What pushed Israel out of herd immunity?

Clinical Trials & Research

Scientists have recently tried to explain the factors that could be responsible for the recent outbreaks of coronavirus disease 2019 (COVID-19) in Israel. Their observations reveal that the prevalence of the more infectious delta variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and waning vaccine efficacy results in the outbreaks. As preventive interventions, they suggest the strict implementation of non-pharmaceutical control measures and administering a third booster dose of the vaccine.

Study: What pushed Israel out of herd immunity? Modeling COVID-19 spread of Delta and Waning immunity. Image Credit: next143/ Shutterstock

A preprint version of the study is available on the medRxiv* server while the article undergoes peer review.

Background

Soon after its first detection in India in October 2020, the delta variant (B.1.617.2) became dominant in many countries, including Israel. The variant is known to have significantly higher infectivity and virulence compared to previously circulating variants of SARS-CoV-2.

Israel had vaccinated more than 60% of the population with Pfizer/BioNTech-developed COVID-19 vaccine BNT162b2 by the end of February 2021. With this high vaccine coverage, the country had initially experienced a significant reduction in infection and mortality rates. Assuming that the country had reached the herd immunity threshold, the health authorities decided to remove non-pharmaceutical control measures, including mandatory mask-wearing. Despite this initial success, a sharp rise in new infections and related mortality was observed in Israel by August 2021, when the delta variant was predominantly circulating across the country.

In the current study, the scientists have investigated whether the predominance of the delta variant is solely responsible for the recent outbreaks in Israel.

Study design

The scientists used a spatial-dynamic model of disease spread to examine the impact of vaccination on both confirmed COVID-19 cases and severe cases that required hospitalization. Moreover, they estimated the current effectiveness of the BNT162b2 vaccine.

They calculated the theoretical reproduction number for the alpha and delta variants of SARS-CoV-2, which defines the contact frequency between infected and healthy individuals that would have resulted in an infection in the absence of vaccines. Unlike the basic reproduction rate of a virus, the theoretical reproduction number considers the effectiveness of non-pharmaceutical control measures.

Important observations

The scientists tested two different scenarios using the spatial-dynamic models. In one model, they considered the predominance of delta variant and no decline in vaccine efficacy with time (delta model). In another model, they considered the predominance of alpha variant and a time-dependent reduction in vaccine efficacy (waning model).

The findings obtained from these models revealed that both waning vaccine efficacy and infectiousness of the delta could independently push Israel above the herd immunity threshold. Both models indicated that a declining vaccine efficacy could result in 2000 confirmed COVID-19 cases a day. Similarly, a predominant circulation of the delta variant could lead to the detection of 3500 daily cases.

Since older adults were prioritized in all vaccination campaigns, they are expected to experience a waning vaccine immunity. Consistent with this hypothesis, the waning model predicted a higher number of daily cases (250 cases) in individuals aged 60 years or above than the delta model (150 cases). When these models were combined, this number increased to 700 cases, equivalent to that observed in the real-world setup (716 cases).

With further analysis, it was observed that the percentage of cases in older adults is significantly lower than that in the total population during the winter outbreaks. This could be because of higher vaccination rates among older adults. In contrast, a relatively increased percentage of cases were observed among older adults in the recent outbreaks, further justifying the effect of waning vaccine efficacy.

Based on these observations, the scientists suggested that countries that have vaccinated a large proportion of their population only recently might effectively control delta outbreaks. Moreover, by analyzing the percentage of recent vaccine recipients and the number of COVID-19 related deaths in 50 countries, they revealed that a higher rate of recent vaccination is associated with a lower rate of COVID-19 related mortality.

Study significance

The study identifies two factors responsible for recent outbreaks of COVID-19 in Israel – higher infectiousness of the delta variant and waning vaccine efficacy. As mentioned by the scientists, future outbreaks can be prevented by administering a third booster dose of the vaccine and implementing non-pharmaceutical control measures (mask-wearing, physical distancing, etc.).

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
  • De-Leon, H. and Aran, D. (2021) “What pushed Israel out of herd immunity? Modeling COVID-19 spread of Delta and Waning immunity”. medRxiv. doi: 10.1101/2021.09.12.21263451.

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