Real time monitoring of COVID-19 intervention effectiveness through contact tracing data

Abstract

Communities worldwide have used vaccines and facemasks to mitigate the COVID-19 pandemic. When an individual opts to vaccinate or wear a mask, they may lower their own risk of becoming infected as well as the risk that they pose to others while infected. The first benefit-reducing susceptibility-has been established across multiple studies, while the second-reducing infectivity-is less well understood. Using a new statistical method, we estimate the efficacy of vaccines and facemasks at reducing both types of risks from contact tracing data collected in an urban setting. We find that vaccination reduced the risk of onward transmission by 40.7% [95% CI 25.8-53.2%] during the Delta wave and 31.0% [95% CI 19.4-40.9%] during the Omicron wave and that mask wearing reduced the risk of infection by 64.2% [95% CI 5.8-77.3%] during the Omicron wave. By harnessing commonly-collected contact tracing data, the approach can broadly provide timely and actionable estimates of intervention efficacy against a rapidly evolving pathogen.

Figure 1

(A): Proportion of contacts successfully investigated across time. There was a significant drop in the contact tracers ability to investigate contacts of index cases during the Omicron surge due to the magnitude of the case load. (B): Distribution of the number of contacts by index case, conditional on an index case having at least one contact, across the time periods when Alpha (02/15/2021–06/15/2021), Delta (06/26/2021–12/10/2021), and Omicron (12/10/2021–03/04/2022) were dominant.

Figure 2

Estimates of intervention effectiveness (in percent) against infection of the contact with 95% credible intervals (vertical black marks). (A): Posterior density of vaccine effectiveness against infection (contact vaccination status). The negative estimate for the time period when Omicron was dominant may stem from excess behavioral risks in the vaccinated group. (B): Posterior density of mask effectiveness against infection (contact masking status). (C): Posterior density of vaccine effectiveness against onward transmission (index vaccination status). (D): Posterior density of mask effectiveness against onward transmission (index masking status).

Figure 3

Risk factors for SARS-CoV-2 transmission on a university campus. (A): Posterior densities or potentially confounding variables across the study period (02/15/2021 through 03/04/2022), including the relationship between the index case and the contact (classmate, roommate, or baseline, which included common relationship types such as friends, partners, co-workers, family), the location of exposure (indoor vs outdoor), the duration of the interaction (longer or shorter than one hour), and whether there was direct physical contact. The odds ratio indicates the odds for those with the indicator relative to those without the indicator. (B): Histogram of the estimated random intercepts of the index cases, representing excess heterogeneity in transmission not explained by the vaccination status of the index case and contact, mask wearing behavior of the index case and contact, relationship between the index case and contact, exposure duration, exposure location, or direct physical contact. Values less than one (greater than one) correspond to index cases who were less (more) infectious than average.