The Omicron variant of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus is many times more contagious than previous variants. During the course of the coronavirus disease 2019 (COVID-19) pandemic, lockdowns have been implemented globally to curb the spread of the virus. Whether such lockdowns have been effective or not remains unknown.
A recent study published in Vaccines aimed to fill this gap in research by evaluating the impact of strict lockdown measures on SARS-CoV-2 Omicron transmission in Shanghai between March and April 2022.
To curb the spread of the SARS-CoV-2 virus, many countries have widely deployed non-pharmaceutical interventions (NPIs) and lockdowns. Despite the development of vaccines, variants of concern (VOC), such as the Delta variant, have driven large and disruptive waves. The Omicron VOC was first identified in South Africa in November 2021, with key mutations responsible for immune escape (K417N, E484A, and T478K).
The Omicron replaced the Delta variant rapidly as the dominantly circulating strain, hinting at its strong selective advantage. Compared to the Delta variant, Omicron has a greater affinity for human angiotensin-converting enzyme 2, and this is due to a large number of mutations in the receptor-binding domain (RBD).
The epidemiology of the early 2022 Hong Kong wave showed that the intrinsic severity of Omicron could be similar to ancestral strains. Given the mutations in the Omicron variant, researchers will need to develop Omicron-specific vaccines, and in the meantime, lockdown measures may still be needed to control the viral spread. But, the efficacy of lockdowns in curbing the spread of the Omicron variant remains unknown.
About the study
The current retrospective study analyzed the strictness of the lockdown policy enacted in Shanghai and contrasted it with other countries. Specifically, the recent Omicron wave was considered by evaluating case numbers in March and April 2022, and critical factors to make lockdowns effective were identified. The efficacy of the lockdown was gauged by calculating the effective reproductive number based on the daily case numbers. Pearson correlation coefficients were computed to highlight the driving factors of strict lockdowns in the 16 different districts of Shanghai.
The correlation analysis revealed that the timely lockdown was instrumental in controlling the Omicron epidemic in urban regions. The lockdown was implemented right before the case numbers surged, which could have stopped the asymptomatic transmission. Consistent with the current study, prior research has also utilized the same Oxford Coronavirus Government Response Tracker project and studied the relationship between social distancing measures and disease incidence in 149 countries. These studies documented that early lockdown implementation was associated with a greater decline in the incidence of COVID-19. Similar results have been documented in the current study in the context of the Omicron variant.
It was hypothesized that preventing community transmission through lockdowns in Shanghai could have mitigated the asymptomatic transmission among young and middle-aged individuals. Insights from a previous New Zealand study were used to support this hypothesis. New Zealand had a low proportion of asymptomatic infection, despite widespread testing, compared to other countries. It was believed that the low level of community transmission mainly drove this.
It was also highlighted that lockdown measures should be implemented differently among young/middle-aged people and the elderly. This is because the former group is more mobile and likely to transmit infection faster. The current study attributed the post-lockdown low asymptomatic viral transmission in Shanghai to low and middle-aged individuals, whose movements were restricted much more.
Strengths and limitations
Most countries adopted looser public health social measures compared to the strict lockdown in Shanghai. Coupled with the Omicron outbreak, this provides an opportunity to accurately study the effectiveness of strict lockdowns in reducing the transmission of the Omicron variant. The study's main limitation was the inability to measure population mobility accurately. Only subway ridership data were used, as information on other public and private transport was unavailable.
The current study confirmed the effectiveness of strict lockdown measures in curbing the spread of the highly contagious SARS-CoV-2 Omicron variant. The success in Shanghai was largely due to the reduction of the asymptomatic spread. Based on the findings, the authors recommended formulating differential lockdown strategies dependent on demographic composition and population density. This could also reduce the economic, social, and psychological costs of city-wide lockdowns.