Herd immunity induced by COVID-19 vaccination programs and suppression of epidemics caused by the SARS-CoV-2 Delta variant in China

Liu, Hengcong; Zhang, Juanjuan; Cai, Jun; Deng, Xiaowei; Peng, Cheng; Chen, Xinghui; Yang, Juan; Wu, Qianhui; Chen, Zhiyuan; Zheng, Wen; Viboud, Cécile; Zhang, Wenhong; Ajelli, Marco; Yu, Hongjie

doi:10.1101/2021.07.23.21261013

Cited by 21 publications

(32 citation statements)

References 50 publications

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“…According to our modelling results, at least 7-days quarantine time should be maintained so that boosting high-risk workers was more effective in reducing the total infections and deaths. Besides, vaccinating 74%-89% of total populations with varying efficiency (70%-50%) can help accomplish the virus-eliminating project of China, which was generally in consistent with the figure suggested by World Health Organization (WHO) and previous estimation conducted by Liu et al [27,28]. Additionally, the required coverage decreased to 71%-84% when the restrictions of keeping social distance and masks wearing were implemented.…”

Section: Discussionsupporting

confidence: 86%

Impact of non-pharmacological interventions on COVID-19 boosting vaccine prioritization and vaccine-induced herd immunity: a population-stratified modelling study

Wang

Yang

et al. 2021

Preprint

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BackgroundWhile the COVID-19 pandemic seemed far from the end, the booster vaccine project was proposed to further reduce the transmission risk and infections. However, handful studies have focused on questions that with limited vaccine capacity ether boosting high-risk workers first or prioritizing susceptible normal individuals is optimal, and vaccinating how many people can lead us to the goal of herd immunity. In this study we aimed to explore the conclusions of such two problems with consideration of non-pharmacological interventions including mandatory quarantine for international entrants, keeping social distance and wearing masks.MethodsBy implementing the corresponding proportion of individuals who remain infectious after four lengths of quarantine strategies to the novel population-stratified model, we quantified the impact of such measures on optimizing vaccine prioritization between high-risk workers and normal populations. Furthermore, by setting the hypothetical COVID-19 transmission severity (reproduction number, R0) to the level of the most contagious COVID-19 variant (B.1.617.2, delta variant, R0 = 5.0), we separately estimated the threshold vaccine coverage of five countries (China, United States, India, South Africa and Brazil) to reach herd immunity, with and without the consideration of interventions including wearings masks and keeping social distance. At last, the sensitive analysis of essential parameter settings was performed to examine the robustness of conclusions.ResultsFor Chinese scenarios considered with moderate hypothetical transmission rate (R0 = 1.15–1.8), prioritizing high-risk workers the booster dose reached lower cumulative infections and deaths if at least 7-days of quarantine for international travelers is maintained, and the required screening time to remain such vaccinating strategy as optimal increased from 7-days to 21-days with the transmission severity. Although simply maintaining at least 7-days quarantine can lead to over 69.12% reduction in total infections, the improvement of longer quarantine strategies was becoming minimum and the least one was 2.28% between the 21 and the 28-days of quarantine. Besides, without the vaccination program, the impact of such measures on transmission control dropped significantly when R0 exceeded 1.5 and reached its minimal level when R0 equal to 2.5. On the other hand, when we combat the delta variant, the threshold vaccine coverage of total population to reach herd immunity lay within 74%–89% (corresponding to the vaccine efficiency from 70% to 50%), and such range decreased to 71%–84% if interventions including wearing mask and keeping social distance were implemented. Furthermore, Results of other countries with 85% vaccine efficiency were estimated at 79%, 91%, 94% and 96% for South Africa, Brazil, India and United States respectively.ConclusionsNon-pharmacological interventions can substantially affect booster vaccination prioritization and the threshold condition to reach herd immunity. To combat the delta variant, restrictions need to be integrated with mass vaccination so that can reduce the transmission to the minimum level, and the 21-days might be the suggested maximum quarantine duration according to the cost-effectiveness. Besides, by implementing interventions, the requirement to reach herd immunity can be lower in all countries. Lastly, the following surveillance after vaccination can help ensure the real-time proportion of vaccinated individuals with sufficient protection.

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Section: Discussionsupporting

confidence: 86%

Impact of non-pharmacological interventions on COVID-19 boosting vaccine prioritization and vaccine-induced herd immunity: a population-stratified modelling study

Wang

Yang

et al. 2021

Preprint

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“…The numbers depicted by the model are related to the relatively low vaccine efficacy of the vaccines in use in China against Delta variant infection (54.3% [35]) as of September 2021. Model projections show that, in the absence of NPIs, even when 97% of the population aged ≥12 years is fully vaccinated, the importation of Delta variant infections could lead to a new COVID-19 wave and substantial health burden.…”

Section: Discussionmentioning

confidence: 99%

“…We consider that susceptible individuals are only eligible for vaccination, mimicking the programme implemented in China. Moreover, we accounted for individuals with contraindications against vaccination and pregnant women as they were excluded from the Chinese vaccination programme (Table S2) [35]. We simulated the daily distribution of vaccine doses according to the observed vaccination capacity (S1 Text).…”

Section: Vaccination Strategy and Vaccine Effectivenessmentioning

confidence: 99%

“…We considered a two-dose vaccine with a 21-day interval between doses. Fourteen days after the second dose, the vaccine efficacy in protecting against an infection caused by the Delta variant was set at 54.3% [35]. The considered vaccine effectiveness (VE) against clinical endpoints such as symptomatic COVID-19, hospitalisations, ICU admissions, and deaths are reported in Table 1.…”

Section: Vaccination Strategy and Vaccine Effectivenessmentioning

confidence: 99%

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Projecting the transition of COVID-19 burden towards the young population while vaccines are rolled out: a modelling study

Cai

Yang

Deng

et al. 2021

Preprint

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SARS-CoV-2 infection causes most cases of severe illness and fatality in older age groups. In China, over 85% of individuals aged ≥12 years have been vaccinated against COVID-19 (albeit with vaccines developed against historical lineages), while children aged 0-11 years are currently not eligible for vaccination (as of September 2021). The aim of this work is to assess whether the importation of Delta variant infections will shift the COVID-19 burden from adults to children. We developed an age-structured susceptible-infectious-removed model of SARS-CoV-2 transmission dynamics to simulate the epidemics triggered by the importation of Delta variant infections and project the age-specific incidence of SARS-CoV-2 infections, cases, hospitalisations, intensive care unit (ICU) admissions, and deaths. In the context of the vaccination programme targeting individuals aged ≥12 years (as of September 2021), and in the absence of non-pharmaceutical interventions, the importation of Delta variant infections could lead to widespread transmission and substantial disease burden in mainland China, even with vaccination coverage as high as 97% across the currently eligible age groups. The symptomatic SARS-CoV-2 infections and hospitalisation are projected to shift towards children and young adolescents, with 13% of symptomatic infections and 30% of hospitalisations occurring in those aged 0-11 years. Extending the vaccination roll-out to include children aged 3-11 years is estimated to dramatically decrease the burden of symptomatic infections and hospitalisations within this age group (54% and 81%, respectively), but would have a low impact on protecting infants (aged 0-2 years). Our findings highlight the need to strengthen vaccination efforts by simultaneously extending the target population and elevating vaccine effectiveness.

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“…In the case of SARS-CoV-2 and vaccine efficacies of ~95% it is estimated that 63-75% of people need to be immune to provide herd immunity [159,160]. Hence, a high acceptance of vaccines, or a widespread natural immunity following infection is required to achieve this [161]. Unfortunately, COVID-19 delta infections currently threaten vaccine strategies to achieve herd immunity [162].…”

Section: What Do We Need To Achieve Herd Immunity?mentioning

confidence: 99%

Immune Responses against SARS-CoV-2—Questions and Experiences

et al. 2021

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Understanding immune reactivity against SARS-CoV-2 is essential for coping with the COVID-19 pandemic. Herein, we discuss experiences and open questions about the complex immune responses to SARS-CoV-2. Some people react excellently without experiencing any clinical symptoms, they do not get sick, and they do not pass the virus on to anyone else (“sterilizing” immunity). Others produce antibodies and do not get COVID-19 but transmit the virus to others (“protective” immunity). Some people get sick but recover. A varying percentage develops respiratory failure, systemic symptoms, clotting disorders, cytokine storms, or multi-organ failure; they subsequently decease. Some develop long COVID, a new pathologic entity similar to fatigue syndrome or autoimmunity. In reality, COVID-19 is considered more of a systemic immune–vascular disease than a pulmonic disease, involving many tissues and the central nervous system. To fully comprehend the complex clinical manifestations, a profound understanding of the immune responses to SARS-CoV-2 is a good way to improve clinical management of COVID-19. Although neutralizing antibodies are an established approach to recognize an immune status, cellular immunity plays at least an equivalent or an even more important role. However, reliable methods to estimate the SARS-CoV-2-specific T cell capacity are not available for clinical routines. This deficit is important because an unknown percentage of people may exist with good memory T cell responsibility but a low number of or completely lacking peripheral antibodies against SARS-CoV-2. Apart from natural immune responses, vaccination against SARS-CoV-2 turned out to be very effective and much safer than naturally acquired immunity. Nevertheless, besides unwanted side effects of the currently available vector and mRNA preparations, concerns remain whether these vaccines will be strong enough to defeat the pandemic. Altogether, herein we discuss important questions, and try to give answers based on the current knowledge and preliminary data from our laboratories.

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Herd immunity induced by COVID-19 vaccination programs and suppression of epidemics caused by the SARS-CoV-2 Delta variant in China

Cited by 21 publications

References 50 publications

Impact of non-pharmacological interventions on COVID-19 boosting vaccine prioritization and vaccine-induced herd immunity: a population-stratified modelling study

Impact of non-pharmacological interventions on COVID-19 boosting vaccine prioritization and vaccine-induced herd immunity: a population-stratified modelling study

Projecting the transition of COVID-19 burden towards the young population while vaccines are rolled out: a modelling study

Immune Responses against SARS-CoV-2—Questions and Experiences

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