Frequency of surveillance testing necessary to reduce transmission of SARS-CoV-2

TL;DR

This study estimates how often surveillance testing should be conducted to effectively reduce SARS-CoV-2 transmission, highlighting that twice weekly testing suffices for original strains but not for the Delta variant, especially without rapid contact tracing.

Contribution

It provides a comparative analysis of testing frequencies needed for different SARS-CoV-2 variants, incorporating viral dynamics and vaccination status to inform effective mitigation strategies.

Findings

Twice weekly testing is sufficient for original strains.

More frequent testing is needed for Delta variant to control spread.

Rapid contact tracing is crucial but challenging for Delta variant.

Abstract

We estimate the reduction in transmission of SARS-CoV-2 achievable by surveillance testing of a susceptible population at different frequencies, comparing the cases of both the original Wuhan strain and the Delta variant. We estimate the viral dynamics using viral copy number at first detection combined with considerations arising from aerosol transmission. We take into account the recent findings that infected vaccinated adults may have live viral loads at the same level as infected unvaccinated adults. Our estimates suggest that twice weekly testing, which was adequate for the original strains of SARS-CoV-2 will be insufficient on its own to contain the spread of the Delta variant of concern. We exclude consideration of contact tracing since the rapidity of the onset of viral titre in the case of the Delta variant suggests that unless contact tracing and quarantine are performed very rapidly (ie. much less than a day), these mitigations will be of minimal impact in reducing transmission. These crude estimates do not take into account heterogeneity of susceptibility, social activity, and compliance, nor do they include the additional reduction in transmission that could be achieved by masking and social distancing. In the setting of a large public university, these considerations suggest that risk-targeted testing of vaccinated students, staff and faculty combined with surveillance testing of all unvaccinated individuals is the most efficient way to reduce transmission of COVID-19.