez; Shashwat Shivam; Conan Zhao

than 60; Figures S2-S3) . Second, shield immunity is robust to variation in asymptomatic infection probabilities, improving outcomes in models with varying baseline levels of asymptomatic transmission (see Figures S4-S6 ). In addition, the present model results assumes that immunity has relatively fast onset and is permanent in duration. Clinical work in Zhejiang University, China, suggests that seroconversion of total antibody (Ab), IgM and IgG antibodies developed with a median period of 15, 18, and 20 days, respectively (albeit for symptomatic patients in a hospital; similar data for seroconversion of asymptomatic individuals was not included [30] ). In the SI we show that impacts of shield immunity is robust insofar as the duration of immunity is 4 months or longer ( Figures S7-S8) ; we note that distinct control measures that extend the epidemic would likely impact effectiveness of shield immunity. For context, the titer of protective antibodies in individuals infected with related betacoronaviruses (causing mild/moderate symptoms) reduced over a one year period such that re-exposure can lead to re-infection [31] , in contrast to evidence of multi-year immunity for individuals recovered from SARS [32] . In addition, we emphasize that the accuracy of serological tests is key. The benefits of shield immunity can be undermined if recovered individuals can be reinfected (even with little danger to them), or potentially misidentified, leading to interaction substitution with individuals that could infect others (a risk reduced by combining serology with PCR).