Association Between the Relaxation of Public Health and Social Measures and Transmission of the SARS-CoV-2 Omicron Variant in South Korea.

Introduction

It is essential to effectively manage the timing of relaxing public health and social measures (PHSMs) during a pandemic while considering the available capacity of the health system.[1] South Korea used a very successful SARS-CoV-2 infection mitigation strategy throughout the COVID-19 pandemic during 2020-2021[2] and began to relax PHSMs during the early phase of the Omicron variant (lineage BA.1) wave (mid-January 2022).[3] On March 16, 2022, the daily number of COVID-19 cases was approximately 600 000, the largest increase in the number of new daily cases worldwide since the pandemic began.[4] This study assessed the association of changing PHSMs with the transmissibility of the Omicron variant in South Korea.

Methods

This observational cohort study was approved and informed consent was waived by Konyang University Institutional Review Board because all data used were anonymous and publicly available on a public health agency website. Data on the daily number of laboratory-confirmed COVID-19 local cases by polymerase chain reaction test or rapid antigen test results and the proportion of Omicron cases were acquired from the Korean public health authorities during the early phase of the Omicron (lineage BA.1) wave: December 1, 2021, to March 16, 2022.[5] We multiplied the daily number of COVID-19 local cases by the rate of positive Omicron test results to obtain a proxy of Omicron activity in the community. We reconstructed the daily number of Omicron cases by using a deconvolution-based method, which adjusts for the delay between the date of infection and confirmation (eMethods in the Supplement). Then, we estimated the time-varying effective reproduction number, Rt, which represents the mean number of infections resulting from a case infected at time t (eMethods in the Supplement). The epidemic in a community is considered under control if Rt remains below 1 sustainably.

Changes in PHSMs nationwide during the study period included relaxation of restrictions for social gatherings, suspension of the need for vaccine passes in public facilities, expansion of community COVID-19 screening, and nationwide mandatory screening for students in schools (Table). To assess the association of relaxing PHSMs with Rt, we fitted a least absolute shrinkage and selection operator regression model to the log of Rt to reduce multicollinearity among the PHSMs (eMethods in the Supplement). Data were analyzed with R, version 4.0.1.

Table.

Overall Changes in Time-Varying Effective Reproduction Number With Changing PHSMs During the Omicron Wave of SARS-CoV-2 Infections in South Korea, 2022

Change in PHSMs (effective period)	Change in Rt associated with changes in PHSMs, % (95% CI)	Change details
Relaxation in restrictions for social gatherings (January 14)	20.4 (5.8 to 36.5)	Restrictions on the number of people in social gatherings were eased from ≤5 to ≤7
Suspension of need for vaccine passes (January 18)	10.9 (3.2 to 24.8)	The need for vaccine pass testing using QR code scanning was suspended in public facilities nationwide
Extension of community COVID-19 screening centers (January 26)	−16.8 (−25.2 to −11.1)	Screening facilities were extended to include primary care clinics in the community
Mandatory nationwide COVID-19 screening for all students in schools (March 2)	−18.8 (−22.2 to −11.1)	In-person classes were resumed, and a mandatory rapid antigen test using a nasal swab for COVID-19 screening was conducted twice a week

Abbreviations: PHSMs, public health and social measures; QR, quick response; Rt, time-varying effective reproduction number of Omicron variant.

Results

The Figure shows the daily number of COVID-19 cases and the estimated Rt of the Omicron variant. The Rt was sustained at greater than 1 during the study period. The relaxation of restrictions for social gatherings and suspension of the need for vaccine passes were associated with increases in Rt of 20.4% (95% CI, 5.8%-36.5%) and 10.9% (3.2%-24.8%), respectively (Table). However, extending community COVID-19 screening centers and mandatory COVID-19 screening for all students in schools were associated with reductions in Rt of −16.8% (−25.2% to −11.1%) and −18.8% (−22.2% to −11.1%), respectively.

Figure.

Daily Number of COVID-19 Cases and Time-Varying Effective Reproduction Number (Rt) of the SARS-CoV-2 Omicron Variant, December 1, 2021, to March 16, 2022

The blue solid line indicates the daily estimated Rt; shading, the pointwise 95% CIs; and horizontal dashed line, the critical Rt threshold of 1, below which an epidemic in a community is considered controlled.

Discussion

South Korea achieved significant success in controlling the COVID-19 pandemic with PHSMs alone without formal lockdown during 2020-2021. However, relaxation of rules for social gatherings and vaccine passes during the early phase of the Omicron wave were associated with an increase in Omicron transmissibility, which may have been associated with the rapid increase in Omicron cases in March 2022. After the relaxation of PHSMs, extending the COVID-19 screening system and mandatory school-based screening measures were found to be associated with reduced transmissibility of Omicron. However, our finding of a mean Rt greater than 1 during the study period suggested there may have been difficulty in controlling the COVID-19 pandemic in South Korea.[6]

As limitations of this study, we did not consider heterogeneity in individuals’ exposure to SARS-CoV-2 infection and could not analyze temporal changes in potential for superspreading, that is, SARS-CoV-2 transmission to many individuals. In addition, we did not account for potential sources of behavioral change and other factors associated with reduced transmissibility of Omicron. The findings suggest awareness of Rt can inform policy planning and practice, including by allowing timely implementation and relaxation of PHSMs.